UNIVERSITY OF CALIFORNIA 
 
 ANDREW 
 
 SMITH 
 
 HALLIDIE: 
 
* 
 
By the Same Author. 
 
 ELECTRIC POWER 
 
 TRANSMISSION 
 
 Third Edition, Revised and Enlarged. 
 632 pages. 285 illustrations. 21 
 plates. Price $}.oo. 
 
 POWER DISTRIBUTION FOR 
 ELECTRIC RAILWAYS 
 
 Second Edition, Extensively Revised 
 and Enlarged. 
 
 33'P^S es t ~ ~ 1 4$ illustrations 
 Price $2.50. 
 
 McGEAW PUBLISHING COMPANY 
 
 114 Liberty St., .NEW YORK. 
 
THE ART 
 
 OF 
 
 ILLUMINATION 
 
 BY 
 
 LOUIS BELL, PH. D. 
 
 OF THE 
 
 UNIVERSITY 
 
 NEW YORK 
 
 McGRAW PUBLISHING CO. 
 
 114 LIBERTY STREET 
 1902 
 
HALUDIE 
 
 COPYRIGHTED, 1902, 
 
 BY THE 
 
 MCGRAW PUBLISHING COMPANY, 
 NEW YORK. 
 
PREFACE. 
 
 THIS volume is a study of the utilization of artificial 
 light. It is intended to deal, not with the problem of dis- 
 tributing illuminants, but with their application, and treats 
 of the illuminants themselves only in so far as a knowl- 
 edge of their peculiarities is necessary to their intelligent 
 use. To compress the subject within reasonable bounds, 
 it has been necessary to discuss general principles rather 
 than concrete examples of artificial lighting. The science 
 of producing light changes rapidly and the apparatus of 
 yesterday may be discarded to-morrow, but the art of 
 employing the materials at hand to produce the required 
 results follows lines which are to a very considerable 
 extent subject to fairly well-defined laws. Sins against 
 these laws are all too common, the more so since artificial 
 light has become relatively cheap and easy of applica- 
 tion. If this outline of a complex art shall tend to avert 
 even some of the commoner errors and failures in illumi- 
 nation, it will have served its purpose. The author here 
 desires to express his obligations to the beautiful treatise 
 of M. Allemagne for illustrations of early fixtures and to 
 numerous friends, notably Mr. Luther Stieringer, for 
 valuable material and suggestions. 
 
 November, 1902. 
 
 1 16753 
 
CONTENTS. 
 
 CHAPTER PAGE 
 
 I. LIGHT AND THE EYE, . . . i 
 
 II. PRINCIPLES OF COLOR, 23 
 
 V III. REFLECTION AND DIFFUSION, 38 
 
 -IV. THE MATERIALS OF ILLUMINATION ILLUMIMANTS OF 
 
 COMBUSTION, . . 56 
 
 V. THE MATERIALS OF ILLUMINATION INCANDESCENT BURN- 
 ERS, 83 
 
 VI. THE ELECTRIC INCANDESCENT LAMP, .... 95 
 
 VII. THE ELECTRIC ARC LAMP, 140 
 
 VIII. SHADES AND REFLECTORS, 163 
 
 IX. DOMESTIC ILLUMINATION, ...... 183 
 
 X. LIGHTING LARGE INTERIORS 211 
 
 . XI. STREET AND EXTERIOR ILLUMINATION, .... 244 
 
 XII. DECORATIVE AND SCENIC ILLUMINATION, . . . 275 
 
 XIII. THE ILLUMINATION OF THE FUTURE, . . . .301 
 
 XIV. STANDARDS OF LIGHT AND PHOTOMETRY, . . 313 
 
 IX 
 
OF THE 
 
 ( UNIVERSITY;) 
 
 I 
 
 THE ART OF ILLUMINATION. 
 
 CHAPTER I. 
 
 LIGHT AND THE EYE. 
 
 WHILE even the Esquimaux and the Patagonian use 
 artificial light and all civilized peoples count it a necessity, 
 it is seldom used skillfully and with proper knowledge of 
 the principles that should govern its employment. Since 
 the introduction of electric lights that very facility of ap- 
 plication which gives them unique value has encouraged 
 more zeal than discretion in their use. It is the purpose 
 of the present volume to set forth some of the fundamental 
 doctrines, optical, physiological, and aesthetic, which 
 underlie the proper use of artificial illuminants, and to 
 point out how they may be advantageously adapted to 
 existing conditions. 
 
 To begin with, there are two general purposes which 
 characterize two quite distinct branches of the art of illu- 
 mination. First comes the broad question of supplying 
 artificial light for carrying on such avocations or amuse- 
 ments as are extended into the hours of darkness. Quite 
 apart from this is the case of scenic illumination directed 
 at special objects and designed to produce particular 
 effects or illusions. Lighting a shop or a house exempli- 
 fies the one, lighting a picture gallery or the stage of a 
 theater the other. Each has a distinct purpose, and re- 
 
2 THE ART OF ILLUMINATION. 
 
 quires special means for its accomplishment. Confusing 
 the purposes or mixing the methods often leads to serious 
 mistakes. Sometimes both general and scenic illumina- 
 tion have to be used coincidently, but the distinction be- 
 tween them should be fully realized even when it cannot 
 fully be preserved. 
 
 General illumination, whether intended to serve the 
 ends of work or play, must fulfill the following condi- 
 tions: it must be amply adequate in amount, suitable in 
 kind, and must be so applied as not to react injuriously 
 upon the eye. 
 
 i/ It must be remembered that the human eye is not merely 
 a rather indifferent optical instrument, but a physical 
 organ which has through unfathom- 
 able ages accumulated the characters 
 wrought upon it by evolution, until it 
 bears the impress and incurs the limita- 
 tions of its environment. It works 
 
 Fi i Indian ^ >es * over a ratner li m ited retinal area 
 Goggles. and through a range in intensity of 
 
 light which, although great, is yet immensely smaller 
 than the range available to nocturnal creatures. It 
 has, moreover, become habituated to, and adapted to, 
 light coming obliquely from above, and resents strong 
 illumination, whether natural or artificial, from any 
 other direction. It seems to be well established, for ex- 
 ample, that the distress caused by the reflected glare from 
 sand, or water, or snow, and the grave results which fol- 
 low prolonged exposure to it, are due not so much to the 
 intensity of the light as to the fact that it is directed up- 
 ward into the eye and is quite insufficiently stopped by 
 the rather transparent lower eyelid. Ordinary dark 
 glasses are small protection in this case, but if the lower 
 
LIGHT AND THE EYE. 3 
 
 part of the eye be thoroughly guarded no difficulty is 
 found. The Alaskan Indians have evolved a very effect- 
 ive protection against snow blindness in the shape of 
 leather goggles with the eye arranged as shown in Fig. i . 
 The eyepiece is merely a round bit of dark leather with a 
 semicircular cut made for the peep hole, the resulting flap 
 being turned outward and downward, so that the eye is 
 fully guarded from brilliant upward beams. Blackening 
 the whole lower eyelid with burnt cork is stated by one 
 distinguished oculist to be completely efficacious for the 
 same reason. 
 
 It is more than likely that the bad effects ascribed to the 
 habit of reading while lying down are due largely to the 
 unwonted direction of the illumination, as well as to the 
 unusual direction of the eye's axis. 
 
 All these matters are of fundamental importance in 
 planning any illumination to facilitate hard visual work. 
 Their significance is that we are not at liberty to depart 
 widely from the distribution and character of natural day- 
 light illumination. Of course,, one realizes immediately 
 that the eye is neither fitted nor habituated to working to 
 advantage in anything like the full strength of sunlight, 
 but its more general properties steadiness, absence of 
 pronounced color, downward cblique direction, wide and 
 strong diffusion, freedom from sharp and black shadows 
 these must be followed rather closely in ordinary artifi- 
 cial illumination, or the eye, that has been taking form 
 through a million years of sunlight and skylight, will re- 
 sent the change. The eye is automatically adjustable, it 
 is true, for wonderfully diverse conditions, but persistent 
 and grave changes in environment are more than it can 
 bear. 
 
 Now from a practical standpoint the key to artificial 
 
4 THE ART OF ILLUMINATION. 
 
 illumination is found in the thoughtful contemplation of 
 what is known as Fechner's law, relating to the sensitive- 
 ness of the eye to visual impressions. It is stated by 
 Helmholtz substantially as follows : " Within very wide 
 limits of brightness, differences in the strength of light 
 are equally distinct or appear equal in sensation, if they 
 form an equal fraction of the total quantity of light com- 
 pared." That is, provided the parts of the visual picture 
 remain of the same relative brightness the distinctness of 
 detail does not vary materially with great changes of ab- 
 solute brightness. Now since, barring binocular vision, 
 our whole perception of visible things depends, in the ab- 
 sence of strong color contrasts, upon differences of illumi- 
 nation, the importance of the law just stated needs little 
 comment. It implies what experience proves, that within 
 a rather wide range of absolute brightness of illumination 
 our vision is about equally effective for all ordinary pur- 
 poses. 
 
 Fechner's law, to be sure, fails when extremely brilliant 
 lights are concerned. Few persons realize, for instance, 
 that the sun is twice as bright at noon as it is when still 
 10 to 15 degrees above the horizon, still less that its bril- 
 liancy is reduced more than a hundred fold as its lower 
 limb touches the horizon. Yet while the eye does not de- 
 tect very small changes or properly evaluate large ones in 
 a body so bright as the sun, the mere fact that one can see 
 to work or read about equally well from sunrise to sunset 
 is most convincing as to the general truth of the law. 
 Full sunlight at noon is generally over-bright for the eye 
 if it falls directly upon the work, but with half of it one 
 can get along very comfortably. 
 
 All this is most important from the standpoint of arti- 
 ficial illumination, since it means that within rather wide 
 
LIGHT AND THE EYE. 5 
 
 limits of intensity artificial lighting remains about equally 
 effective for most practical purposes. 
 
 The actual amount of illurriination necessary and desir- 
 able, the terms by which we measure it, and the laws that 
 govern its intensity are matters of primary importance 
 which must now occupy our attention. 
 
 A simple and definite standard of light is greatly to be 
 desired, but we do not yet possess it. The p/actical and 
 generally legal standard in English-speaking countries is 
 the standard candle. This is defined to be a spermaceti 
 candle of certain definite dimensions, weighing one-sixth 
 of a pound avoirdupois, and burning 120 grains per hour. 
 Such a candle is a fairly steady and uniform source of 
 light, and although far less precise than would be desira- 
 ble, has served a most useful purpose as a standard of light. 
 From this a standard of illumination is derived by defin- 
 ing the distance at which this standard intensity produces 
 a certain definite illumination, which forms an arbitrary 
 unit. Thus one candle-foot has come to be a definite unit 
 of illumination, i. e., the direct illumination given by a 
 standard candle one foot from the object illuminated. Of 
 course, it is entirely empirical, but it serves the practical 
 purpose of comparing and defining amounts of illumina- 
 tion just as well as if it were a member of the C. G. S. 
 system in good and regular standing. 
 
 A unit of illumination frequently used abroad is the 
 bougie-meter, similarly derived, with the meter as unit dis- 
 tance. This is sometimes known as the lux, but un- 
 happily there is neither any convenient and practicable 
 absolute standard of light nor any definitely settled nofnen- 
 clature of the subject, so that to save confusion the writer 
 prefers to adhere for the present to candle-foot, which is 
 at least specific, and bears a determinable relation to the 
 
6 THE ART OF ILLUMINATION. 
 
 bougie-meter. (Approximately the candle-foot equals 
 eleven bougie-meters. ) 
 
 For any light the illumination at one foot distance is 
 obviously a number of candle feet numerically equal to the 
 candle power of the light. 
 
 At distances other than one foot the illuminating power 
 is determined by the well defined, but often misapplied, 
 " law of inverse squares." This .law states that the in- 
 tensity of light from a given source varies inversely as the 
 square of the distance from that source. Thus if we have 
 a radiant point (P, Fig. 2) it will shine with a certain in- 
 tensity on a surface a b c d at a distance e P. If we go to 
 double the distance (E P) the same light which fell on 
 abed now falls on the area A B C D of twice the linear 
 dimensions and four times the area, and consequently the 
 intensity is reduced to one-fourth of the original amount. 
 Thus if P be one candle and e P one foot, then the illumi- 
 nation at e will be one candle-foot, and at E one-fourth 
 candle-foot. 
 
 This law of inverse squares is broadly true of every case 
 of the free distribution of energy from a point within a 
 homogeneous medium, for reasons obvious from the in- 
 spection of Fig. 2. It does not hold in considering a 
 radiant surface as a whole, nor for any case in which the 
 medium is not homogeneous within the radii considered. 
 
 By reason of these limitations, in problems of practical 
 illumination the law of inverse squares can be considered 
 only as a useful guide, for it is far from infallible, and 
 may lead to grossly inaccurate results. It is exact only in 
 the rare case of radiation from a minute point into space 
 in which there is no refraction or reflection. A room 
 with dead black walls, lighted by a single candle, would 
 furnish an instance in which the illumination could be 
 
LIGHT AND THE EYE. 
 
 computed by the law of inverse squares without an error 
 of- more than say 2 or 3 per cent., while a white and gold 
 room lighted by a well shaded arc light would illustrate 
 an opposite condition, in which the law of inverse squares 
 alone would give a result grossly in error. 
 
 Fig. 3 shows how completely deceptive the law of in- 
 verse squares may become in cases complicated by refrac- 
 tion or reflection. Here one deals with an arc light of 
 perhaps 10,000 nominal cp. as the source of radiation, 
 
 A 
 
 Fig. 2. Illustrating Law of Inverse Squares. 
 
 but a very large proportion of the total luminous energy is 
 concentrated' by the reflector or lens system into a nearly 
 parallel beam which maintains an extremely high lumi- 
 nous intensity at great distances from the apparatus. If 
 the beam were actually of parallel rays its resultant illumi- 
 nation would be uniform at all distances, save as dimin- 
 ished by the absorption of the atmosphere, probably not 
 over iQper cent, in a mile in ordinary clear weather, since 
 the absorption of the entire thickness of the atmosphere 
 for the sun's light .is only about 16 per cent. 
 
 The searchlight furnishes really a special case of scenic 
 illumination, which frequently depends upon the use of 
 concentrated beams in one form or another, so that one 
 must realize that a very considerable branch of the art of 
 illumination imposes conditions not reconcilable with the 
 ordinary application of the law of inverse squares. 
 
8 THE ART OF ILLUMINATION. 
 
 It is worth while thus to examine the law in question, 
 because it is a specially flagrant example of a principle 
 absolutely and mathematically correct within certain rigid 
 limitations, but partially or wholly inapplicable in many 
 important cases. 
 
 Having considered the unit strength of light and the 
 
 Fig. 3- Beam From Searchlight. 
 
 unit strength of illumination, the other fundamental of 
 artificial lighting is the intensity of the luminous source 
 generally known as intrinsic brightness. Optically this 
 has no very great or direct importance, but physiologically 
 it is of the most serious significance, and perhaps deserves 
 more thoughtful attention than any other factor in prac- 
 tical illumination. It is of the more consequence, as it is 
 the one thing which generally receives scant considera- 
 tion, and is left to chance or convenience. 
 
 By intrinsic brightness is meant the strength of light 
 per unit area of light-giving surface. If we adopt the 
 
LIGHT AND THE EYE. 9 
 
 standard candle as the unit of light, and adhere to English 
 measures, the logical unit of intrinsic brightness is one 
 candle power per square inch. One then may conven- 
 iently express the brightness of any luminous surface in 
 candle power per square inch, and thus obtain a definite 
 basis of comparison. 
 
 Although a measure of intrinsic brightness is obtained 
 by dividing the candle-power of any light by the area of 
 the luminous surface, this latter quantity is very difficult 
 to determine accurately, since with the exception of the 
 electric incandescent filament no source of light is any- 
 where nearly of uniform brilliancy over its entire surface. 
 For the sake of comparison we can, however, draw up an 
 approximate table by assuming equal brightness over the 
 generally effective lighting area of any radiant. It should 
 be distinctly understood that the values tabulated are only 
 average values of quantities, some of which are incapable 
 of exact determination and others of which vary over a 
 
 .wide range according to conditions. 
 j ^^ 
 
 INTRINSIC BRILLIANCIES IN CANDLE POWER PER 
 
 SQUARE INCH 
 SOURCE. BRILLIANCY. NOTES. 
 
 horizon 
 
 Arc light 10,000 to 100,000 Maximum about 200,000 in crater. 
 
 Calcium light 5,ooo 
 
 Nernst " glower" 1,000 Unshaded. 
 
 Incandescent lamp 200-300 Depending on efficiency. 
 
 Melting platinum. 130 I sq. cm. = 18.5 c.p. 
 
 Enclosed arc 75-100 Opalescent inner globe. 
 
 Acetylene flame 75-ioo 
 
 Welsbach light 20 to 25 
 
 Kerosene light 4 to 8 Very variable. 
 
 Candle 3 to 4 
 
 Gas flame 3 to 8 Very variable. 
 
 Incandescent (frosted). .. 2 to 5 
 
 Opal shaded lamps, etc. . 0.5 to 2 
 
 The striking thing about this table is the enormous dis- 
 crepancy between electric and other lamps of incandes- 
 
io THE ART OF ILLUMINATION. 
 
 cence and flames of the ordinary character. The very 
 great intrinsic brilliancy of the ordinary incandescent lamp 
 is particularly noteworthy and, from the oculist's stand- 
 point, menacing. 
 
 Everyone is familiar with the distress caused the eye by 
 sudden alternations of light and darkness, as in stepping 
 from a dark room into full sunlight, or even in lighting 
 the gas after the eye has become habituated to the dark- 
 ness. The eye is provided with a very wonderful auto- 
 matic " iris diaphragm " for its adjustment to various 
 degrees of illumination, but it is by no means instanta- 
 neous, although very prompt, in its action. Moreover, the 
 eye after resting in darkness is in an extremely sensitive 
 and receptive state, and a relatively weak light will then 
 produce very noticeable after-images. These after- 
 images, such as are seen in vivid colors after looking at 
 the sun, are due to retinal fatigue. 
 
 If the image of a brilliant light is formed upon the 
 retina, it produces certain very considerable chemical 
 changes, akin to those produced by light upon sensitized 
 paper. In so doing it temporarily exhausts or weakens 
 the power of the retina to respond at that point to further 
 visual impressions, and when the eye is turned away the 
 image appears, momentarily persistent, and then reversed, 
 dark for a white image, and of the complementary hue for 
 a colored one. This after-image fades away more or less 
 slowly, according to the intensity of the original impres- 
 sion, as the retina recovers its normal sensitiveness. 
 
 A strong after-image means a serious local strain upon 
 the eye, and shifting the eye about when brilliant light can 
 fall upon it implies just the same kind of strain that one 
 gets in going out of a dark room into bright sunshine. 
 The results of either may be very serious. In one case 
 
LIGHT AND THE EYE. n 
 
 recently reported a strong side light from an unshaded in- 
 candescent lamp set up an inflammation that resulted in 
 the loss of an eye. The light was two or three feet from 
 the victim, whose work was such that the image of the 
 filament steadily fell on about the same point on the retina, 
 at which point the resulting inflammation had its focus. 
 A few weeks' exposure to these severe conditions did the 
 mischief. This is an extreme case, but similar conditions 
 may very quickly cause troubleTj A year or two since the 
 writer was at lunch facing a window through which was 
 reflected a brilliant beam from a white painted sign in full 1 
 sunlight just across the street. No especial notice was 
 taken of this, until on glancing away a strong after-image 
 of the sign appeared, and although the time of exposure 
 was only ten or fifteen minutes, the net result was inability 
 to use the eyes more than a few minutes at a time for a 
 fortnight afterwards. 
 
 To certain extent the eye can protect itself from too 
 brilliant general illumination by closing up the iris, and it 
 always does so, reducing the general brightness of the 
 retinal images, as one regulates the illumination on a 
 photographic plate. The following results of experi- 
 ments by Lambert will give an idea of the way in which 
 the pupil reacts to variations of light. The radiant used 
 was a hole in a shutter admitting bright skylight to a 
 darkened room. 
 
 RELATIVE DISTANCE. AREA OF PUPIL IN SQ. MM. 
 
 1 7-3 
 
 2 13.0 
 
 3 16-6 
 
 4 20.5 
 
 5 25.0 
 
 6 30.6 
 
 7 36.8 
 
 44-5 
 
 9 48.0 
 
 10 57.1 
 
12 THE ART OF ILLUMINATION. 
 
 But a light of great intrinsic brilliancy produces so 
 strong an image that it may cause trouble even when 
 the aperture of the eye is stopped to the utmost limit 
 provided by nature. In the effort to accomplish this 
 adjustment the iris closes so far, when a brilliant light is 
 in the field of vision, that the rest of the field may be 
 dimmed so much as to interfere with proper vision, quite 
 aside from any question of fatigue induced by the bright 
 image wandering over the retina as the eye is shifted. 
 
 In general terms the iris adjusts itself with reference to 
 the brightest light it has to encounter, so that if there is in 
 the field of vision a source of light of great intrinsic bril- 
 liancy, the working illumination may be highly unsatisfac- 
 tory. The same principle coupled with retinal fatigue 
 accounts for one's inability to see beyond a brilliant light, 
 as in driving towards an arc lamp hung low over the 
 street. 
 
 A very simple experiment, showing the effect of a bril- 
 liant source of light on the apparent illumination, may be 
 tried as follows : Light a brilliant lamp, unshaded, in a 
 good-sized room, preferably one with darkish paper. 
 Then put on the light an opal or similar shade.. It will 
 be found that the change has considerably improved the 
 apparent illumination of the room, although it has really 
 cut off a good part of the total light. Moreover, at points 
 where there remains a fair amount of illumination, the 
 shade has improved the reading conditions very materi- 
 ally. If one is reading where the unshaded light is at or 
 within the edge of the field of vision, the improvement 
 produced by the shade is very conspicuous. Lowering" 
 the intrinsic brilliancy of the light has decreased the strain 
 upon the eye and given it a better working aperture. 
 
 As a corollary to these suggestions on the effect of 
 
LIGHT AND THE EYE. 13 
 
 bright lights on our visual apparatus should be mentioned 
 the fact that sudden variations in the intensity of illumi- 
 nation seriously strain the eye both by fatigue of the 
 retina, due to sudden changes 'from weak to strong light, 
 and by keeping the eye constantly trying to adjust itself 
 to changes in light too rapid for it properly to follow. 
 
 A flickering gaslight, for example, or an incandescent 
 lamp run at very low frequency, strains the eye seriously 
 and is likely to cause temporary, even if not permanent, 
 injury. 
 
 The persistence of visual impressions whereby the 
 retinal image remains steady for an instant after the ob- 
 ject ceases to affect the eye furnishes a certain amount of 
 protection in case of very rapid changes of brilliancy. It 
 acts like inertia in the visual system. 
 
 In the case of arc and incandescent lamps the thermal 
 inertia of the filament or carbon rod also tends physically 
 to minimize the changes, but with a low frequency alter- 
 nating current they may still be serious. 
 
 The exact frequency at which an incandescent lamp on 
 an alternating circuit begins to distress the eye by the flick- 
 ering effect depends somewhat on the individual eye and 
 somewhat on the mass of the filament. In general, a i6-cp 
 lamp of the usual voltages, say 100 to 120 volts, begins to 
 show flickering at or sometimes a little above 30 cycles per 
 second; one foreign authority noting it even up to 40 
 cycles. At 25 cycles the flickering is very troublesome to 
 most eyes, and at 20 cycles or below it is generally quite 
 intolerable. In looking directly at the lamp the filament is 
 so dazzling that the fluctuations are not always in evidence 
 at their full value, and a low frequency lamp is quite likely 
 to be the source of trouble to the eye even when at first 
 glance it -appears to be quite steady. 
 
i 4 THE ART OF ILLUMINATION. 
 
 Lamps having relatively thick filaments can be worked 
 at lower frequencies than those of the common sort, so 
 that 5O-volt lamps, particularly of large candle-power, 
 may be worked at 30 cycles or thereabouts rather well, and 
 out of doors even down to 25 cycles. That is, at a pinch 
 one can do satisfactory work when current is available at 
 25 cycles or so, by using low voltage lamps of 32, 50, or 
 100 cp, which, by the way, are capable of giving admirable 
 results in illumination if properly disposed. Of course, 
 such practice is bad in point of efficient distribution of cur- 
 rent, but on occasion it may be useful. 
 
 As to arc lamps, conditions are not so favorable. The 
 fluctuations of an alternating arc lamp are easily detected, 
 even at 60 cycles, by moving a pencil or the finger quickly 
 when strongly illuminated. The effect is a series of 
 images along the path of motion, corresponding to the 
 successive maxima of light in the arc. At 40 to 45 cycles 
 the flickering becomes evident even when viewing station- 
 ary objects, the exact point where trouble begins depend- 
 ing upon the adjustment of the lamp, the hardness of the 
 carbons, and various minor factors. Enclosing the 
 arc mitigates the difficulty somewhat, but does not re- 
 move it. 
 
 In working near the critical frequency the best results 
 are attained by using an enclosed arc lamp taking all the 
 current the inner globe will stand, with as short an arc as 
 will work steadily. 
 
 When polyphase currents are available, as is usually the 
 case where rather low frequencies are involved, some relief 
 may be obtained by arranging the arcs in groups consist- 
 ing of one from each phase. At a little distance from 
 such a group the several illuminations blend so as to par- 
 tially suppress the fluctuations of the individual arcs. 
 

 LIGHT AND THE EYE. 15 
 
 This device makes it possible to obtain fairly satisfactory 
 lighting between 35 and 40 cycles. At these frequencies, 
 however, the arcs should not be used except when a very 
 powerful light is necessary, or when the slightly yellowish 
 tinge of incandescents would interfere with the proper 
 judgment of colors. Powerful incandescents are gener- 
 ally better, and are but little less efficient, particularly when 
 one takes into account proper distribution of the light. 
 In using incandescents in large masses, particularly on 
 polyphase circuits, the flickering of the individual lights 
 is lost in the general glow, so that even at 25 cycles the 
 light may be steady enough for general purposes, as was 
 the case with the decorative lighting at the Pan-American 
 Exposition. The fluctuations due to low frequency are 
 usually very distressing to the eye, and should be sedu- 
 lously avoided. Fortunately, save in rare instances, the 
 frequency can be and should be kept well above the dan- 
 ger point. 
 
 The same considerations which forbid the use of very 
 intense lights, unshaded, flickering lights, and electric 
 lights at too low frequency, render violent contrasts of 
 brilliant illumination and deep shadows highly objection- 
 able. It should be remembered that in daylight the 
 general diffusion of illumination is so thorough that such 
 contrasts are very much softened, even in full sunlight, 
 and much of the time the direct light is modified by clouds. 
 In situations where the sun shines strongly down through 
 interstices in thick foliage, the effect is decidedly un- 
 pleasant if one wishes to use the eyes steadily, and if in 
 addition the wind stirs the leaves and causes flickering the 
 strain upon the eyes is most trying. 
 Pin artificial lighting one should carefully avoid the con- 
 ditions that are objectionable in nature, which can easily 
 
1 6 THE ART OF ILLUMINATION. 
 
 be done by a little foresight. If for any purpose very 
 strong illumination becomes necessary at a certain point, 
 the method of furnishing it which is most satisfactory 
 from a hygienic standpoint is to superimpose it upon a 
 moderate illumination well distributed. If a brilliant 
 light is needed upon one's work, start with a fairly well 
 lighted room and add the necessary local illumination, in- 
 stead of concentrating all the light on one spot. This 
 procedure avoids dense shadows and dark corners, and 
 enables the eye to work efficiently in a much stronger 
 illumination than would otherwise be practicable. 
 
 It should not be understood that the complete abolition 
 of shadows is desirable. On the contrary, since much of 
 our perception of form and position depends upon the 
 existence of shadows, the entire absence of them is 
 troublesome and annoying. This is probably due to two 
 causes. First, the absence of shadows gives an appear- 
 ance of flatness, out of which the eye vainly struggles to 
 select the wonted degrees of relief. In a shadowless space 
 we have to depend upon binocular vision to locate points 
 in three dimensions, and the strain upon the attention is 
 severe and quickly felt. 
 
 iSecond, the existence of a shadowless space presupposes 
 a nearly equal illumination from all directions. If it be 
 strong enough from any particular direction to be con- 
 venient for work requiring close attention of mind and 
 eye, then, if there be no shadows, equally strong light will 
 enter the eye from directions altogether unwonted. This 
 state of things we have already found to be objectionable 
 in the highest degree. 
 
 The best illustration of this latter condition may be 
 found in nature during a thin fog which veils the sun while 
 diffusing light with very great brilliancy. Try to read at 
 
LIGHT AND THE EYE. 17 
 
 such a time out of doors, and although there is no direct 
 light on the page to dazzle you, and there is in reading no r 
 trouble from the sense of flatness, yet there is a distinctly 
 painful glare which the eyes cannot long endure without 
 serious strain. 
 
 In artificial lighting the same complete diffusion is com- 
 petent to cause the same results, so that while contrasts of 
 dense shadows and brilliant light must be avoided, it is 
 generally equally important to give the illumination a 
 certain general direction to relieve the appearance of flat- 
 ness and to save the eye from crosslights. S 
 
 With respect to the best direction of illumination, only 
 very general suggestions can be given. Brilliant light,/ 
 direct or reflected, should be kept out of the eye and upon 
 the objects to be illuminated. In each individual case the 
 nature and requirements of the work must determine the 
 direction of lighting. 
 
 The old rule given for reading and writing, that the 
 light should come obliquely over the left shoulder, well 
 illustrates ordinary requirements. By receiving the light 
 from the point indicated direct light is kept out of the eyes, 
 and any light regularly reflected is generally out of the 
 way. The eye catches then only diffused light from the 
 paper before it, and if the light comes from the left (for a 
 right-handed person) the shadow of hand and arm does 
 not interfere with vision. If work requiring both hands 
 is under way the chances are that the best illumination will 
 be obtained by directing it downwards and slightly from 
 the front, in which case care must be exercised to avoid 
 strong direct reflection into the eyes. The best simple ^ 
 rule is, avoid glare direct or reflected, and get strong dif- 
 fused lio-ht from the object illuminated. 
 
 This brings us at once to the very important but ill- 
 
i8 THE ART OF ILLUMINATION. 
 
 defined question of the strength of illumination required 
 for various kinds of work. 
 
 (^Fortunately, the eye works well over a wide range of 
 brightness, but there is a certain minimum illumination 
 which should be exceeded if one is to work easily and 
 without undue strain. The matter is much complicated 
 by questions of texture and color, which will be taken up 
 presently, so that only general average results can be con- 
 sidered. For reading and writing experience has shown 
 that an intensity of about one candle-foot is the minimum ^- 
 suitable amount with ordinary type and ink, such as is 
 here used, for instance. With large, clear type 
 
 like that used for this particular line 
 
 half a candle-foot enables one to read rather easily, while 
 with ordinary type set solid or in type of the smaller sizes, 
 
 such type as is employed in this line as a horrible example, 
 
 two candle- feet is by no means an unnecessary amount of 
 lighting. Dense black ink and clear white paper not 
 highly calendered, such as some of the early printers knew 
 well how to use, make vastly easier reading than the 
 grayish-white stuff and cheap muddy-looking ink to be 
 found in the average newspaper. 
 
 Illumination of less than half a candle-foot usually ren- 
 ders reading somewhat difficult and slow, the more diffi- 
 cult and slower as the illumination is further reduced. 
 At one-tenth or two-tenths of a candle-foot reading is by 
 no means easy, and there is a strong tendency to bring the 
 book near the eye, thereby straining one's power of ac- 
 commodation, and to concentrate the attention upon 
 single words, a tendency which increases as the light is 
 still further lessened. 
 
 In fact, when the illumination falls to the vicinity of 
 
LIGHT AND THE EYE. 
 
 one-tenth candle- foot it is of very little use for the purpose 
 of reading or working. *^J 
 
 One may get a fair idea of the strength of illumination 
 required for various purposes by a consideration of that 
 actually furnished by nature. To get at the facts in the 
 case, we must make a little digression in the direction 'of 
 photometry, a subject which will be more fully discussed 
 later. 
 
 To get an approximate measure of the illumination fur- 
 nished by daylight, one can conveniently use what is 
 
 Fig. 4. Principle of the Photometer. 
 
 known as a daylight photometer. This instrument fur- 
 nishes a means for balancing the illumination due to any 
 source against that due to a standard candle at a known 
 distance. Like most common forms of photometer it 
 consists of a screen illuminated on its two sides by the two 
 sources of light respectively. Equality of illumination is 
 determined by the disappearance of a grease spot upon the 
 screen. A spot of grease on white paper produces, as is 
 well known, a highly transparent spot, which looks bright 
 if illuminated from behind, and dark when illuminated 
 from the front. 
 
 Thus, if one sets up such a screen C between and equi- 
 
20 THE ART OF ILLUMINATION. 
 
 distant from a candle A and an incandescent lamp B, and 
 then looks at the screen obliquely from the same side as 
 B, the appearance is that shown in Fig. 4. Moving 
 around to the other side of the screen one gets the effect 
 shown in Fig. 5. By moving the candle A nearer or the 
 incandescent B farther off, a point will be found where the 
 spot becomes nearly invisible on account of the equal 
 illumination on the two sides. This " Bunsen photometer 
 screen " requires very careful working to get highly accu- 
 rate results, but gives closely approximate figures readily. 
 
 Fig. 5. Principle of the Photometer. 
 
 The daylight photometer, Fig. 6, is the simplest sort of 
 adaptation of this principle. It consists of a box, say five 
 or six feet long and fifteen inches square. In one end is 
 a hole B filled with the photometer screen just described, 
 and a slot to receive a graduated scale A carrying a socket 
 for a standard candle. The interior of the box is painted 
 dead black, so as to avoid increasing the illumination at B 
 by light reflected within the box. 
 
 Setting up the box with the end B pointing in the direc- 
 tion of the illumination to be estimated, the candle is slid 
 back and forth until the grease spot disappears, when the 
 
LIGHT AND THE EYE. 21 
 
 distance from the candle to B gives the required illumina- 
 tion, by applying the law of inverse squares, which holds 
 sufficiently well for approximate purposes if the box is 
 well blackened. 
 
 Of course the results of such measurements vary enor- 
 mously with different conditions of daylight. A few 
 
 Fig. 6. Daylight Photometer. 
 
 * 
 
 measurements made in a large, low room with windows on 
 two sides, culled from the writer's notebook, give the 
 following results, the day being bright, but not sunny, and 
 the time early in the afternoon : 
 
 Facing south window 6 candle-feet 
 
 Facing east window 2.2 
 
 Facing north wall 0.7 
 
 And again, 10 feet from south window, on a misty 
 April day, 5 P. M 0.5 
 
 On a clear day the diffused illumination near a window, 
 while the sun is still high, will generally range from 5 to 
 10 candle- feet, while in cases where there are exception- 
 ally favorable conditions for brilliant illumination it may 
 rise to twice or even four times the amount just stated. 
 
 Now, these figures for the lighting effects of diffused 
 daylight give a good clew, if nothing more, to the intensity 
 of illumination required for various purposes. In point 
 of fact, reading and writing require less light than almost 
 any other processes which demand close ocular attention. 
 
22 THE ART OF ILLUMINATION. 
 
 Everything is black and white, there is no delicate shad- 
 ing of colors, nor any degrees of relief to> be perceived in 
 virtue of differences of light and shade. Moreover, the 
 characters are sharply defined and not far from the eye. 
 It is therefore safe to say that for any work requiring 
 steady use of the eyes at least one candle-foot is demanded. 
 If practicable, this minimum should be doubled for really 
 effective lighting, while for much fine detail and for work 
 on colored materials not less than five candlesfeet shOul'd 
 be provided. Even this amount may advantageously be 
 doubled for the finest mechanical work, such as engraving, 
 watch repairing, and similar delicate operations. In fact, 
 for such cases the more light the better, provided the 
 source of light and direct undiffused rjeflections therefrom 
 are kept out of the eyes. 
 
 These estimates have taken no account of the effect of 
 color, which sometimes is a most important factor, alike in 
 determining the amount of illumination necessary and in 
 prescribing the character and arrangement of the sources 
 of light to be employed. 
 
CHAPTER II. 
 
 PRINCIPLES OF COLOR. 
 
 THE relation of color to practical illumination is some- 
 what intricate, for it involves considerations physical, 
 physiological, and aesthetic, but it is well worth studying, 
 for while in some departments of illumination, such as 
 street lighting, it is of little consequence, in lighting in- 
 teriors it plays a very important part. In lighting a shop 
 where colored fabrics are displayed, for example, it is 
 necessary to reproduce as nearly as may be the color 
 values of diffused daylight, even at considerable trouble. 
 Such illumination, however, may be highly undesirable in 
 lighting a ballroom, where the softer tones of a 
 light richer in yellow and orange are generally far prefer- 
 able. 
 
 In certain sorts of scenic illumination strongly colored 
 lights must be employed, but always with due understand- 
 ing of their effect on neighboring colored objects. Some- 
 times, too, the natural color of a light needs to be slightly 
 modified by the presence of tinted shades, serving to 
 modify both the intrinsic brilliancy and the color. 
 
 The fundamental law with respect to color is as follows : 
 Every opaque object assumes a hue due to the sum of the 
 colors which it reflects. A red book, for instance, looks 
 red because from white light it selects mainly the red 
 for reflection, while strongly absorbing the green and 
 blue. 
 
24 THE ART OF ILLUMINATION. 
 
 White light, as a look through a prism plainly shows, is 
 a composite of many colors, fundamentally red, green, and 
 blue, incidentally of an almost infinite variety of transi- 
 tion tints. If a narrow beam of sunlight passes through 
 a prism it is drawn out into a many-colored spectrum in 
 which the three colors mentioned are the most prominent. 
 Closer inspection detects a rather noticeable orange region 
 passing from red to green by way of a narrow space of 
 pure yellow, which is never very conspicuous. The green 
 likewise shades into pure blue through a belt of greenish 
 blue, and the blue in turn shades off into a deep violet. If 
 the slit which admits the sunlight is made very narrow, 
 certain black lines appear crossing the spectrum the 
 Fraunhofer lines due to the selective absorption of vari- 
 ous substances in the solar atmosphere, These lines are 
 for the purpose in hand merely convenient landmarks to 
 which various colors may be referred. They were desig- 
 nated by Fraunhofer by the letters of the alphabet, begin- 
 ning at the red end of the spectrum. 
 
 Fig. 7 shows in diagram the solar spectrum with these 
 lines and the general distribution of the colors. The A 
 line, really a broad dark band of many lines, is barely visi- 
 ble save in the most intense light, and the eye can detect 
 little or nothing beyond it. At the other end of the spec- 
 trum the H lines are in a violet merging into lavender, are 
 not easy to see. and there is but a narrow region visible be- 
 yond them pale lavender, as generally seen. The spec- 
 trum in Fig. 7 is roughly mapped out to show the extent 
 of the various colors as distributed in the ordinary pris- 
 matic spectrum. 
 
 At A, Fig. 7, is shown the spectrum of the light reflected 
 from a bright red book. i. e., the color spectrum which de- 
 fines that particular red. It extends from a deep red into 
 
PRINCIPLES OF COLOR. 
 
 2 5 
 
 clear orange, while the absorption in the yellow and yel- 
 lowish green is by no means complete. 
 
 At B, is the color spectrum from a green book. 
 Here there is considerable orange and yellow, a little red 
 
 ~r 
 
 r 
 
 
 Fig. 7. Solar and Reflected Spectra. 
 
 and much bright green, together with rather weak absorp- 
 tion in the bluish green. 
 
 C shows a similar diagram from a book apparently of a 
 clear, full blue. The spectrum shows pretty complete ab- 
 sorption in the red and extending well into the orange. 
 The orange-yellow and yellowish-green remain, however, 
 as does all the deep blue, while there is a perceptible ab- 
 sorption of the green and bluish-green. 
 
 Now, these reflected spectra are thoroughly typical of 
 those obtained from any dyed or painted surfaces. The 
 colors obtained from pigments are never the simple hues 
 they appear to be, but mixtures more or less complex, 
 sometimes of colors from very different regions of the 
 spectrum. Most of the commoner pigments produce ab- 
 
26 THE ART OF ILLUMINATION. 
 
 sorption over rather wide regions of the spectrum, but 
 some of the delicate tints found in dyed fabrics show sev- 
 eral bands of absorption in widely separated portions of 
 the spectrum. These are the colors most seriously affected 
 by variations in the color of the illuminant when viewed by 
 artificial light. Fig. 8 is a case in point, a color spectrum 
 taken from a fabric which in daylight was a delicate corn- 
 flower blue. The absorption begins in the crimson, leav- 
 ing much of the red intact, is partial in the orange and 
 yellow, stronger in the green, and quite complete in the 
 bluish-green region. The blue well up to the violet is 
 freely reflected, and then the violet end of the spectrum is 
 
 Fig. 8. Spectrum Reflected From Blue Silk. 
 
 considerably absorbed. Most of the reflected light is blue, 
 but if the illumination is conspicuously lacking in blue 
 rays, as is the case with candle light or common gaslight, 
 the blue light reflected is necessarily weak, while the red 
 component comes out at its full strength, and the visible 
 color of the fabric is distinctly reddish. 
 
 A similar condition is met in certain blues which in day- 
 light reflect a large proportion of blue and bluish violet, 
 but in which some green rays are left, just as was the clear 
 red in Fig. 8. By gaslight the blue becomes relatively 
 very much weakened, and the apparent color is unmistak- 
 ably green. Such changes in hue are in greater or less de- 
 gree very common, and furnish some very curious effects. 
 Sometimes a color clear by daylight appears dull and 
 
PRINCIPLES OF COLOR. 
 
 27 
 
 muddy by artificial light, and in general the quality of the 
 illumination requires careful attention whenever one deals 
 with delicate colors. 
 
 The absorption found in the pigments used in painting 
 is seldom so erratic as that shown in Fig. 8, but pictures 
 often show very imperfectly under ordinary artificial 
 illumination. 
 
 It is no easy matter to get a clear idea of the color 
 properties of various illuminants. O course, one can 
 form spectra from each of the lights to be compared, and 
 compare the relative strengths of the red, green, blue, and 
 other rays in each, but this gives but an imperfect idea of 
 the relative color effects produced, for the results them- 
 selves are rather discordant, and the relative brightness 
 thus measured does not correspond accurately with the 
 visual effect. Probably a better plan from the standpoint 
 of illumination is to match the visible color of a given 
 illuminant accurately by mixtures of the three primary 
 spectral colors, red, blue-violet and green, and to deter- 
 mine the exact proportions of each constituent required to 
 give a match. Even this evidently does not tell the whole 
 story, but it gives an excellent idea of the color differences 
 found in various lights. Such work has been very beauti- 
 fully carried out by Abney, from whose results the follow- 
 ing table is taken : 
 
 
 SUNLIGHT 
 
 SKY LIGHT 
 
 ARCLIGHT 
 
 GASLIGHT 
 
 Red 
 
 IOO 
 
 IOO 
 
 IOO 
 
 IOO 
 
 Green ... 
 
 IQ-2 
 
 2^6 
 
 2O -5 
 
 05 
 
 Violet 
 
 228 
 
 760 
 
 2CQ 
 
 27 
 
 
 
 
 
 
 Incandescent lamps are not here included, but give enor- 
 mously different results according to the degree of incan- 
 
28 THE ART OF ILLUMINATION. 
 
 descence to which they are carried. If burned below 
 candle-power they give a light not differing widely from 
 gaslight; while if pushed far above candle-power the light 
 is far richer in violet rays, and becomes pure white. 
 Unfortunately, however, the lamp does not reach this 
 point save at a temperature that very quickly ends its life. 
 
 The effects of the selective absorption which so deceives 
 the eye when colored objects are viewed in colored lights 
 are shown in a variety of ways according to the colors in- 
 volved, but the net result of them all is 'to show the neces- 
 sity of looking out for the color of artificial lights. Of 
 course, a really strong color may produce very fantastic 
 results. For example, in the rays of an ordinary green 
 lantern, such as is used for railway signals, greens gener- 
 ally appear of nearly their natural hues; but greens, yel- 
 lows, browns, and grays all match pretty well, although 
 they may appear darker or lighter in shade. Pink looks 
 gray, darkening in shade as it gets redder, and red is 
 nearly black, for the green light which falls upon it is al- 
 most totally absorbed. 
 
 Practical illuminants do not often present so violent 
 deceptions, and yet gas or candle light is certain to change 
 the apparent hue of any delicate colors containing bluish 
 green, blue, or violet rays. An old Welsbach mantle 
 which gives a light of a strongly greenish cast is pretty 
 certain to change the color of everything not green upon 
 which it falls. Incandescent electric lights affect colors 
 in much the same way as brilliant gaslight, while arc 
 lights give a fair approximation to daylight. It by no 
 means follows, however, that all colors should be matched 
 by arc lights in preference to other sources of illumina- 
 tion. A match so made stands daylight, but may be most 
 faulty when viewed by gaslight. * 
 
PRINCIPLES OF COLOR. 29 
 
 If matching colors has to be done, it is a safe rule to 
 match them by the kind of light by which they are in- 
 tended to be viewed. Moreover, different shades of the 
 same color are differently affected in artificial light. As a v 
 rule, deep, full colors are far less affected than light tones 
 of the same general hue. Clear yellows, reds, and blues 
 not verging on green are usually little altered, but pale 
 pinks, violets, and " robin's-egg " blues quite generally 
 suffer. Very often when a color is not positively altered 
 it is made to appear gray and muddy. 
 
 For while in a green light greens look particularly bril- 
 liant, red may be practically extinguished, absorbing all 
 the rays which come to it, so that a deep red will be nearly 
 black, and a very light red merely a dirty white, tinged 
 with green if anything. 
 
 Quite apart from any effect of colored illumination, 
 colors seem to change in very dim light. This is a purely 
 physiological matter, the eye itself differing in its sensi- 
 bility to different colored lights. In very faint illumina- 
 tion no color of any kind is perceptible everything ap- 
 pears of uncertain shades of gray. As the light fades 
 from its normal intensity, as in twilight, red disappears 
 first, then violet and deep blue follow, settling like the red 
 into murky blackness; then the bluish green and green 
 shade off into rapidly darkening gray, and finally the yel- 
 low and yellowish orange lose their identity and merge 
 into the night. At the same time the hues even of simple 
 colors change, scarlet fading into orange, orange into yel- 
 low, and green into bluish green. 
 
 Obviously, complicated composite colors must vary 
 widely under such circumstances, for as the light grows 
 dimmer their various components do not fade in equal 
 measure. Pinks, for instance, generally turn bluish gray 
 
3 o THE ART OF ILLUMINATION. 
 
 at a certain stage of illumination, owing to the extinction 
 of the red rays. In fact, in a dim light the normal eye 
 is color blind as regards red, and one can get a rather 
 good idea of the sensations of the color blind by study- 
 ing a set of tinted wools or slips of paper in the late 
 twilight. 
 
 The similarity of the conditions is strikingly illustrated 
 in Fig. 9, which shows in No. i the distribution of lumi- 
 
 i 2 3 
 
 .100 
 90 
 80 
 70 
 60 
 50 
 40 
 30 
 20 
 10 
 
 
 \/ 
 
 \\ 
 
 \\ 
 
 \ 
 
 ABCD E6 F T G H^ 
 
 Fig. 9. Effect of Faint Light on Color. 
 
 nosity in the spectrum of bright white light to the normal 
 eye, and in No. 2 the luminosity of the same as seen by a 
 red-blind eye. No. 3 shows the luminosity of the spec- 
 trum when reduced to a very small intensity and seen by 
 the normal eye. The data are from Abney's experiments, 
 and the intensity of No. 3 was such that the yellow com- 
 ponent of the light corresponding to D of the spectrum 
 was 0.006 candle-foot. The ordinates of No. 2 and No. 
 3 have been multiplied by such numbers as would bring 
 their respective maxima to equal the maximum of No. i, 
 
PRINCIPLES OF COLOR. 31 
 
 as the purpose is to show their relative shapes only. The 
 " red-blind " curve No. 2 shows very faint luminosity in 
 the scarlet and orange and absence of sensation in the 
 crimson, while the maximum luminosity is in the greenish 
 yellow. It is easy to see that the sensation of red is prac- 
 tically obliterated. 
 
 But in No. 3 every trace of red is gone, and the maxi- 
 mum brilliancy has moved up into the clear green of the 
 spectrum at the line E. With a still further reduction of 
 intensity, the spectrum would fade into gray as just noted, 
 while a slight increase of light would cause No. 3 closely 
 to approximate No. 2. 
 
 Starting with the normal curve of luminosity No. i, the 
 peak of the curve being one candle-power, the light at B 
 would disappear if the illumination were reduced to .01 
 of its initial value, that at C at about .001 1, at D .00005, at 
 E .0000065, at F .000015, arj d at G .0003. 
 
 Now the practical application of these facts is manifold. 
 Not only do they explain the odd color effects at twilight 
 and dawn, but it is worth noting that the cold greenish 
 hue of moonlight on a clear night means simply the ab r 
 sence of the red and orange from one's perception of a 
 very faint light, for dim moonlight is ordinarily not much 
 brighter than the light of curve No. 3. For the same rea- 
 son a red light fades out of sight rather quickly, so that a 
 signal of that color is not visible at a distance at which 
 one of another color and equal brightness would be easily 
 seen. 
 
 Not only is the eye itself rather insensitive to red, but 
 the luminosity of the red part of the spectrum of any light 
 is rather weak, so that when the other rays are cut off by 
 colored glass, the effective light is greatly reduced. 
 About 87 per cent, of the effective luminosity of white 
 
3 2 THE ART OF ILLUMINATION. 
 
 light lies between the lines C (scarlet) and E (deep 
 green), the relative luminosities at various points being 
 about as follows : 
 
 LINE. LUMINOSITY. 
 
 B 3 
 
 C 20 
 
 D 98.5 
 
 E 50 
 
 b 35 
 
 F 7 
 
 G 0.6 
 
 The luminosities of light transmitted through ordi- 
 nary colored glasses of various colors is about as fol- 
 lows, following Abney's experiments, clear glass being 
 
 COLOR OF GLASS. LIGHT TRANSMITTED. 
 
 Ruby 13-1 
 
 Canary 82.0 
 
 Bottle green 10.6 
 
 Bright green (signal green No. 2) 19.4 
 
 Bluish green (signal green No. i) 6.9 
 
 Cobalt blue 3-75 
 
 These figures emphasize the need of a very powerful 
 source if it is necessary to get a really bright-colored 
 flight. It is worth noting that red is a particularly 
 bad color for danger signals on account of its low lumi- 
 nous effect, and were it not for the danger of changing a 
 universal custom, red should be the " clear " signal and 
 green the danger signal, the latter color giving a much 
 brighter light, and thus being on the average more easily 
 visible. 
 
 It is easy to see that any artificial illuminant is at a con- 
 siderable disadvantage if at all strongly colored, for not 
 only does a preponderance of red or green rays injure color 
 perception, but the luminosity of such rays is rather low, 
 
PRINCIPLES OF COLOR. 33 
 
 and they do not compensate for their presence by giving 
 greatly increased illumination. 
 
 Owing to this fact the effective illumination derived 
 from various sources of light is pretty nearly proportional 
 to the intensity of the yellow component of each. Crova 
 has based on this rule an ingenious approximate method 
 of comparing the total intensity of colbred lights by com- 
 paring the intensities of their yellow rays, either from 
 their respective spectra or by sifting out all but the t 
 yellow and closely adjacent rays by means of a colored 
 screen. 
 
 Certainly for practical purposes the rays at the ends of 
 the spectrum are not very useful. So far as the ordinary 
 work of illumination goes, white or yellowish white 
 light should be used, and the only practical function oL 
 strongly colored lights is for signaling and scenic illu- 
 mination. 
 
 The general effect of strongly colored lights is to ac- 
 centuate objects colored like the light and to change or dim 
 all others. Lights merely tinted produce a similar effect 
 in a less degree. Bluish and greenish tinges in the light 
 give a cold, hard hue to most objects, and produce on the 
 face an unnatural pallor; in fact, on the stage they are 
 used to give in effect the pallor of approaching dissolu- 
 tion. Naturally enough such light is unfitted for interior 
 illumination, as, aside from its effect on persons, it makes 
 a room look bare, chill, and unfurnished. In a less degree , 
 a similar effect is produced by moonlight, which, from a ^ 
 clear sky, is distinctly cold, the white light growing faintly 
 greenish blue as its diminishing intensity causes the red to 
 disappear. 
 
 On the other hand, a yellow-orange tinge in the light 
 seems to soften and brighten an interior, giving an effect 
 
34 THE ART OF ILLUMINATION. 
 
 generally warm and cheery. This result is extremely 
 well seen in stage fire-light effects. Strongly red light is, 
 however, harsh and trying and particularly difficult 
 to see well by, so that it should generally be carefully 
 avoided. 
 
 While it is not easy to predict accurately the effect of 
 tinted lights upon various delicate shades without a careful 
 study of the light rays forming each, the average effects 
 relating to the simpler colors are summarized in the fol- 
 lowing table. It is compiled from the experiments of the 
 late M. Chevreul, for many years director of the dyeworks 
 of the Gobelins tapestries. The colored lights were from 
 sunlight sifted through colored glass, and the effects were 
 upon fabrics dyed in plain, simple colors. 
 
 The facts set forth in this table show well what should 
 be avoided in colored illumination. As regards various 
 shades of the same colors it must be remembered that light 
 shades are merely the full deep ones diluted with white, 
 which is itself affected by the color of the incident light. 
 In a general way, therefore, one can use this table over a 
 wider range than that written down. 
 
 For instance, a very light red in blue light would look 
 blue with a mere trace of violet, while in yellow light it 
 would be bright yellow with a very slight orange cast. 
 Generally a very light color viewed by colored light will 
 be between the effect produced on the full color, and that 
 produced by the light on a white surface. Similarly a 
 light only tinged with color will only slightly modify the 
 tone of a colored object in the direction indicated for the 
 full-colored light in the table. 
 
 But delicate shades from modern dyestuffs, which often 
 absorb the light in very erratic ways, as in Fig. 8, are a 
 different matter, and do not obey any simple laws. On 
 
PRINCIPLES OF COLOR. 
 
 35 
 
 ORIGINAL 
 COLOR OF 
 FABRIC 
 
 Black 
 
 COLOR OF LIGHT FALLING UPON FABRICS 
 
 RED 
 
 ORANGE 
 
 YELLOW 
 
 GREEN 
 
 BLUE 
 
 VIOLET 
 
 Purplish 
 Black 
 
 Deep 
 Maroon 
 
 Yellow 
 Olive 
 
 Greenish 
 Brown 
 
 Blue- 
 Black 
 
 Faint Vio- 
 let Black 
 
 White 
 
 Red 
 
 Orange 
 
 Light 
 Yellow 
 
 Green 
 
 Blue 
 
 Violet 
 
 Red 
 Orange 
 
 Intense 
 Red 
 
 Orange 
 Red 
 
 Scarlet 
 
 Intense 
 Orange 
 
 Orange 
 
 Yellow- 
 Orange 
 
 Brown 
 
 Faint Yel- 
 low slight- 
 ly Green- 
 ish 
 
 Violet 
 Brown 
 
 
 
 Red-Violet 
 Purple 
 
 Light Red 
 
 Yellow 
 
 Orange 
 
 Yellow 
 Orange 
 
 Orange- 
 Yellow 
 
 Yellowish 
 Green 
 
 Green 
 
 Brown 
 tinged 
 with 
 faint Red 
 
 Light 
 Green 
 
 Reddish- 
 Gray 
 
 Yellow 
 Green 
 
 Greenish 
 Yellow 
 
 Intenser 
 Green 
 
 Blue- 
 Green 
 
 Light 
 Purple 
 
 Deep 
 
 Green 
 
 Reddish 
 Black 
 
 Rusty 
 Green 
 
 Yellowish 
 Green 
 
 Intenser 
 Green 
 
 Greenish 
 Blue 
 
 
 Light 
 Blue 
 
 Violet 
 
 Orange 
 Gray 
 
 Yellowish 
 Green 
 
 Green 
 
 Blue 
 
 Vivid Blue 
 
 
 Deep 
 
 Blue 
 
 Indigo 
 Blue 
 
 
 Gray 
 slightly on 
 Orange 
 
 Orange- 
 Maroon 
 
 Green- 
 Slate 
 
 Orange- 
 Yellow 
 (very dull) 
 
 Blue 
 Green 
 
 Dull 
 Green 
 
 Intenser 
 Blue 
 
 Dark 
 Blue- 
 Indigo 
 
 Bright 
 Blue- 
 Violet 
 
 Deep 
 Blue- 
 Violet 
 
 Violet 
 
 Purple 
 
 Red- 
 Maroon 
 
 Yellow- 
 Maroon 
 
 Bluish 
 Green- 
 Brown 
 
 Deep 
 
 Bluish 
 Violet 
 
 Deep 
 Violet 
 
 the other hand, pure colors, in the sense in which the 
 scarlet around the C line of the spectrum is pure, act in a 
 fashion rather different from that shown in the table, 
 which pertains to standard dyestuffs which never are any- 
 where near being pure colors. However, as artificial 
 illumination has to do only with commercial pigments and 
 dyes, the table serves as a useful guide in judging the 
 effects produced on interior furnishings by change in the 
 color of the light. 
 
 Of common illuminants, none have any very decided 
 
36 THE ART OF ILLUMINATION. 
 
 color, yet most are somewhat noticeably tinged. One 
 can tabulate them roughly as follows : 
 
 ILLUMINANT. COLOR. 
 
 Sun (high in sky). White. 
 
 Sun (near horizon). Orange red. 
 
 Sky light. Bluish white. 
 
 Electric arc (short). White. 
 
 Electric arc (long). Bluish white to violet. 
 
 Nernst lamp. White. 
 
 Incandescent (normal). Yellow-white. 
 
 Incandescent (below voltage). Orange to orange-red. 
 
 Acetylene flame. Nearly white. 
 
 Welsbach light. Greenish white. 
 
 Gaslight (Siemens burner). Nearly white, faint yellow tinge. 
 
 Gaslight, ordinary. Yellowish white to pale orange. 
 
 Kerosene lamp. Yellowish white to pale orange. 
 
 Candle. Orange yellow. 
 
 Outside the earth's atmosphere the sun would look dis- 
 tinctly blue, while its light, after thorough absorption in 
 the earth's atmosphere, gets the blue pretty completely 
 sifted out, so that the light from the eclipsed moon, once 
 refracted by the earth's atmosphere and then reflected 
 through it again, is in color a deep coppery red. 
 
 Arc lights vary much in color, from clear white 
 in short arcs with comparatively heavy current to bluish 
 white or whitish violet in long arcs carrying rather small 
 current. The modern enclosed arcs tend in the latter 
 direction, and give their truest color effects with yellowish 
 white inner globes or. shades. Incandescents, as gener- 
 ally worked, verge upon the orange. Of the luminous 
 flames in use, only acetylene comes anywhere near being 
 white, although the powerful regenerative burners are a 
 close second. Incandescent gas lamps, at first showing 
 nearly white with a very slight greenish cast, acquire a 
 greenish or yellowish green tinge after burning for some 
 time. 
 
 It is evident then that a study of the color effects pro- 
 
PRINCIPLES OF COLOR. 37 
 
 duced by colored illuminants is by no means irrelevant, for 
 distinct tinges of color are the rule rather than the ex- 
 ception. 
 
 But this is not at all the whole story, for the general 
 color of the illumination in a given space depends not only 
 on the hue of the illuminant, but upon the color of the 
 surroundings. Colored shades, of course, are in common 
 use; sometimes with a definite purpose, more often from a 
 mistaken notion of prettiness. Used intelligently, as we 
 shall presently see, they may prove very valuable adjuncts 
 in interior illumination. 
 
 But far more important than shading is the modification 
 in the color of the light which comes from selective reflec- 
 tion at surfaces upon which the light falls. In every en- 
 closed space light is reflected in one way or another from 
 all the bounding surfaces, and at each reflection not only 
 is the amount of light profoundly modified, but its color 
 may undergo most striking changes. It is this phenome- 
 non that gives its greatest interest to the study of color in 
 illumination. Its importance is not always readily recog- 
 nized, for few persons pay really close attention to the 
 matter of colors, but now and then it obtrudes itself in a 
 way that forces attention. 
 
 Take for example a display window lined with red cloth 
 and brightly illuminated. Passing along the sidewalk 
 one's attention is immediately drawn to a red glow upon 
 the street, while the lights themselves may be ordinary gas 
 jets. To get at the significance of this matter, we must 
 take up the effect of reflection and diffusion in modifying 
 the amount and quality of light. 
 
CHAPTER III. 
 
 REFLECTION AND DIFFUSION. 
 
 To begin with, reflection is of two kinds in their 
 essence the same, yet exhibiting very different sets of 
 properties. The first, or regular reflection, may be best 
 exemplified by the reflection which a beam of light under- 
 goes at the surface of a mirror. The beam strikes the 
 surface and is reflected therefrom as sharp and as distinct 
 as it was before its incidence, and in a perfectly definite 
 direction. 
 
 The character of this regular reflection is very clearly 
 shown in Fig. 10. Here B is the reflecting surface a 
 plane, polished bit of metal, for instance. AB is the inci- 
 dent ray and BC the reflected ray. In such reflection two 
 principal 'facts characterize the nature of the phenomenon. 
 In the first place, if a perpendicular to the surface of the 
 mirror as BH -is erected at the point of incidence, the 
 angle ABJ} is always precisely equal to the angle DBC. f 
 In other words, the angle of incidence is equal to the angle 
 of reflection, which is the first law of regular reflection. 
 Moreover, the incident ray AB, the normal to the surface* 
 at the point of incidence BD, and the reflected ray BC are 
 aH in the same plane. 
 
 In this ordinary form of reflection, such as is familiar 
 in mirrors, the direction of the reflected ray is entirely 
 determinate, and, in general, although the reflected ray 
 has lost in intensity, it is not greatly changed in color. A 
 polished copper surface, to be sure, shows a reddish reflec- 
 
 38 
 
REFLECTION AND DIFFUSION. 
 
 39 
 
 tion, and polished gold a distinctly yellowish reflection. 
 Only in certain dye stuffs which exhibit a brilliant metallic 
 reflection is the color strongly marked. In other words, 
 a single reflection from a good, clean, reflecting surface 
 does not very greatly ctiange either the intensity or the 
 color of the reflected beam. The angle of incidence 
 
 Fig. 10. Regular Reflection. 
 
 affects the brilliancy of the reflection somewhat, but the 
 color only imperceptibly. In the art of practical illumina- 
 tion regular reflection comes into play only in a rather 
 helpful way, and kindly refrains from complicating the 
 situation with respect to color or intensity. 
 
 The second sort of reflection is what is technically 
 known as diffuse reflection. This term does not mean 
 that the phenomenon itself is of a totally different kind 
 from regular reflection, but nevertheless, its results are 
 totally different. No surface is altogether smooth. 
 Even with the best polished metallic mirrors, while the re- 
 flected image is perfectly distinct at ordinary angles of 
 reflection, it is apt to become slightly hazy at grazing inci- 
 
40 THE ART OF ILLUMINATION. 
 
 dence that is, when the incident and reflected beams are 
 nearly parallel to the surface. This simply means that 
 under such conditions the infinitesimal roughness of the 
 reflecting surface begin to be in evidence. 
 
 To get an idea of the nature of diffuse reflection, ex- 
 amine Fig. ii. In this case a section of the reflecting 
 surface is rough, showing grooves and points of every 
 
 Fig. ii. Diffuse Reflecti($n. 
 
 description in fact, nearly everything except a plane 
 surface. Consider now the effect of a series of parallel 
 incident beams numbered in the figure from i to 10 
 falling upon the surface. Each one of them is reflected 
 from its own point of incidence in a perfectly regular man- 
 ner; yet the reflected rays, on account of the irregularity 
 of the surface, lie in all sorts of directions, and moreover, 
 in all sorts of planes, according to the particular way in 
 which the surface at the point of incidence is distorted. 
 Diffuse reflection, therefore, scatters the incident beam in 
 all directions, for the roughnesses of an unpolished surface 
 are generally totally devoid of any regularity. The point 
 
REFLECTION AND DIFFUSION. 41 
 
 of incidence upon which a beam falls, therefore, radiates 
 light in a diverging cone and behaves as if it were really 
 luminous. 
 
 Some consideration of the nature of this diffuse reflec- 
 tion will bring to light a fact which in itself seems rather 
 surprising: namely, that the total intensities of the two 
 kinds of reflection are not so different from each other as 
 might appear probable at first thought provided the 
 roughness of the unpolished surface is not on too small a 
 scale; for each of the incident rays in Fig. n is reflected 
 from the surface just as in the case of Fig. 10, in a per- 
 fectly clean, definite way, and there is no intrinsic reason 
 why the intensity of this elementary ray should be any 
 more diminished than in the case of regular reflection. 
 
 A little inspection of Fig. n, however, shows that rays 
 Nos. 5 and 10 are twice reflected before they get fairly 
 clear of the surface, and if one went on drawing still more 
 incident rays and following out the figure on a still finer 
 scale, a good many other rays would be found to be re- 
 flected two or more times before finally escaping from the 
 surface. Such multiple reflection, of course, diminishes 
 the intensity of the light just as in the multiple reflection 
 from mirrors, for there is always a little absorption, 
 selective or otherwise, at any surface however apparently 
 opaque. Thus, while the difference in the final intensi- 
 ties of light regularly and diffusely reflected is not so great 
 as might be imagined, it still does exist, and for a perfectly 
 logical reason. 
 
 To go into the matter a little further suppose the 
 rough surface of Fig. n to be not heterogeneous, but 
 made up of a series of grooves having Cross- sect ions like 
 saw teeth. On examining the reflection from such a 
 surface we should find a rather remarkable state of affairs, 
 
42 THE ART OF ILLUMINATION. 
 
 for the course of reflection would then vary very greatly 
 with the relation between the direction of the incident 
 light and the surfaces of the grooves in the reflecting 
 surface. 
 
 Light coming in one direction, i. e., so as to strike the 
 inclined surfaces of the grooves, would get clear of the 
 surface at the first reflection, and the intensity of the re- 
 flected beam would have a very marked maximum in one 
 particular direction. A beam falling on the reflecting 
 surface in the other direction, however that is, on the 
 perpendicular sides of the saw-tooth grooves, would suffer 
 several reflections before escaping from the grooves, and 
 hence would lose in intensity, might be changed in color, 
 and might be considerably diffused. This sort of phe- 
 nomenon one may call asymmetric reflection. As we 
 shall presently see, it plays a somewhat important part in 
 some very familiar phenomena. 
 
 Reflection from ordinary smooth but not polished 
 surfaces partakes both of the nature of regular and diffuse 
 reflection, and is, in fact, a mixture of the two phenomena, 
 there being a general predominant direction of reflection 
 plus a certain amount of diffuse reflection. This sort of 
 thing is most commonly met with in practical illumina- 
 tion. The light from artificial illuminants usually falls 
 on painted walls, on tinted papers with surfaces more or 
 less regular, on fabrics and on various rough or smooth 
 objects in the vicinity. If these surrounding surfaces are 
 colored as in the case discussed a little while ago some 
 curious results may be produced. Of course, light re- 
 flected from a colored surface is colored, as we have seen 
 already, but the manner in which it is colored is by no 
 means obvious. 
 1 When white light falls upon a colored surface, the re- 
 
REFLECTION AND DIFFUSION. 
 
 43 
 
 flection is generally highly selective as regards color. 
 Fig. 12, from Abney's data, shows clearly enough the sort 
 of thing which occurs. It exhibits the intensity of the re- 
 flected light in each part of the spectrum when the reflect- 
 ing surface is colored. The surfaces in this case were 
 smooth layers of pigment. Curve No. i is the light re- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 /\ 
 
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 N, 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 H 
 
 
 
 
 
 
 
 
 
 
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 V 
 
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 X 
 
 
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 M *n 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 v^ 
 
 
 
 ^ 
 
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 G 
 
 Fig. 12. Selective Reflection. 
 
 fleeted from a surface painted cadmium-yellow; No. 2, 
 Antwerp blue; No. 3, emerald green. Each curve shows 
 a principal reflection of the color of the pigment, reaching 
 a rather high maximum value, but falling off rapidly in 
 parts of the spectrum other than that to which the pre- 
 dominant pigment color belongs. As has been already 
 shown, pigment colors are nearly always impure, and this 
 fact is strikingly exhibited in the shape of the curves. It 
 is clear enough what will be the color of the main body of 
 Alight reflected from any one of these surfaces. 
 Vy The visible color of the light is, however, strongly in- 
 / fluenced by the character of the surface. A shiny enamel 
 
 \ 
 
44 THE ART OF ILLUMINATION. 
 
 paint, for example, will reflect a good deal of light which 
 is not strongly influenced by the pigment, but is reflected 
 from the surface of the medium without much selective 
 action; consequently, there will be in the reflected light 
 both light which has taken the color of the pigment and 
 light unchanged in color. In other words, when viewed by 
 reflected light, the pigment color is mixed with white, and 
 when we have a perfectly simple pigment color such as 
 is not found in practice this would lead merely to light- 
 ening the tint. It may, however, have results much more 
 far-reaching for an admixture of white light in sufficient 
 quantity would shut out the distinct perception of any 
 color, diluting it until it becomes invisible. 
 
 The effects of this dilution are most marked in the ends 
 of the spectrum the colors at the middle being least 
 affected by the admixture of white light; hence, the fact 
 that such a surface as we have been considering, reflecting 
 a mixture of white and colored light, may produce a 
 change not only in tint, but in the hue of the color, if the 
 color, as usual, is composite. For example, a purple in 
 enamel paint might according to its composition look 
 pinkish or light blue if the surface reflection of white light 
 were particularly strong. If the pigmented surface is not 
 shiny and capable of considerable reflection of uncolored 
 light, another phenomenon may appear. 
 
 Fig. 13 shows curve No. 3 of Fig. 12, emerald green 
 pigment and below it a similar curve, resulting from a 
 second reflection of the light selectively reflected from a 
 pigment of that color. Assuming what is nearly in ac- 
 cordance with the fact that the second reflection follows 
 closely the properties of the first the result is obviously 
 to intensify the green of the reflected light. The clear 
 green portion of the light reflected from this particular 
 
REFLECTION AND DIFFUSION. 
 
 45 
 
 pigment is practically embraced between the dotted lines 
 P and Q of Fig. 13. After one reflection the area under 
 the curve embraced by these two lines is about 42 per cent, 
 of the whole. After two reflections it has risen to 55 per 
 cent., and each successive reflection while greatly reduc- 
 
 Fig. 13. Effect of Multiple Reflection. 
 
 -will 
 
 ing the intensity of the reflected light as a whol< 
 leave it greener and greener. 
 
 Consequently in diffuse reflection those rays which are 
 reflected several times before escaping from the surface 
 are strongly colored, and the more such multiple reflec- 
 tions there are the more pronounced is the Selective 'color- 
 ation due to reflection; hence, ordinary colored surfaces, 
 from which diffuse reflection takes place, are apt to take 
 very strongly the color^ of the pigment more strongly, 
 perhaps, than a casual inspection of the pigment would 
 suggest. 
 
 Now, as we shall presently see, in any enclosed space 
 the light reflected from the bounding surfaces is a very 
 
46 THE ART OF ILLUMINATION. 
 
 considerable portion of the whole, and, therefore, if these 
 surfaces are colored, the general illumination is strongly 
 colored also, whatever the illuminant may be; in other 
 words, colored surroundings will modify the color of the 
 illumination just as definitely as a colored shade over the 
 source of light. In planning the general color tone of a 
 room to be illuminated, it must be remembered that if the 
 walls are strongly colored the dominant tone of the illu- 
 mination will be that of the walls rather than that of the 
 light. 
 
 An interesting corollary resulting from Fig. 13 some- 
 times appears in the colors of certain fabrics. If the 
 surface fibers of the fabric lie in one general direction the 
 light reflected from that fabric, which determines its visi- 
 ble color, follows somewhat the same laws laid down for 
 asymmetric reflection, discussed in the case of Fig. n. 
 
 Light falling on the fabric from the direction toward 
 which the surface fibers run does not escape without pro- 
 fuse multiple reflection, and hence takes strongly the color 
 of the pigment. Light, however, falling on the fabric 
 reversely to the direction of the fibers undergoes much less 
 multiple reflection, and is likely to be mixed with a large 
 amount of white light hardly affected by pigment at all; 
 hence, the curious phenomenon of changeable color in 
 fabrics for instance, a fine purple from one direction of 
 illumination and perhaps very light pink from another. 
 
 If, in addition to the effects resulting from an admix- 
 ture of white light in certain directions of incidence, one 
 also has the curiously composite colors sometimes found 
 in modern dye stuffs, the changeable color effects may be 
 and often are very conspicuous ; the more so, since in such 
 colors, by multiple reflection, or what amounts to the 
 same thing by more or less complete absorption of cer- 
 
REFLECTION AND DIFFUSION. 47 
 
 tain rays, the resultant color may be very profoundly 
 changed. 
 
 Absorbing media sometimes show these color changes 
 very conspicuously; as, for example, chlorophyll, the green 
 coloring matter of leaves, which in a weak solution is 
 green, but of which a very strong solution of considerable 
 thickness transmits only the dark red rays. Similar char- 
 acteristics pertain to many modern dye stuffs, and result, 
 in connection with the composite reflection which has just 
 been explained, in some very extraordinary and very beau- 
 tiful effects. 
 
 From what has just been said about color reflection it is 
 obvious enough that the loss in intensity in a reflected ray 
 may be very considerable, even from a single regular re- 
 flection under quite favorable conditions. Many experi- 
 ments have been made to find the absolute loss of inten- 
 sity due to reflection. This absolute value of what is called 
 the coefficient of reflection that is to say, the ratio be- 
 tween the intensities of the incident and reflected light 
 varies very widely according to the condition of the re- 
 flecting surface. It also, in case the surfaces are not with- 
 out selective reflection in respect to color, varies notably 
 with the color of the incident light. 
 
 The following table gives a collection of approximate 
 results derived from various sources. The figures show 
 clearly enough the uncertain character of the data : 
 
 MATERIAL COEFFICIENT 
 
 OF REFLECTION. 
 
 Highly polished silver .92 
 
 Mirrors silvered on surface 70 .85 
 
 Highly polished brass , 70 .75 
 
 Highly polished copper 60 .70 
 
 Highly polished steel .60 
 
 Speculum metal 60 .80 
 
 Polished gold 50 .55 
 
 Burnished copper 40. 50 
 
48 THE ART OF ILLUMINATION. 
 
 The losses in reflection are due to absorption and to a 
 certain amount of diffuse reflection mixed with the regular 
 reflection. The above figures are for light in the most in- 
 tense part of the spectrum and for rather small angles of 
 incidence. For large angles of incidence 85 degrees 
 and more the intensity of the reflected beam is materially 
 diminished, owing probably both to increase in absorption 
 and to diffuse reflection. 
 
 Mirrors silvered with amalgam on the back, and various 
 burnished metals sometimes used for reflectors, belong 
 near the bottom of the table just given. Silver is dis- 
 tinctly the best reflecting surface; under very favorable 
 circumstances the coefficient of reflection of this metal is 
 in excess of .90. A very little tarnishing of the sur- 
 face results in increased absorption and diffusion and a 
 still further reduction of the intensity of the reflected ray. 
 The values of these coefficients show plainly the consider- 
 able losses which may be incurred in using reflectors in 
 connection with artificial lighting. 
 
 So far as general illumination is concerned, the light 
 diffused at the reflecting surfaces is not altogether lost, 
 but that absorbed is totally useless. In the case of or- 
 dinary reflecting surfaces one deals with a mixture of 
 regular and diffused reflection, and in practical illumina- 
 tion the .latter is generally more important than the for- 
 mer, for it determines the amount of light which reaches 
 the surface to be illuminated in ways other than direct 
 radiation from the illuminant. 
 
 Obviously, if one were reading a book in a room com- 
 pletely lined with mirrors, the effect of the illumination 
 upon the page would be vastly greater than that received 
 directly from the source of light itself. On the other 
 hand, a room painted black throughout would give very 
 
REFLECTION AND DIFFUSION. 49 
 
 little assistance from reflection, and the illumination upon 
 the page would be practically little greater than that re- 
 ceived directly from the lamp. Between these limits falls 
 the condition of ordinary "illumination in enclosed spaces. 
 Generally speaking, there-is very material assistance from 
 reflection at the bounding surfaces. The amount of such 
 assistance depends directly upon the coefficient of diffuse 
 
 Fig. 14. Asymmetric Reflection from a Fabric. 
 
 reflection of the various surfaces concerned, varying with 
 the color and texture of each. 
 
 As has been already indicated, diffuse reflection is 
 rough, heterogeneous, regular reflection, more or less com- 
 plicated, according to the texture of the reflecting surface, 
 by multiple reflections in the surface before the ray finally 
 escapes, and therefore, the coefficients of diffuse reflection 
 are not so widely different from those of direct reflection 
 as might at first sight appear probable, so far at least as 
 the total luminous effect is concerned. 
 
 In certain kinds of diffuse reflection there is consider- 
 able loss from absorption as well as from multiple reflec- 
 tions. This is conspicuously the case in the light reflected 
 from fabrics, where there is not only reflection from the 
 surface fibers, but where the rays before escaping are more 
 than likely to have to traverse some of them. This is 
 
5 o THE ART OF ILLUMINATION. 
 
 illustrated in a rather crude but typical way in Fig. 14, 
 which gives a characteristic case of asymmetric reflection. 
 We may suppose that the beam of light falls upon a surface 
 of fabric having a well-marked nap. In the cut aa is the 
 fabric surface composed of inclined fibers or bunches of 
 fibers. These fibers, although colored, are more or less 
 translucent and are not colored uniformly throughout 
 their substance. Owing to their direction, rays i, 2, and 
 3 get completely clear of the surface of the fabric by a 
 single reflection. These rays are but slightly colored, be- 
 cause of the comparatively feeble intensity of the colora- 
 tion of the individual fibers, which have a strong tendency 
 to reflect white light from the shiny surface. 
 
 On the other hand, rays 4, 5, and 6, inclined from the 
 other direction, are several times reflected before clearing 
 the surface, and in emerging therefrom have to pass 
 through the bunches of translucent fibers that form the 
 nap. As the result they are strongly colored. The 
 amount of white light is very small and the structure of 
 the surface has produced a marked changeable coloration. 
 
 In reality, of course, few rays actually escape on a single 
 reflection, and those striking almost in line with the direc- 
 tion of the fibers, as 4, 5, and 6 in the figure, may be re- 
 flected many times, so that the actual effect is an exaggera- 
 tion of that illustrated. 
 
 Moreover, the material of the surface fibers exercises a 
 considerable influence on the amount and character of the 
 selective coloration. Silk is especially well adapted to 
 show changeable color effects, since its fibers can be made 
 to lie more uniformly in the same direction than the fibers 
 of any other substance, and they are themselves naturally 
 lustrous, so as to be capable even when strongly dyed of 
 reflecting, particularly at large angles of incidence, a very 
 
REFLECTION AND DIFFUSION. 51 
 
 considerable proportion of white light. Being thus lus- 
 trous they form rather good reflecting surfaces, and hence 
 the light entangled in their meshes can undergo a good 
 many reflections without losing so much in intensity as to 
 dull conspicuously the resulting color effect; besides, silk 
 takes dyes much more easily and permanently than other 
 fibers and, hence, can be made to acquire a very fine color- 
 ation. 
 
 Wool takes dye less readify, and it is not so easy to give 
 the surface fibers a definite direction. They are, however, 
 quite transparent and lustrous enough to give fine rich 
 colors. Cotton is inferior to 'both silk and wool in these 
 particulars; hence, the phenomena we have been investi- 
 gating are seldom marked in cotton fabrics. 
 
 In velvet, which is a very closely woven cut pile fabric, 
 the surface fibers forming the pile stand erect and very 
 closely packed together. It is difficult, therefore, for 
 light to undergo anything except a very complex reflec- 
 tion, and practically all the rays which come from the 
 surface have penetrated into the pile and acquired a strong 
 coloration. The white light reflected from the surface of 
 the fibers hardly comes into play at all except at large 
 angles of incidence, so that the result is a particularly 
 strong, rich effect from the dyes, particularly in silk 
 velvet. 
 
 Cotton velvet, with its more opaque fibers, seems duller, 
 and, particularly if a little worn, reflects enough light from 
 the surface of the pile to interfere with the purity and in- 
 tensity of the color. Much of the richness in color of 
 rough colored fabrics and surfaces is due to the complete- 
 ness of the multiple reflections on the dyed fibers, which 
 produces an effect quite impossible to match with a smooth 
 surface unless dyed with the most vivid pigments. 
 
52 THE ART OF ILLUMINATION. 
 
 In practical illumination one seldom deals with fabrics 
 to any considerable extent, but almost always with papered 
 or painted surfaces. These are generally rather smooth, 
 except in the case of certain wall papers which have a silky 
 finish. Smooth papers and paint give a very considerable 
 amount of surface reflection of white light, in spite of the 
 pigments with which they may be colored. The diffusion 
 from them is very regular, except for this surface sheen, 
 and may be exceedingly strong. When light from the 
 radiant point falls on such a surface it produces a very 
 wide scattering of the rays, and an object indirectly illumi- 
 nated therefore receives in the aggregate a very large 
 amount of light. 
 
 A great many experiments have been tried to determine 
 the amount of this diffuse reflection which becomes avail- 
 able for the illumination of a single object. The general 
 method has been to compare the light received directly 
 from the illuminant with that received from the same 
 illuminant by one reflection from a diffusing surface. 
 
 The following table gives an aggregation of the results 
 obtained by several experimenters, mostly from colored 
 papers. 
 
 COEFFICIENT OF 
 MATERIAL DIFFUSE REFLECTION. 
 
 blotting paper ................................ ......... 82 
 
 N White cartridge paper .................................. ,. .80 
 
 Ordinary foolscap ........................................... 70 
 
 Chrome yellow paper ............... ......................... 62 
 
 Orange paper ............................................. 50 
 
 Plain deal (clean).. ... ................................... 45 
 
 Yellow wall paper ......................................... 40 
 
 ^Yellow painted wall (clean; ................................... 40 
 
 Light pink paper ........................................... 36 
 
 Yellow cardboard .......................................... 30 
 
 Light blue cardboard ....................................... 25 
 
 Brown cardboard ........................................... 20 
 
 _^ Plain deal (dirty) ................................. .......... .20 
 
 Yellow painted wall (dirty) ................................. 20 
 
 Emerald green paper ..................................... , .18 
 
 Dark brown paper ................... . ............ ........... 13 
 
REFLECTION AND DIFFUSION. 53 
 
 COEFFICIENT OF 
 MATERIAL. DIFFUSE REFLECTION. 
 
 Vermilion paper 12 
 
 Blue-green paper .12 
 
 Cobalt blue paper 12 
 
 "^ Black paper 05 
 
 Deep chocolate paper 04 
 
 French ultramarine blue paper 035 
 
 \Black cloth 012 
 
 X Black velvet 004 
 
 At the head of the list stands white blotting paper, 
 which is really a soft mass of lustrous white fibers. Its 
 coefficient of reflection .82 is comparable with the co- 
 efficient of direct reflection from a mirror; so far, at least, 
 as lights of ordinary intensity are concerned. 
 
 White cartridge paper is a good second, and partakes of 
 the same general characteristics. 
 
 Of the colored papers only the yellows, and pink so light 
 as to give a strong reflection of white light from the un- 
 colored fibers, have coefficients of diffuse reflection of any 
 considerable magnitude. Very light colors in general 
 diffuse well owing to the uncolored component of the re- 
 flected light, but of those at all strongly colored only the 
 yellows are conspicuously luminous. 
 
 Of course, all of the papers when at all dirty diffuse 
 much less effectively than when clean, and the rough 
 papers, which have the highest coefficients of diffusion, 
 are particularly likely to become dirty. 
 
 A smooth, clean white board and white painted surfaces 
 generally diffuse pretty well, but lose rapidly in effective- 
 ness as they become soiled. Greens, reds, and browns, in 
 all their varieties, have low coefficients, and it is worth 
 noticing that deep ultramarine blue, diffuses even less 
 effectively than black paper coated with lamp-black, which 
 has a diffusion of .05 as against .035 for the blue. Black 
 cloth, with a surface rough compared with the black paper, 
 
5 4 THE ART OF ILLUMINATION. 
 
 diffuses very much less light; while black velvet of which 
 the structure is, as just explained, particularly adapted to 
 suppress light has a coefficient of diffusion conspicu- 
 ously less than any of the others. A little dust upon its 
 surface, however, is capable of reflecting a good deal of 
 light. 
 
 These coefficients of diffusion have a very important 
 bearing on the illumination of interiors. It is at once ob- 
 vious that except in the case of a white interior finish or 
 a very pale shade of color the illumination received by 
 any object is not very greatly strengthened by diffused 
 light from the walls. All of the strong colors, particu- 
 larly if very dark, cut down diffusion to a relatively small 
 amount, although it is very difficult to suppress diffusion 
 with anything like completeness. 
 
 One of the standing difficulties in photometric work is 
 to coat the walls of the photometer room with a substance 
 so non-reflecting as not to interfere with the measure- 
 ments. Even lamp-black returns as diffused light one- 
 twentieth of that thrown upon it, and painting with any- 
 thing less lusterless than lamp-black would increase the 
 proportion of diffused light very consideraby. Walls 
 painted dead black, and auxiliary screens, also dead black, 
 to cut off the diffused light still more, are the means gen- 
 erally taken to prevent the interference of reflected light 
 with the accuracy of the photometric measurements. 
 
 In the case of any diffusing surface, or any reflecting 
 surface whatever, for that matter, a second reflection has, 
 at least approximately; the same coefficient of reflection as 
 the first, so that for trie two reflections the intensity of the 
 beam that finally escapes is that of the incident beam mul- 
 tiplied by the square of the coefficient of diffusion, and so 
 on for higher powers. 
 
REFLECTION AND DIFFUSION. 55 
 
 Inasmuch as in any enclosed space there is considerable 
 cross-reflection of diffused light, the difference in the total 
 amount of illumination due to reflection is even more vari- 
 able than would be indicated by the table of coefficients 
 given; for while the amount of light twice diffused from 
 white paper or paint would be very perceptible in the 
 illumination, that twice diffused from paper of a dark 
 color would be comparatively insignificant. 
 
 The color of the walls, therefore, plays a most impor- 
 tant part in practical illumination, for rooms with dark or 
 strongly-colored walls require a very much more liberal 
 use of illuminants than those with white or lightly-tinted 
 walls. The difference is great enough to be a considerable 
 factor in the economics of the question in cases where 
 artistic considerations are not of prime importance. The 
 nature and amount of the effect of the bounding surfaces 
 on illumination will be discussed in connection with the 
 general consideration of interior lighting. 
 
CHAPTER IV. 
 
 THE MATERIALS OF ILLUMINATION ILLUMINANTS 
 OF COMBUSTION. 
 
 AT root, all practical illuminants are composed of solid 
 particles, usually of carbon, brought to vivid incandes- 
 cence. We may, however, divide them into two broad 
 classes according as the incandescent particles are heated 
 by their own combustion or by extraneous means. The 
 first class, therefore, may be regarded as composed of 
 luminous flames, such as candles, lamps, ordinary gas 
 flames, and the like, while the second consists of illumi- 
 nants in which a solid is rendered incandescent, it is true, 
 but not by means of its own combustion. 
 
 The second class thus consists of such illuminants as 
 mantle gas burners, electric incandescent lamps, and the 
 electric arcs, which really give their light in virtue of the 
 intense heating of the tips of the carbons by the arc, which 
 in itself is relatively of feeble luminosity. 
 
 Illumination based on incandescent gas, phosphores- 
 cence, and the like is in a very early experimental stage, 
 and while it is in this direction that we must look for in- 
 creased efficiency in illumination, nothing of practical mo- 
 ment has yet been accomplished. To the examination of 
 flame illuminants, then, we must first address ourselves. 
 
 They are interesting as being the earliest sources of 
 artificial light, and while of much less luminous efficiency 
 than the second class referred to, still hold their own in 
 
THE MATERIALS OF ILLUMINATION. 57 
 
 point of convenience, portability, and ease of extreme sub- 
 division. 
 
 We have no means of knowing the earliest sources of 
 artificial light as distinguished from heat. The torch of 
 fat wood was a natural development from the fire on the 
 hearth. But even in Homeric times there is clear evi- 
 dence of fire in braziers for the purpose of lighting, and 
 there is frequent mention of torches. The rope link satu- 
 rated with pitch or bitumen was a natural growth from the 
 pine wood torch, and was later elaborated into the candle. 
 
 It is clear that both lamps and candles date far back 
 toward prehistoric times, the lamp being perhaps a little 
 the earlier of the two. At the very dawn of ancient civili- 
 zation man had acquired the idea of soaking up animal 
 or vegetable fats into a porous wick and burning it to ob- 
 tain light, and the use of soft fats probably preceded the 
 use of those hard enough to form candles conveniently. 
 
 The early lamps took the form of a small covered basin 
 or jar with one or more apertures for the wick and a sepa- 
 rate aperture for filling. They were made of metal or pot- 
 tery, and by Roman times often had come to be highly 
 ornamented. Fig. 15 shows a group of early Roman 
 lamps of common pottery, and gives a clear idea of what 
 they were. They rarely held more than one or two gills, 
 and must have given at best but a flickering and smoky 
 light. Fig. 1 6 shows a later Roman lamp of fine work- 
 manship in bronze. 
 
 In very early times almost any fatty substance that 
 would burn was utilized for light, but in recent centuries 
 the cruder fats have largely gone out of use, and new 
 materials have been added to the list. It would be a 
 thankless task to tabulate the properties of all the sub- 
 stances which have been burned as illuminants, but those 
 
5 8 THE ART OF ILLUMINATION. 
 
 in practical use within the century just passed may for 
 convenience be classified about as follows : 
 
 FLAME ILLUMINANTS. 
 
 FATS AND WAXES. 
 
 Tallow (stearin). 
 Sperm oil. 
 Spermaceti. 
 Lard oil. 
 
 FATS AND WAXES. 
 
 Olive oil. 
 Whale oil. 
 Beeswax. 
 Vegetable waxes. 
 
 The true fats are chemically glycerides, i. e., combina- 
 tions of glycerin with the so-called fatty acids, mainly 
 
 Fig. 15. Early Roman Lamps. 
 
 stearic, oleic, and palmetic. The waxes are combinations 
 of allied acids with bases somewhat akin to glycerin, but 
 of far more complicated composition. Technically, sper- 
 maceti is allied to the waxes, while some of the vegetable 
 waxes properly belong to the fats. 
 
 All these substances, solid or liquid, animal or vege- 
 table, are very rich in carbon. They are composed en- 
 tirely of carbon, hydrogen, and oxygen, and as a class have 
 
THE MATERIALS OF ILLUMINATION, 
 
 59 
 
 about the following percentage composition by weight: 
 Carbon, 76 to 82 per cent.; hydrogen, n to 13 per cent; 
 oxygen, 5 to 10 per cent. 
 
 They are all natural substances which merely require to 
 
 Fig. 16. Roman Bronze Lamp. 
 
 go through a process of separation from foreign matter, 
 and sometimes bleaching, to be rendered fit for use. 
 
 An exception may be made in favor of " stearin," which 
 is obtained by breaking up chemically the glycerides of 
 animal fats and separating the fatty acids before men- 
 tioned from the glycerin. The oleic acid, in which liquid 
 fats are rich, is also gotten rid of in the commercial prepa- 
 ration of stearin in order to raise the melting point of the 
 product. 
 
 In a separate class stand the artificial " burning fluids " 
 
60 THE ART OF ILLUMINATION. 
 
 used considerably toward the middle of the century. As 
 they are entirely out of use, they scarcely deserve particu- 
 lar classification. Their base was usually a mixture of 
 wood alcohol and turpentine in varying proportions. 
 From its great volatility such a compound acted almost 
 like a gas generator ; the flame given off was quite steady 
 and brilliant, with much less tendency to smoke than the 
 natural oils, but the " burning fluids " as a class were out- 
 rageously dangerous to use, and fortunately were driven 
 out by the advent of petroleum and its products. 
 
 Petroleum, which occurs in one form or another at 
 many places on the earth's surface, has been known for 
 many centuries, although not in large amounts until 
 recently. Bitumen is often mentioned by Herodotus and 
 other early writers, and in Pliny's time mineral oil from 
 Agrigentum was even used in lamps. 
 
 But the actual use of petroleum products as illuminants 
 on a large scale dates from a little prior to 1860, when the 
 American and Russian fields were developed with a com- 
 mon impulse. Crude petroleum is an evil smelling liquid, 
 varying in color from very pale yellow to almost black, 
 and in specific gravity from 0.77 to i.oo, ranging com- 
 monly from 0.80 to 0.90. 
 
 Chemically it is composed essentially of carbon and 
 hydrogen, its average percentage composition being about 
 as follows: carbon, 85 per cent.; hydrogen, 15 per cent. 
 It is composed in the main of a mixture of the so-called 
 paraffin hydrocarbons, having the general formula 
 C n H. jn _j_ 3 , and the members of this series found in 
 ordinary American petroleum vary from methane ( CH 4 ) 
 to pentadecane (C 1B H 32 ), and beyond to solid hydro- 
 carbons still more complicated. 
 
 To fit petroleum for use as an illuminant, these com- 
 
THE MATERIALS OF ILLUMINATION. 61 
 
 ponent parts have to be sorted out, so that the oil for 
 burning shall neither be so volatile as to have a danger- 
 ously low flashing point nor so stable as not to burn 
 clearly and freely. 
 
 This sorting is done by fractional distillation. The 
 following table gives a general idea of the products ar- 
 ranged according to their densities : 
 
 SUBSTANCE. 
 
 \ Cymogene, 
 Petroleum ether. . . j Rhigoline, 
 
 ' Gasoline, 
 
 ( Benzine naphtha, 
 Petroleum spirit. . ) Naphtha, 
 
 ( Benzine, 
 
 Kerosene J Kerosene of va- 
 
 1 rious grades, 
 
 Oils. . . . \ Lubricating oils 
 
 f of various grades. 
 
 USE. 
 
 Small. 
 
 Solids 
 
 ( Vas< 
 
 ids ] 
 
 ( Pars 
 
 Vaseline, 
 I'araffin, 
 
 Gas, explosion engines. 
 Gas lamps, engines. 
 Cleaning, engines. 
 Varnish, etc. 
 
 Illumination. 
 
 Lubrication. 
 
 Emollient. 
 Candles, insulation, 
 waterproofing, etc. 
 
 " Petroleum ether " and " petroleum spirit " find little 
 use in illumination, for they are so inflammable as to be 
 highly dangerous, and form violently explosive mixtures 
 with air at ordinary temperatures. 
 
 Kerosene should be colorless, without a. very penetrat- 
 ing odor, which indicates too great volatility, and should 
 not give off inflammable vapor below a temperature of 
 120 F., or, better still, below 140 F. to 150 F. Oils of 
 the latter grades are pretty safe to use, and are always to 
 be preferred to those more volatile. The yield of kero- 
 sene from crude oil varies from place to place, but with 
 good American oil runs as high as 50 to 75 per cent. 
 
 Paraffin is sometimes used unmixed for making candles, 
 
62 THE ART OF ILLUMINATION. 
 
 but is preferably mixed with other substances, like stearin, 
 to give it a higher melting point. 
 
 Having thus casually looked over the materials burned 
 in candles and lamps, the results may properly be con- 
 sidered. 
 
 Candles. These are made usually of stearin, paraffin, 
 wax, or mixtures of the two first named. They are 
 molded hot in automatic machines, and, as usually sup- 
 plied in this country, are made in weights of 4, 6, and 12 
 to the pound. Spermaceti candles are also made, but are 
 little used except for a standard of light. The English 
 standard candle is of spermaceti, weighing one-sixth of a 
 pound and burning at the rate of 120 grains per hour. 
 
 Commercial candles give approximately one candle- 
 power, sometimes rather more, and burn generally from 
 no to 130 grains per hour. As candles average from 
 15 to 1 8 cents per pound, the cost of one candle-hour from 
 this source amounts to about 0.25 cent to 0.30 cent. This 
 is obviously relatively very expensive, although it must 
 not be forgotten that candles subdivide the light so effect- 
 ively that for many purposes 16 lighted candles are very 
 much more effective in producing illumination than a gas 
 flame or incandescent lamp of 16 candle power. 
 
 The present function of candles in illumination is con- 
 fined to their use as portable lights, for which, on the score 
 of safety, they are far preferable to kerosene lamps, and to 
 cases in which, for artistic purposes, thorough sub- 
 division of the light is desirable. Where only a small 
 amount of general light is needed, candles give a most 
 pleasing effect and are, moreover, cleanly and odorless. 
 
 In efficiency candles leave much to be desired. For, 
 taking the ordinary stearin candle as a type, it requires in 
 dynamical units 90 watts per candle-power, consumes per 
 
THE MATERIALS OF ILLUMINATION. 63 
 
 hour the oxygen contained in 4.5 cu. ft. of air, and gives 
 off about 0.6 cu. ft. of carbonic acid gas. In these re- 
 spects the candle is inferior to the ordinary lamp, and still 
 more inferior to gas or electric lights. Nevertheless, it is 
 oftentimes a most convenient illuminant. 
 r~Qil Lamps. Oils other than kerosene are used in this 
 country only to a very slight extent, the latter having 
 driven out its competitors. Sperm oil and, abroad, colza 
 oil (obtained from rape seed) are valued as safe and re- 
 liable illuminants for lighthouses, and in some parts of the 
 Continent olive oil is used in lamps, as it has been from 
 time immemorial.^? 
 
 Here, kerosene is still the general illuminant outside of 
 the cities and larger towns. It has the merits of being 
 cheap (on the average 12 cents to 15 cents per gallon in 
 recent years), safe, if of the best quality, and of giving, 
 when properly burned, a very steady and brilliant light. 
 UAH oils require a liberal supply of air for their combus- 
 tion, particularly the heavier oils, and many ingenious 
 forms of lamp have been devised to meet the requirements. 
 On the whole, the most successful are on the Argand prin- 
 ciple, using a circular wick with air supply both within and 
 without, although some of the double flat wick burners 
 are admirable in their results. A typical lamp^the fa- 
 miliar " Rochester," is shown in Fig. 17, which sufficiently 
 shows the principle involved. In kerosene lamps the 
 capillary action of the wick affords an ample supply of oil, 
 but with some other oils it has proved advantageous to 
 provide a forced supply. The so-called " student lamp," 
 with its oil reservoir, is the survival of an early form of 
 Argand burner designed to burn whale oil. In other in- 
 stances clock-work is employed to pump the oil, and some- 
 times a forced air supply is used. 
 
 TY ) 
 
64 THE ART OF ILLUMINATION. 
 
 Kerosene lamps usually are designed to give from 10 
 to 20 candle-power, and occasionally more, special lamps 
 giving even up to 50 or 60 candle-power. The consump- 
 tion of oil is generally from 50 to 60 grains per hour per 
 
 Fig. 17. "Rochester" Kerosene Burner. 
 
 candle-power. As kerosene weighs about 6.6 pounds per 
 gallon, the light obtained is in the neighborhood of 800 
 candle-hours per gallon. 
 
 This brings the cost of the candle-hour down to about 
 0.018 cent, taking the oil at 15 cents per gallon. No 
 illuminants save arc lights and mantle burners with 
 cheap gas can compare with it in point of economy^ 
 
 A very interesting and valuable application of oil light- 
 ing is found in the so-called " Lucigen " torch and several 
 kindred devices. The oil, generally one of the heavier 
 petroleum products, is carried under air pressure in a good- 
 sized portable reservoir, and the oil is led, with the com- 
 pressed air highly heated by its passage through the appa- 
 ratus, to an atomizing nozzle, from which it is thrown out 
 
THE MATERIALS OF ILLUMINATION. 65 
 
 Fig. 1 8. " Lucigen " Torch. 
 
 in a very fine spray, and is instantly vaporized and burned 
 under highly efficient conditions. 
 
 These " Lucigen " torches give nearly 2000 candle- 
 
66 THE ART OF ILLUMINATION. 
 
 power on a consumption of about two gallons of oil per 
 hour, burning with a tremendous flaring flame three feet 
 or more in length and six or eight inches in diameter. 
 They are very useful for lighting excavations and other 
 rough works for night labor, being powerful, portable, and 
 cheap to operate. Fig. 18 gives an excellent idea of this 
 apparatus in a common form. Such a light is only suited 
 to outdoor work, but it forms an interesting transitional 
 step toward the air-gas illuminants which have come into 
 considerable use for lighting where service mains for gas 
 or electricity are not available, or where the conditions call 
 for special economy. 
 
 ^Air Gas. It has been known for seventy years or more 
 that the vapor of volatile hydrocarbons could be used to 
 enrich poor coal gas, and that even air charged with a 
 large amount of such vapor was a pretty good illuminant. 
 Of late years this has resulted 5 in the considerable use of 
 "carbureters," which saturate air with hydrocarbon 
 vapor, making a mixture too rich to be in itself explosive 
 and possessing good illuminating properties when burned 
 as gas in the ordinary way. The usual basis of opera- 
 tions is commercial gasoline, which consists of a mixture 
 of the more volatile paraffin hydrocarbons, chiefly pen- 
 tane, hexane and iso-hexane. 
 
 The process of gas-making is very simple, consisting 
 merely of charging air with the gasoline vapor. Fig. 19 
 shows in section a typical air-gas machine. It consists of 
 a large metal tank holding a supply of gasoline, a carburet- 
 ing chamber of flat trays over which a gasoline supply 
 trickles, a fan to keep up the air supply, and a little gas 
 reservoir in which the pressure is regulated and from 
 which the gas is piped. The fan is driven by heavy 
 weights, wound up at suitable intervals. 
 
THE MATERIALS OF ILLUMINATION. 67 
 
 The whole gas machine is usually put in an under- 
 ground chamber, both for security from fire and to aid in 
 maintaining a steady temperature. About six gallons of 
 gasoline are required per 1000 cu. ft. of air, and the result 
 
 Fig. 19. Gasoline Gas Machine. 
 
 is a gas of very fair illuminating power, rather better 
 than ordinary city gas. 
 
 The cost of this air gas is very moderate, but on account 
 of the cost of plant and some extra labor, it is materially 
 greater than the cost of direct lighting by kerosene lamps. 
 It is a means of lighting very useful for country houses 
 and other places far from gas or electric supply companies. 
 
68 THE ART OF ILLUMINATION. 
 
 The principal difficulty is the variation of the richness of 
 the mixture with the temperature, owing to change in the 
 volatility of the gasoline, a fault which is very difficult to 
 overcome. At low temperatures there is a tendency to 
 carburet insufficiently and to condense liquid in the cold 
 pipes. The gas obtained from these machines is burned 
 in the ordinary way, although burners especially adapted 
 for it are extensively employed. Recently such gas has 
 been considerably used with mantle burners, obtaining 
 thus a very economical result. 
 
 Coal Gas. In commercial use for three-quarters of a 
 century, coal gas was, until about twenty years ago, the 
 chief practical illuminant. Little need here be said of its 
 manufacture, which is a department of technology quite 
 by itself, other than that the gas is obtained from the de- 
 structive distillation of rich coals enclosed in retorts, from 
 which it is drawn through purifying apparatus and re- 
 ceived in the great gasometers familiar on the outskirts of 
 every city. 
 
 The yield of gas is about 10,000 cu. ft. per ton of coal of 
 good quality. The resulting gas consists mainly of 
 hydrogen and of methane (CH 4 ) with small amounts of 
 other gases, the composition varying very widely in de- 
 tails while preserving the same general characteristics. 
 A typical analysis of standard coal gas giving 16 to 17 
 candle-power for a burner consuming 5 cu. ft. per hour 
 would be about as follows : 
 
 Hydrogen 53.0 
 
 Paraffin hydrocarbons 33.0 
 
 Other hydrocarbons 3. 5 
 
 Carbon monoxide 5.5 
 
 Carbon dioxide , 0.6 
 
 Nitrogen 4.2 
 
 Oxygen 0.2 
 
 100. 
 
THE MATERIALS OF ILLUMINATION. 69 
 
 Ammonia compounds, carbon dioxide, and sulphur 
 compounds are the principal impurities which have to be 
 removed. Traces of these and of moisture are often 
 found in commercial gas. 
 
 In point of fact, at the present time but a small propor- 
 tion of the illuminating gas used in this country is un- 
 mixed coal gas, such as might show the analysis just 
 given. Most of it is water gas, or a mixture of coal gas 
 and water gas. Water gas is produced by the simple 
 process of passing steam through a mass of incandescent 
 coal or coke, and thus breaking up the steam into hy- 
 drogen and oxygen, which latter unites with the carbon 
 of the coal, forming carbon monoxide. 
 
 At moderate temperatures considerable carbon dioxide 
 would be formed, but, as this is worse than useless for 
 burning purposes, the heat is always carried high enough 
 to insure the formation of the monoxide. The hypo- 
 thetical chemical equation is : 
 
 The reaction is never clean in so complete a sense as 
 this, some CO 3 always being formed. This water gas as 
 thus formed is useless as an illuminant, and requires to 
 be enriched by admixture of light-producing hydro- 
 carbons carbureted, in other words. This is done by 
 treating it to a spray of petroleum in some form, and at 
 once passing the mixture through a superheater, which 
 breaks down the heavier hydrocarbons and renders the 
 mixture stable. 
 
 There are many modifications of this system worked on 
 the same general lines. The enriching is carried to the 
 extent necessary to meet the legal requirements, usually 
 producing gas of 15 to 20 candle-power for a 5-ft. jet. 
 
7 o THE ART OF ILLUMINATION. 
 
 A typical analysis of the water gas after enriching would 
 show about the following by volume : 
 
 Hydrogen 34-O 
 
 Methane 1 5 -O 
 
 Enriching hydrocarbons 12.5 
 
 Carbon monoxide 33- 
 
 Oxygen, nitrogen, etc 5-5 
 
 The latter part of the enriching process, i. e., superheat- 
 ing and breaking up the heavy hydrocarbons while in the 
 form of vapor, is substantially that used in making 
 Pintsch and allied varieties of oil gas, so that commercial 
 water gas may be regarded as a mixture of water gas and 
 oil gas. 
 
 Water gas, when properly enriched, is fully the equiva- 
 lent of coal gas for illuminating purposes. The main dif- 
 ference between them is the very large proportion of car- 
 bon monoxide in the water gas, which adds greatly to the 
 danger of leaks. 
 
 For this carbon monoxide is an active poison, not kill- 
 ing merely by asphyxia, but by a well-defined toxic action 
 peculiar to itself. Hence persons overcome by water gas 
 very frequently die under circumstances which, if coal gas 
 were concerned, would result only in temporary insensi- 
 bility. As the enriched water gas is cheaper than coal 
 gas, however, the gas companies, maintaining, with some 
 justice, that gas is not furnished for breathing purposes, 
 supply it unhesitatingly sometimes openly, sometimes 
 without advertising the fact. 
 
 Very commonly so-called coal gases contain enriched 
 water gas to bring up their illuminating power. In these 
 cases the carbon monoxide is in much less proportion, 
 perhaps only 12 to 15 per cent. 
 
 It is often stated that water gas is doubly dangerous 
 
THE MATERIALS OF ILLUMINATION. ?i 
 
 from its lack of odor. The unenriched gas is practically 
 odorless, but when enriched the odor, while less penetrat- 
 ing than that of coal gas, is sufficiently distinctive to make 
 a leak easily perceptible. 
 
 Lras burners for ordinary illuminating gas are of three 
 general types : flat flame, Argand, and regenerative. The 
 first named is the most common and least efficient form. 
 It consists of two general varieties, known respectively as 
 the " fishtail " and " bat's-wing." The former has a con- 
 cave tip, usually of steatite or similar material, containing 
 two minute round apertures, so inclined that the two little 
 jets meet and flatten out crosswise into a wide flame. 
 This form is now relatively little used save in dealing 
 with some special kinds of gas. 
 
 The bat's-wing burner, with a dome-shaped tip, having 
 a narrow slit for the gas jet, is the usual form employed 
 with ordinary gas. Flat-flame burners work badly in 
 point of efficiency unless of fairly large size. On ordinary 
 gas of 14 to i7-cp nominal value on a 5-ft. burner, 
 burners taking less than about 4 cubic ft. per hour are 
 decidedly inefficient. A 4-ft. burner will give about 2.5 
 candles per foot, while a 5-ft. burner will give 2.75 to 3 
 candle-power per foot. 
 
 The Argand burners give considerably better results, 
 their flames being inclosed and protected from draughts 
 by a chimney; and the air supply being good the tempera- 
 ture of the flame is high and the light is whiter than in the 
 flat-flame burners. The principle is familiar, the wick of 
 the Argand oil lamp being replaced in the gas burner by a 
 hollow ring of steatite connected with the supply, and 
 perforated with tiny jet holes around the upper edge. 
 Fig. 20 shows in section an Argand burner (Suez's) of a 
 standard make used in testing London gas. This burner 
 
7* THE ART OF ILLUMINATION. 
 
 uses 5 cubic feet per hour, and the annular chamber has 24 
 holes, each 0.045" m diameter. The efficiency is a little 
 better than that of the flat-flame burners, running, on good 
 
 Fig. 20. Section of Argand Gas Burner. 
 
 gas, from 3 to 3.5 candle-power per foot. The London 
 legal standard gas is of 16 candle-power in this 5-ft. 
 burner. 
 
 On rich gas the flat-flame burners, particularly the fish- 
 tail, work better than the Argand, the fishtail being better 
 on very rich gas than is the bat's-wing form. With 
 
THE MATERIALS OF ILLUMINATION. 73 
 
 ordinary qualities of gas, however, the Argand burner is 
 vastly more satisfactory than the flat flames/^ 
 
 For very powerful lights the so-called regenerative 
 burners are generally preferred. These are based on the 
 
 Fig. 21. Wenham Regenerative Burner. 
 
 general principle of heating both the gas and the air fur- 
 nished for its combustion prior to their reaching the 
 flame. The burner proper is something like, an inverted 
 Argand, so arranged as to furnish a circular sheet of flame 
 convex downward, and with, of course, a central cusp. 
 Directly above the burner and strongly heated by the 
 flame, are the air and gas passages. 
 
 Fig. 21 shows in section the Wenham burner of this 
 
74 
 
 THE ART OF ILLUMINATION. 
 
 class. The arrows show the course of the air and the gas, 
 the latter being burned just below the iron regenerative 
 chamber and the products of combustion passing upward 
 
 Fig. 22. Siemens Regenerative Gas Burner. 
 
 through the upper shell of the lamp, and preferably to a 
 ventilating flue. The globe below prevents the access of 
 cold air, and the annular porcelain reflector surrounding 
 the exit flue turns downward some useful light. 
 
 The Siemens regenerative burner, shown in Fig. 22, is 
 
THE MATERIALS OF ILLUMINATION. 75 
 
 arranged upon a similar plan and gives much the same 
 effect. The regenerative burners of this class give a very 
 brilliant yellow-white light with a generally hemispherical 
 distribution downward. They work best and most eco- 
 nomically in the larger sizes, 100 to 200 candle-power, 
 and must be placed near the ceiling to take the best advan- 
 tage of their usual distribution of light. 
 
 With gas of about i6-cp standard these regenerative 
 burners consume only about i cubic foot per hour for 5 to 
 7 candle-power. They are thus nearly twice as economi- 
 cal as the best Argand burners. Their chief disadvantage 
 lies in the fact that to get this economy very powerful 
 burners must be used, of a size not always conveniently 
 applicable. 
 
 From such a powerful center of light a large amount of 
 heat is thrown off, obviously less per candle-power of light 
 than in other gas burners, but, in the aggregate, large. 
 Regenerative burners are well suited, however, to the 
 illumination of large spaces, although at the present time 
 the greater economy of the mantle burner has rather 
 pushed the regenerative class into the background. Their 
 light, nevertheless, is of a very much more desirable color 
 than that given by the mantle burners. 
 
 The most recent and in some respects mo^t important 
 addition to the list of flame illuminants is acetylene. This 
 gas is a hydrocarbon having the formula C 2 H 2 , which has 
 been well known to chemists for many years, but which 
 until recently has not been preparable by any convenient 
 commercial process. It is a rather heavy gas, of evil odor, 
 generally somewhat reminiscent of garlic/ and, being very 
 rich in carbon uncombined with oxygen (nearly 93 per 
 cent, by weight) It burns very brilliantly when properly 
 supplied with air. Its flame is intensely bright, nearly 
 
76 THE ART OF ILLUMINATION. 
 
 white in color, and for the light given it vitiates the air in 
 comparatively small degree. 
 
 Acetylene is made in practice from calcic carbide, 
 Ca C 2 , a chemical product prepared by subjecting a mix- 
 ture of powdered lime and carbon (coke) to the heat of 
 the electric furnace. By this means it can be prepared 
 readily in quantity at moderate cost. The acetylene is 
 made from the calcic carbide by treating it with water, 
 lime and acetylene being the results of the reaction, which, 
 in chemical terms, is as follows : 
 
 Ca C a + 2 H 2 O = Ca (OH) a -f C, H 2 . 
 
 Commercial calcic carbide is far from being chemically 
 pure, so that the acetylene prepared from it contains vari- 
 ous impurities, and is neither in quantity nor quality just 
 what the equation would indicate. The carbide is ex- 
 tremely hygroscopic, and hence not very easy to transport 
 or keep, and the upshot of this property and the inherent 
 impurities is that the practical yield of acetylene is only 
 about 4.5 to 5.0 cubic feet per pound of carbide, 4.75 cubic 
 feet being an extremely good average unless the work is 
 on a very large scale, though 4.5 cubic feet is the more 
 usual yield. In theory the yield should be nearly 5.5 
 cubic feet Qer pound. 
 
 The gaseous impurities are quite varied and by no 
 means uniform in amount or nature, but the most objec- 
 tionable ones may be removed by passing the gas in fine 
 bubbles through water. If the gas is being prepared on 
 a large scale it can readily be purified. 
 
 Acetylene has the disadvantage of being somewhat un- 
 stable. It forms direct compounds with certain metals, 
 notably copper, these compounds being known as acety- 
 lides, and being themselves so unstable as to be easily ex- 
 
THE MATERIALS OF ILLUMINATION. 77 
 
 plosive. Acetylene should be therefore kept out of con- 
 tact with copper in storage, and even in fixtures. 
 
 The gas itself is easily dissociated with evolution of 
 heat into carbon and hydrogen, and hence may be inher- 
 ently explosive under certain conditions, fortunately not 
 common. 
 
 At atmospheric pressure, or at such small increased 
 pressures as are employed in the commercial distribution 
 of gas, acetylene, unmixed with air, cannot be exploded 
 by any means ordinarily at hand. 
 
 Above a pressure of about two atmospheres acetylene 
 is readily explosive from high heat and from a spark or 
 flame, and grows steadily in explosive violence as the 
 initial pressure rises, until when liquefied it detonates with 
 tremendous power if ignited. At ordinary temperatures it 
 can be liquefied at a pressure of about 80 atmospheres, 
 and it has been proposed to transport and store it in 
 liquid form. But, although even when liquefied it will not 
 explode from mechanical shgck alone, it is in this condi- 
 tion an explosive of the same order of violence as gun- 
 cotton or nitro-glycerine, and should be treated as such. 
 
 Mixtures of acetylene and air explode violently, just as 
 do mixtures of illuminating gas and air. The former be- 
 gin to explode rather than merely burn, when the mix- 
 ture contains about one volume of acetylene to three of 
 air, detonate very violently with about nine volumes of 
 air, and cease to explode with about twenty volumes of 
 air. 
 
 Ordinary coal gas begins to explode when mixed with 
 three volumes of air, reaches a maximum of violence with 
 about five to six volumes, and ceases to explode with 
 eleven volumes. Of the two gases, the acetylene is rather 
 the more violently explosive when mixed with air, and it 
 
7 8 THE ART OF ILLUMINATION. 
 
 becomes explosive while the mixture is much leaner. The 
 difference is not of great practical moment, however, ex- 
 cept as acetylene generators, being easily operated, are 
 likely to get into unskillful hands. This fact has already 
 resulted in many disastrous explosions. 
 
 As regards its poisonous properties, acetylene seems to 
 be somewhat less dangerous than coal gas and very much 
 less dangerous than water gas. Properly speaking, acety- 
 lene is very feebly poisonous when pure, and has such an 
 outrageous smell when slightly impure that the slightest 
 leak attracts attention. Some early experiments showed 
 highly toxic properties, but these have not been fully con- 
 firmed, and may have been due to impurities in the gas 
 possibly to phosphine, which is a violent poison. 
 
 The calcic carbide from which the acetylene is pre- 
 pared is so hygroscopic and gives off the gas so freely that 
 it has to be stored with great care on account of possible 
 danger from fire. Fire underwriters are generally united 
 in forbidding entirely the use or storage of liquid or com- 
 pressed acetylene, or the storage of any but trivial amounts 
 of calcic carbide (a few pounds) except in detached fire- 
 proof buildings. 
 
 Acetylene is, when properly burned, a magnificent 
 illuminant. It will not work in ordinary burners, for un- 
 less very liberally supplied with air it is so rich in carbon 
 as to burn with a smoky flame and a deposit of soot. It 
 must actually be mixed with air at the burner in order to 
 be properly consumed. When so utilized its illuminating 
 power is very great. The various experiments are not 
 closely concordant, but they unite in indicating an illumi- 
 nating power of 35 to 45 candle-hours per cubic foot, ac- 
 cording to the capacity of the burner, the larger burners, 
 as usual, working the more economically. 
 
THE MATERIALS OF ILLUMINATION. 79 
 
 This means that the acetylene has nearly fifteen times 
 the illuminating power of a good quality of ordinary 
 illuminating gas when burned in ordinary burners. It 
 will, consequently, give about eight to ten times more light 
 per cubic foot than gas in a regenerative burner, and, it 
 may be mentioned, about three to four times more light 
 than gas in a mantle (Welsbach) burner. 
 
 Fig. 23 shows a common standard form of acetylene 
 burner, intended to consume about 0.5 cubic foot per hour. 
 
 Fig. 23. Acetylene Burner. 
 
 It is a duplex form akin in its production of flame to a 
 common fishtail. Each of the two burners is formed with 
 a lava tip having a slight constriction close to its point. 
 In this is the central round aperture for the gas, and just 
 ahead of it are four lateral apertures for the air supply. 
 The acetylene and air mix just in front of the constric- 
 tion and the two burners unite their jets to form a small, 
 flat flame. It is in effect a pair of tiny Bunsen burners 
 inclined to produce a fishtail jet. 
 
 Larger acetylene burners are worked on a similar prin- 
 ciple, all having the air supply passages characteristic of 
 
8o 
 
 THE ART OF ILLUMINATION. 
 
 the Bunsen burner. Too great air supply for the acety- 
 lene gives the ordinary colorless Bunsen flame, but on re- 
 ducing the amount the acetylene burns with a singularly 
 white, brilliant, and steady flame. 
 
 Of acetylene generators designed automatically to 
 supply gas at constant pressure from the calcic carbide the 
 
 Fig. 24. Small Acetylene Generator. 
 
 name is legion. A vast majority of those in use at present 
 are of rather small capacity, being designed for a few 
 lights locally or as portable apparatus for lamps used for 
 projection. Generators on a large scale have hardly come 
 
THE MATERIALS OF ILLUMINATION. 81 
 
 into use, and the problems of continuous generation have 
 consequently not been forced into prominence. 
 
 A very useful type of the small generator is shown in 
 Fig. 24, a form devised by d'Arsonval. It consists of a 
 small gasometer with suitable connections for taking off 
 the gas and drawing off the water. The bell of the 
 gasometer is furnished at the top with a large aperture 
 closed by a water seal. Through this is introduced a deep 
 iron wire basket containing the charge of carbide. 
 
 The acetylene is generated very steadily after the appa- 
 ratus gets to working and the pressure is quite uniform. 
 The water in the gasometer of the d'Arsonval machine is 
 covered by a layer of oil, which serves an important pur- 
 pose. When one ceases using the gas the bell rises, and as 
 the carbide basket rises out of the water the oil coats it and 
 displaces the water, checking further evolution of gas. 
 The oil also checks evaporation, so that there is no slow 
 evolution of gas from the absorption of aqueous vapor. 
 
 As to the value of acetylene, it is evidently worth 
 about fifteen times as much per cubic foot as gas burned 
 in ordinary burners, or three to four times as much as 
 gas, assuming it to be burned in Welsbach burners. 
 Now one ton of calcic carbide of high quality, efficiently 
 used, will produce nearly 10,000 cu. ft. of acetylene, 
 equal in illuminating v,alue to 150,000 cu. ft. of gas in the 
 one case or to 30,000 to 40,000 cu. ft. in the other. 
 
 The cost of the calcic carbide is a very uncertain quan- 
 tity at present. The best authorities bring the manu- 
 facturing cost, on a large scale and under very favorable 
 circumstances, somewhere between $30 and $40 per ton. 
 It is doubtful if any finds its way into the hands of bona 
 fide users at less than about $60 per ton, and the current 
 price in small lots is much higher, and naturally so, by 
 
82 THE ART OF ILLUMINATION. 
 
 reason of troublesome storage and the cost of transporta- 
 tion. Adding the necessary allowance for the cost of 
 producing the gas from the carbide, it is at once evident 
 that the cost of lighting by acetylene falls below that of 
 lighting by common gas in ordinary burners at the com- 
 mon price of $i to $1.50 per 1000 ft. 
 
 It is equally evident that it considerably exceeds the cost 
 of gas lighting by Welsbach burners. There seems to be 
 small chance of its coming into general competition with 
 either at present. Its cost of production and distribution 
 does not yet render it commercially attractive under ordi- 
 nary conditions. 
 
 Nevertheless, acetylene is for use in isolated places one 
 of the very best and most practical illuminants, for it is 
 fairly cheap, easily made, and gives a light not surpassed 
 in quality by any known artificial illuminant. It is 
 peculiarly well adapted for temporary and portable use, 
 giving as it does a very brilliant and steady light, well 
 suited for use with reflectors and projecting apparatus, 
 admirable in color, and very easy of operation. 
 
CHAPTER V. 
 
 THE MATERIALS OF ILLUMINATION INCANDESCENT 
 
 BURNERS. 
 
 THE general class of illuminants operative by the in- 
 candescence of a fixed solid body would include in prin- 
 ciple both arc and incandescent electric lamps, as well as 
 those in which the radiant substance is heated by ordinary 
 means. In this particular place, however, it seems appro- 
 priate to discuss the latter forms only, leaving the electric 
 lights for a separate chapter. 
 
 Incandescent radiants brought to the necessary high 
 temperature by a non-luminous flame have their origin in 
 the so-called " Drummond " or " lime " light, which has 
 been used for many years as the chief illuminant in pro- 
 jection, scenic illumination on the stage, and such like pur- 
 poses, and which has only recently been extensively re- 
 placed by the electric arc. The limelight consists of a 
 short pencil of lime against which is directed the colorless 
 and intensely hot flame from a blast lamp fed with pure 
 oxygen and hydrogen, or more commonly with oxygen 
 and illuminating gas. 
 
 The general arrangement of the oxy-hydrogen burner 
 is shown in Fig. 25. Here A and B are the supply pipes 
 for the oxygen and hydrogen, fitted with stop-cocks. 
 These unite in a common jet in the burner E, which is 
 usually inclined so as to bring the burner where it will not 
 cast a shadow. Sometimes the two gases are mixed in 
 the burner tube C f and sometimes the hydrogen is deliv- 
 
8 4 
 
 THE ART OF ILLUMINATION. 
 
 ered through an annular orifice about a central tube which 
 supplies the oxygen. The pencil of lime is carried on a 
 holder D, and the whole burner is often carried on an ad- 
 justable stand E, so that it can be raised, lowered, or 
 
 Fig. 25. Oxy-hydrogen Burner. 
 
 turned, as occasion demands. Themixea gases unite in a 
 colorless, slender flame of enormously high temperature, 
 and when this impinges on the lime the latter rises in a 
 small circular spot to the most brilliant incandescence, 
 giving an intense white light of, generally, 200 to 400 
 candle-power. 
 
 The light, however, falls off in brilliancy quite rapidly, 
 
THE MATERIALS OF ILLUMINATION. 85 
 
 particularly when the initial incandescence is very intense, 
 losing something like two-thirds of its candle-power in an 
 hour, so that it is the custom for the operator to turn the 
 pencil from time to time so as to expose new portions to 
 the oxy-hydrogen jet. 
 
 At the highest temperatures the calcium oxide is some- 
 what volatile and the surface seems to change and lose its 
 radiative power. Sometimes pencils of zirconium oxide 
 are used instead of lime, and this substance has proved 
 more permanently brilliant and does not seem to volatilize. 
 When properly manipulated, the calcium light is beauti- 
 fully steady and brilliant, and being very portable, is well 
 adapted for temporary use. 
 
 From time to time attempts were made to produce a 
 generally useful incandescent lamp in which the oxy- 
 hydrogen jet should be replaced by a Bunsen burner re- 
 quiring only illuminating gas and air. 
 
 Platinum gauze and other substances were tried as the 
 incandescent materials, but the experiments came to noth- 
 ing practically until the mantle burner of Auer von Wels- 
 bach appeared. This is generally known in this country 
 as the Welsbach light, but on the Continent as the Auer 
 light. In this burner the material brought to incandes- 
 cence is a mantle, formed like a little conical bag, of thin 
 fabric thoroughly impregnated with the proper chemicals 
 and then ignited, leaving a coarse gauze formed of the 
 active material. 
 
 The composition of this material has been kept more or 
 less secret, and has been varied from time to time as the 
 burner has gradually been evolved into its present state, 
 but is well known to consist essentially of the oxides of 
 the so-called "metals of the rare earths," chiefly thorium 
 and yttrium. 
 
86 THE ART OF ILLUMINATION. 
 
 These rare earths, zirconia, thoria, glucina, yttria, and 
 a half-dozen others still less well known, form a very curi- 
 ous group of chemical substances. They are whitish or 
 yellowish very refractory oxides occurring as components 
 of certain rare minerals, and most of them rise to magnifi- 
 cent incandescence when highly heated. The hue of this 
 incandescence differs slightly for the different earths and 
 they are very nearly non-volatile except at enormous 
 temperatures. One, erbia, has the extraordinary property 
 of giving a spectrum of bright bands when highly heated 
 instead of the continuous spectrum usual to incandescent 
 solids, a property which is shared in less degree by a few 
 of its curious associates. 
 
 The mantle burners of the Welsbach type are formed of 
 various blends of the more accessible of these rare earths, 
 and when brought to incandescence by the flame of a Bun- 
 sen burner within the mantle, give a most brilliant light 
 with a very small expenditure of gas. 
 
 As first manufactured the mantles were very fragile, 
 breaking on the smallest provocation, but they have gradu- 
 ally been increased in strength until those now made gen- 
 erally hold together for many hundred hours, and usually 
 should be discarded for inefficiency long before they break. 
 This statement refers to mantles burned indoors and not 
 subjected to any unusual vibration, which greatly shortens 
 their life. 
 
 As at present manufactured the standard Welsbach 
 burner complete is shown in Fig. 26, of which the several 
 parts are distinctly labeled in the cut. It consists es- 
 sentially of a Bunsen burner with provisions for regu- 
 lating the flow of both air and gas, capped by fine wire 
 gauze to prevent the flame striking back, and the mantle 
 within which the Bunsen flame burns. There are suita- 
 
THE MATERIALS OF ILLUMINATION. 87 
 
 ble supports for the chimney and shades and for the 
 mantle. 
 
 The mantle carrier is permanently attached to a cap 
 with a wire gauze top, and this cap goes into place on 
 
 CHIMNEY 
 
 SHADE SUPPOn 
 
 MANTLE 
 MANTLESUPPORT 
 CHIMNEY SUPPORT 
 GAUZE TIP 
 SOCKET 
 
 SHADE ' SUPPORT 
 ERY 
 
 BUNSENTUBE 
 R SHUTTER 
 
 GAS 
 
 REGULATOR 
 
 3UNSENTUBE 
 Fig. 26. Standard Welsbach Burner. 
 
 the burner tube with a bayonet joint so that the mantle 
 is brought exactly to the right place, instead of having 
 to be adjusted over a permanent cap. This is one of the 
 most important recent improvements in this type of 
 burner, since previously the risk of breakage in adjusting 
 a new mantle had been very considerable. 
 
 Several makes of mantle burners are in use at the 
 present time, but the ordinary Welsbach may be con- 
 
88 
 
 THE ART O.F ILLUMINATION. 
 
 sidered as a type of the best modern practice, and the 
 data here given refer to it, and are at least as favorable 
 as would be derived from any other form. 
 
 As in most other burners, the efficiency of the mantle 
 burner increases somewhat with the capacity, but the 
 general result reached in common practice with 16 
 candle-power (nominal) gas is 12 to 15 candle-power 
 per cubic foot of gas, assuming the mantle to be new. 
 In other words, at the start the mantle burner is nearly 
 
 Fig. 27. Life Curves, Welsbach Mantles. 
 
 five times as efficient as an Argand burner, about six 
 times as efficient as an ordinary burner, and two to three 
 times as efficient as the powerful regenerative burners. 
 This economy is not maintained, the efficiency of the 
 mantle falling off with use, rapidly at first, more slowly 
 afterwards. This is due in part to actual diminution in 
 the radiating surface of the mantle from surface disin- 
 tegration and in part to real decrease in the radiant ef- 
 ficiency. Fig. 27 shows a set of life curves from Wels- 
 bach burners, which are self-explanatory. In about 300 
 hours the efficiency has fallen off nearly one-third, after 
 
THE MATERIALS OF ILLUMINATION. 89 
 
 which it decreases much less rapidly during the re- 
 mainder of the life of the mantle. 
 
 This decrease in efficiency with age is similar to that 
 found in incandescent electric lamps, but is initially more 
 rapid. Nevertheless even after 300 hours the mantle is 
 still good for 8 or 10 candle-power per cubic foot of 
 gas, and remains far more efficient than any other class 
 of gas burner. Some recent mantles are even more ef- 
 ficient than these figures would indicate. 
 
 The working life of the mantle is stated by Dr. Fahn- 
 drich, director of gas at Vienna, to be about 350 hours, 
 taking due account of the decrease in efficiency. It is 
 safe to say that averaging the working efficiency over 
 this term of life the mantle burner with gas at $i per 
 thousand cubic feet can be operated at a cost not ex- 
 ceeding o.oi cent per candle-hour for gas. This should 
 not be raised by more than 0.0025 cent for mantle re- 
 newals per candle-hour. The upshot of the matter is 
 that the mantle burner is by far the cheapest known il- 
 luminant except the electric arc at a rather low rate for 
 electrical energy. Obviously it uses up the oxygen and 
 contaminates the air only in proportion to the gas used, 
 and hence far less than other burners. 
 
 The chief objection to the mantle burner is the un- 
 pleasant greenish tinge of its light. With the early 
 burners this was very offensive, and even with the latest 
 forms it is so noticeable that one can walk along the 
 street and pick out the mantle burners by the greenish 
 cast of the illumination long before reaching the window 
 from which they are shining. 
 
 The exact tinge of the light varies a little with the 
 kind of mantle and the particular period of its life, but 
 it is always distinctly greenish, sometimes bluish green, 
 
9 o THE ART OF ILLUMINATION. 
 
 and in recent mantles sometimes a very curious shade of 
 yellowish green, but never yellowish like a gas flame or 
 an incandescent lamp, or white or bluish white like an 
 electric arc. 
 
 This color seems thus far to be inseparable from the 
 radiation derived from any feasible combination of the 
 rare earths used to form the mantle. Sometimes in the 
 youth of the mantle the light seems to be nearly free 
 from this tinge, but through change in the specific nature 
 of the radiation or dissipation of some of the components 
 the greenish light soon gains prominence. Whether this 
 difficulty can be overcome in the manufacture of the man- 
 tles it is impossible to predict, but it can to a certain ex- 
 tent be avoided by proper shading, and shading is nearly 
 always necessary in using mantle burners on account of 
 their great intrinsic brilliancy. 
 
 If the exploiters of these mantle burners had spent half 
 the time in devising remedial measures that they have 
 wasted in denying the greenish hue of the light or in ex- 
 plaining that it is quite artistic and really good for the 
 eyes, the ordinary gas burner would now be practically 
 driven out of use. 
 
 As regards the actual color of the light from mantle 
 burners, it varies somewhat, as already explained, but the 
 following table is typical of the peculiarities of the light 
 as compared with that from an ordinary gas flame. In 
 the table the light of the gas flame is supposed to be 
 unity for each of the colors concerned, when the light 
 from the mantle has the given relative values. 
 
 As the actual luminosity of the deep reel, blue, and 
 violet is comparatively small in either burner, the pre- 
 ponderance of green in the light from the mantle is very 
 marked. 
 
THE MATERIALS OF ILLUMINATION. 91 
 
 Color 
 
 FULL 
 
 YELLOW 
 
 YELLOWISH 
 
 BLUISH 
 
 BLUF 
 
 VIOLET 
 
 
 RED 
 
 
 GREEN 
 
 GREEN 
 
 
 
 Light from Mantle 
 
 .71 
 
 I 47 
 
 I 76 
 
 2 TQ 
 
 2 74 
 
 o no 
 
 Ar^and taken as 
 
 I OO 
 
 I OO 
 
 I OO 
 
 I OO 
 
 
 ><->y 
 
 
 
 
 
 
 
 
 To correct this it is necessary to use a shade of 
 such color as to absorb some of the green rays. The 
 actual percentage of light absorbed need not be at all 
 large, provided the absorption is properly selective. 
 The general color of the shade to effect this absorption 
 will generally be a light rose pink, and the result is a 
 fairly white light, better in color than an ordinary gas 
 flame. 
 
 The advantage of the mantle burner in steadiness and 
 economy is so great that there would be little reason for 
 using the more common forms of gas burner indoors, ex- 
 cept for their better artistic effects and for their con- 
 venience for very small lights. The color question and 
 the fragility of the mantle have been the chief hindrances 
 to the general introduction of the Welsbach type, and 
 these are certainly in large measure avertable. 
 
 Recently there have been introduced several forms of 
 mantle burner worked with gas generated on the spot 
 from gasoline or similar petroleum products. Some- 
 times these are operated as individual lamps and some- 
 times as small systems to which the gas-forming fluid 
 is piped. They give, of course, a fine, brilliant light, and 
 at a low cost cheaper than ordinary mantle burners 
 worked with any except rather cheap gas. Where gaso- 
 line gas would be cheaper than gas taken from the near- 
 est available main, such gasoline mantle burners will 
 prove economical. 
 
 But, as a matter of fact, lamps locally generating and 
 
92 THE ART OF ILLUMINATION. 
 
 burning their own petroleum gas have been pretty thor- 
 oughly tried from time to time during the past twenty- 
 five years, and have never taken a strong or permanent 
 hold on the public. It is therefore difficult to see how 
 mantle burners worked in similar fashion are likely to take 
 a material hold upon the art, although in special cases they 
 may prove very useful, when illuminating gas is not 
 available at a reasonable price. 
 
 It must be constantly borne in mind that the lighter 
 petroleum oils are dangerous and must be used with 
 extreme care, and also that they are just now rapidly 
 rising in price, owing to the increasing use of explosion 
 engines and gas machines. 
 
 In using any mantle burner it is good economy to 
 replace the mantle after three or four hundred hours of 
 burning, if it is in regular use to any considerable extent. 
 Of course, in cases when a burner is not regularly used 
 and its maximum brilliancy is not at all needed the man- 
 tle may properly be used until it shows signs of break- 
 ing. In other words, as soon as a mantle which is needed 
 at its full efficiency gets dim, throw it promptly away; 
 but so long as it gives plenty of light for its situation, 
 your consumption of gas will not be diminished by a 
 change. 
 
 The commonest trouble with mantles is blackening 
 from a deposit of soot owing to temporary derangement 
 of the burner. This deposit can generally be burned off 
 by slightly, not considerably, checking the air supply 
 so as to send up a long, colorless flame which will soon 
 get rid of the carbon, after which the full air supply 
 should be restored. Too great checking of the air sup- 
 ply produces a smoky flame. 
 
 It should finally be noted that the mantle burners are 
 
THE MATERIALS OF ILLUMINATION. 
 
 93 
 
 particularly useful in cases of troublesome fluctuations 
 in the gas supply, since while they may burn more or 
 less brightly according to circumstances, they are en- 
 tirely free from flickering when properly adjusted. 
 
 In leaving now the illuminants which depend upon 
 the combustion of a gas of liquid, a brief summation of 
 some of their properties may not come amiss. 
 
 The replacement of candles and lamps by gas worked 
 a revolution, not only in the convenience of artificial 
 lighting, but in its hygienic relations. The older illumi- 
 nants in proportion to their luminous effect removed pro- 
 digious amounts of oxygen from the air and gave off 
 large quantities of carbonic acid. In the days of candles 
 a brilliantly lighted room was almost of necessity one 
 in which the air was bad. The following table, due to 
 a well-known authority on hygiene, gives the approxi- 
 mate properties of the common illuminants of com- 
 bustion as regards their effects on the air of the space 
 in which they are burned. 
 
 
 Q 
 
 
 
 
 Q 
 
 
 O 
 
 
 3 
 
 
 
 
 U 
 
 rt 
 
 H 
 
 
 s 
 
 Ctf 
 
 
 Q 
 
 u 
 
 a 
 
 
 
 E> 
 
 5 
 
 
 U 
 
 D 
 
 y 
 
 ^, 5^ 
 
 
 en & 
 
 1 
 
 Q 
 
 U 
 
 Q 
 O . 
 
 D C/3 
 
 D W 
 
 0- 
 
 
 OQ 
 
 p 
 
 ^. H 
 
 Q H 
 
 3i H 
 
 
 Ld T 
 
 
 u a 
 
 ft< 
 
 ^ 
 
 O ^ 
 
 ft. fc. 
 
 05 O 
 
 ss' 1 ' 
 
 
 ^ 0$ 
 
 jj 
 
 sg 
 
 OH S 
 
 S "^ 
 
 ^ J 
 
 OH 
 
 
 Ha! 
 
 Q 
 
 go 
 
 O 
 
 
 
 g U 
 
 U 
 
 ig 
 
 
 g 
 
 u 
 
 
 O 
 
 O 
 
 X 
 
 ** 
 
 
 & 
 
 
 
 
 - 
 
 
 > 
 
 Tallow candles 
 
 2200 grains 
 
 16 
 
 10.7 
 
 7-3 
 
 8.2 
 
 1400 
 
 12. 
 
 Sperm candles 
 
 1740 " 
 
 16 
 
 9.6 
 
 6-5 
 
 
 1137 
 
 II. 
 
 Paraffin oil . .... 
 
 QQ2 " 
 
 16 
 
 6.2 
 
 4.5 
 
 3-5 
 
 1030 
 
 7 5 
 
 Kerosene oil 
 
 w 
 
 QOO " 
 
 16 
 
 5.9 
 
 4.1 
 
 3-3 
 
 1030 
 
 7.0 
 
 Coal gas, batwing 
 
 V W 7 
 
 5.5 cu.ft. 
 
 16 
 
 6.5 
 
 2.8 
 
 7-3 
 
 1194 
 
 5.0 
 
 Coal gas Argand . 
 
 48 " 
 
 16 
 
 5.8 
 
 2.6 
 
 6.4 
 
 1240 
 
 4-3 
 
 Coal gas Regenerative 
 
 *r* w 
 
 3.2 " 
 
 32 
 
 3.6 
 
 1.7 
 
 
 760 
 
 2.8 
 
 Coal gas, Welsbach 
 
 & m . 
 
 3.5 
 
 J 
 
 50 
 
 4.1 
 
 1.8 
 
 4.7 
 
 763 
 
 3.0 
 
 
 
 3 
 
 
 
 
 
 
94 THE ART OF ILLUMINATION. 
 
 To this it may be added that acetylene in these rela- 
 tions is- about on a parity with the Welsbach burner, and 
 that oil lamps other than kerosene, burning whale oil, 
 colza oil, etc., would fall in just after candles. It is 
 somewhat startling to realize, but very desirable to re- 
 member, that a common gas burner will vitiate the air 
 of a room as much as four or five persons, in so far, at 
 least, as vitiation can be defined by change in the chemical 
 composition of the air. 
 
 In cost also the modern illuminants have a material 
 advantage. In order of cost the list would run at cur- 
 rent American prices of materials about as follows : Can- 
 dles, animal and vegetable oils, gas in ordinary burners, 
 kerosene, acetylene, Welsbachs. Incandescent electric 
 lamps, it may be added, are about equivalent in cost to 
 ordinary gas, with a tremendous hygienic advantage in 
 their favor, while arc lamps would be the lowest on the 
 list, assuming electrical energy relatively as cheap as dol- 
 lar gas would be. As to the quality of the illumination, 
 incandescent lamps, regenerative gas burners, and acety- 
 lene lead the list, while Welsbachs, by reason of their 
 color, and arc lamps, from their lack of steadiness, would 
 take a low rank. 
 
CHAPTER VI. 
 
 THE ELECTRIC INCANDESCENT LAMP. 
 
 AT the present time the mainstay of electric illumina- 
 tion is the incandescent lamp, in which a filament of 
 high electrical resistance is brought to vivid incandes- 
 cence by the passage of the electric current. To prevent 
 the rapid oxidation of the filament at the high tem- 
 perature employed, the filament is mounted in an ex- 
 hausted glass globe, forming the familiar incandescent 
 lamp of commerce. 
 
 The first attempts at incandescent lamps were made 
 with loops or spirals of platinum wire heated by the 
 electric current, either in the air or in vacuo, but the 
 results were highly unsatisfactory, since in the open air 
 the wire soon began to disintegrate, and even in the 
 absence of air its life was short. Moreover, the metal 
 itself, being produced in very limited quantities, was 
 expensive at best, and rose very rapidly in price under 
 a small increase of demand. Having a fairly low specific 
 electrical resistance, the wire used had either to be very 
 thin, which made it extremely fragile, or long, which 
 greatly increased its cost. 
 
 Following platinum came carbon in the form of slen- 
 der pencils mounted in vacuo. These, however, were of 
 so low resistance that the current required to heat them 
 was too great to allow of convenient distribution. 
 
 To get a practical lamp it was necessary to use a fila- 
 
 95 
 
96 THE ART OF ILLUMINATION. 
 
 ment of really high resistance, and which was yet strong 
 enough to keep down the cost of replacements. 
 
 Without going into the details of the many experi- 
 ments on incandescent lamps, it is sufficient to say that 
 after much labor the problem of getting a fairly worka- 
 ble filament was solved through the persistent efforts of 
 Edison, Swan, Maxim, Weston, and others, about twenty 
 years ago, the modern art dating from about 1880. 
 
 All the recent filaments are based on the carbonization, 
 out of contact with air, of thin threads of cellulose 
 the essential constituent of woody fiber. The early work 
 was in the direction of carbonizing thread in some form, 
 or even paper, but Edison, after an enormous amount of 
 experimenting, settled upon bamboo fiber as the most 
 uniform and enduring material, and the Edison lamp 
 came to the front commercially. 
 
 In point of fact, it soon became evident that art could 
 produce a far more uniform carbon filament than nature 
 has provided, so that of late years bamboo, thread, 
 paper, and the rest have been abandoned, and all fila- 
 ments, save those for some special lamps of large candle- 
 power, are made from soluble cellulose squirted into 
 threads, hardened, carbonized, and " treated." 
 
 Fig. 28 shows a typical modern incandescent lamp. 
 It consists essentially of four parts; the base adapted 
 to carry the lamp in its socket, the bulb, the filament, 
 and the filament mounting, which includes the lead- 
 ing-in wires. In its original form the bulb has an open- 
 ing at each end, one at the base end through which the 
 filament and its mounting are put in place, and another 
 in the form of a narrow tube a few inches long, which 
 when sealed off produces the tip at the end of the bulb. 
 
 The filament is made in slightly different ways in dif- 
 
THE ELECTRIC INCANDESCENT LAMP. 97 
 
 ferent factories, and the exact details of the process, 
 constantly subject to slight improvements, are unneces- 
 
 Fig. 28. Typical Incandescent Lamp. 
 
 sary here to be described. Substantially it is as follows: 
 The basis of operations is the purest cellulose con- 
 
98 THE ART OF ILLUMINATION. 
 
 venient to obtain, filter paper and the finest absorbent 
 cotton being common starting points. The material is 
 pulped, as in paper making, dissolved in some suitable 
 substance, zinc chloride solution being one of those used, 
 evaporated to about the consistency of thick molasses, 
 and then squirted under air pressure into a fine thread, 
 which is received in an alcohol bath to harden it. 
 
 Thus squirted through a die the filament is of very 
 uniform constitution and size, and after carbonization 
 out of contact with air it forms a carbon thread that is 
 wonderfully flexible and strong. But even so, there is 
 not yet a perfectly uniform filament, and the carbon is 
 not dense and homogeneous enough to stand protracted 
 incandescence. 
 
 On passage of current portions of the filament may 
 show too low resistance, so as to be dull, or too high 
 resistance, so as to get too hot and burn off. It is hard, 
 too, to produce a durable filament of the somewhat 
 porous carbon obtained in the way described. 
 
 In making up the filaments they are therefore sub- 
 jected prior to being sealed into the lamp to what is 
 known as the flashing process. This has a twofold ob- 
 ject, to build up the filament with dense carbon, and to 
 correct any lack of uniformity which may exist. The 
 latter purpose is far less important to the squirted fila- 
 ments than to the old filaments of bamboo fiber or 
 thread, but the former is important in securing a uni- 
 form product. The filaments are mounted and then are 
 gradually brought to vivid incandescence in an atmos- 
 phere of hydrocarbon vapor, produced from gasoline or 
 the like. 
 
 The heated surface decomposes the vapor, and the 
 carbon is deposited upon the filament in the form of a 
 
THE ELECTRIC INCANDESCENT LAMP. 99 
 
 smooth uniform coating almost as dense as graphite, 
 and a considerably better conductor than the original 
 filament. If, as in the early bamboo filaments, there are 
 any spots of poorer conductivity or smaller cross section 
 than is proper, these become hot first and are built up 
 toward uniformity as the current is gradually raised, 
 so that the filament is automatically made uniform. 
 
 The flashing process is actually quick, the gradual rise 
 of current being really measured by seconds. With the 
 squirted filaments now used the main value of the flash- 
 ing process is to enable the conductivity of the filament 
 to be quite accurately regulated, at the same time giving 
 it a firm, hard coating of carbon that greatly increases 
 its durability. The finished filaments are strong and 
 elastic, generally a fine steely-gray in color, with a pol- 
 ished surface, and for lamps of ordinary candle-power 
 and voltage vary from 6 to 12 ins. in length, with a 
 diameter of 5 to 10 one-thousandths of an inch. 
 
 The filaments are joined near the base of the lamp to 
 two short bits of thin platinum wire which are sealed 
 through one end of a short piece of glass tube. Some- 
 times these platinum leading-in wires are fastened di- 
 rectly to the ends of the filament and sometimes to an 
 intermediary terminal of copper wire attached to the fila- 
 ment. Within the tube the platinum wires are welded 
 to the copper leads which pass down the mounting tube 
 and are attached to the base. The filament itself is 
 cemented to its copper or platinum wires by means of a 
 little drop of carbon paste. 
 
 No effective substitute for platinum in sealing through 
 the glass has yet been found, although many have been 
 tried. Platinum and glass have very nearly the same 
 coefficient of expansion with heat, so that the seal re- 
 
ioo THE ART OF ILLUMINATION. 
 
 mains tight at all temperatures without breaking away. 
 It is possible to find alloys with nearly the right coefficient 
 of expansion, but they have generally proved unsatis- 
 factory either mechanically or electrically, so that the 
 line of improvement has mainly been in the direction of 
 making a very short seal with platinum wires. 
 
 The filament thus mounted is secured in the bulb by 
 sealing the base of the mounting tube or lamp stem into 
 the base of the bulb. This leaves the bulb closed except 
 for the exhaustion tube at its tip. 
 
 The next step is the exhaustion of the bulb. This 
 used to be done almost entirely by mercury pumps, and 
 great pains was taken to secure a very high degree of 
 exhaustion. It was soon found that there was such a 
 thing as too high exhaustion, but the degree found to 
 be commercially desirable is still beyond the easy capa- 
 bilities of mechanical air pumps, at least for regular and 
 uniform commercial practice, although they have been 
 sometimes successfully used. 
 
 At the present time the slow though effective mer- 
 cury pump is being to a very large extent superseded 
 by the Malignani process, or modifications thereof. The 
 bulbs are rapidly exhausted by mechanical air pumps, 
 and when these have reached the convenient limit of 
 their action the residual oxygen is chemically absorbed 
 by the gas produced by the vaporization of a small quan- 
 tity of a solution previously placed in a tubulaire con- 
 nected with the exhaustion tube. The exact nature of 
 the solution used is at the present time a trade secret, 
 but phosphorus and iodine are said to form the basis 
 of its composition. The process is cheap, rapid, and 
 effective, and with a little practice the operator can 
 produce exhaustion that is almost absolutely uniform. 
 
THE ELECTRIC INCANDESCENT LAMP. 101 
 
 Whatever be the method of exhaustion, during its 
 later stages current is put on the filaments both to heat 
 them, and thus to drive out the occluded gases, and to 
 serve as an index of the exhaustion. When exhaustion 
 is complete the leading-in tube is quickly sealed off, and 
 the lamp is done, save for cementing on the base and 
 attaching it to the leads that come from the seal. After 
 this the lamps are sorted, tested, and made ready for 
 the market. 
 
 The shape of the filament in the lamp was originally 
 a simple U, later often modified to a U with a quarter- 
 twist so that the plane of the loop at the top was 90 de- 
 grees from its plane at the base. As the voltage of 
 distribution has steadily crept upwards from 100 to no, 
 1 20, 140, and even 250 volts, it has been necessary either 
 to increase the specific resistance of the filament, to de- 
 crease its diameter, or to increase its length, in order to 
 get the necessary resistance to keep the total energy, 
 and likewise the temperature of the filament, down to 
 the desired point. 
 
 But the modern flashed filament cannot be greatly in- 
 creased in specific resistance without impairing its sta- 
 bility, so the filaments have been growing steadily finer 
 and longer. At present their form is various, accord- 
 ing to the judgment of the maker in stowing away- the 
 necessary amount of filament within the bulb. 
 
 One very common form is that of Fig. 28, where the 
 filament has a single long convolution anchored to the 
 base at its middle point for mechanical steadiness. 
 Sometimes there are two convolutions, or even more, 
 and sometimes there is merely a reduplication of the old- 
 fashioned simple loop, as in Fig. 29. 
 
 The section of the filaments is now always circular, 
 
102 THE ART OF ILLUMINATION. 
 
 although in the early lamps they were sometimes rec- 
 tangular or square. 
 
 There has been a considerable fog of mystery about 
 incandescent lamp practice for commercial purposes, but 
 
 Fig. 29. Lamp with Double Filament. 
 
 the general facts are very firmly established and by no 
 means complicated, and a little consideration of them 
 will clear up much of the haze. 
 
 To begin with, it is not difficult to make a good fila- 
 
THE ELECTRIC INCANDESCENT LAMP. 103 
 
 ment, but it takes much skill and practice to produce, in 
 quantity, one that shall be uniformly good. The quality 
 of the lamps as to durability and other essentials de- 
 pends very largely on the care and conscientiousness of 
 the maker in sorting and rating his product. 
 
 It is practically impossible, for example, to make, say, 
 10,000 filaments, all of which shall give 15 to 17 hori- 
 zontal candle-power at a particular voltage, say, no. 
 With great skill in manufacture, half or rather more will 
 fall within these limits, the rest requiring anywhere be- 
 tween 100 and 120 volts to give that candle-power. 
 Only a few will reach these extremes, the rest being 
 clustered more or less closely around the central point. 
 
 The value of the lamps as sold depends largely on 
 what is done with the varying ones and how carefully 
 they are sorted and rated. If the lamps demanded on 
 the market were all of no volts, then there would be 
 a large by-product which would either have to be thrown 
 away, sold for odd lamps of uncertain properties, or 
 slipped surreptitiously into lots of standard lamps. 
 
 But some companies use lamps of 1 08 or 1 12, or some 
 neighboring voltage, and part of the product is exactly 
 fitted to their needs, and so forth, there being involved 
 only some slight difference in efficiency, not important if 
 similar lamps from other lots are conscientiously rated 
 along with them. 
 
 The basic facts in incandescent lamp practice are two: 
 First, the efficiency, i. c., the ratio of energy consumed 
 to light given per unit of surface, depends mainly on the 
 temperature to which the filament is carried; second, the 
 total light given is directly proportional to the filament 
 surface which radiates this light. The specific radiating 
 power of modern carbon filaments is substantially the 
 
io 4 THE ART OF ILLUMINATION. 
 
 same, so that if one has two filaments of the same sur- 
 face brought to the same temperature of incandescence 
 they will work at substantially the same efficiency and 
 give substantially the same amount of light. 
 
 And if a filament of a certain surface be brought to a 
 certain temperature it will give a definite total amount of 
 light, utterly irrespective of the form in which the fila- 
 ment is disposed. Changes in the form of the filament 
 will produce changes in the distribution of the light in 
 different directions around the lamp, but will not in the 
 least change the total luminous radiation. Much of the 
 current misunderstanding is due to neglect of this sim- 
 ple fact. 
 
 The nominal candle-power of the lamp depends upon 
 a pure convention as to the direction and manner in 
 which the light shall be measured in rating the lamp, 
 and makers have often sought to beat the game by dis- 
 posing the filament so as to exaggerate the radiation in 
 the conventional direction of measurement. 
 
 For example: Many early incandescent lamps had 
 filaments of square cross section bent into a single sim- 
 ple U. These gave their rated candle-power in direc- 
 tions horizontally 45 degrees from the plane of the fila- 
 ments, and this was the maximum in any direction, so 
 that the lamp when thus measured was really credited 
 with its maximum candle-power, and fell below its rat- 
 ing in all directions save the four horizontal directions 
 just noted. 
 
 It is customary to delineate the light from an incan- 
 descent lamp in the form of closed curves, of which the 
 various radii represent in direction and length the rela- 
 tive candle-power in those various directions. Such 
 curves may be made to show accurately the distribution 
 
THE ELECTRIC INCANDESCENT LAMP. 105 
 
 of light in a horizontal plane about the lamp, or the 
 distribution in any vertical plane, and from the average 
 radii in any plane may be deduced the mean candle- 
 power in that plane, while from a combination of the 
 
 Fig. 30. Distribution of Light from Flat Filament. 
 
 radii in the various planes may be obtained the mean 
 spherical candle-power which measures the total lumi- 
 nous radiation in all directions. 
 
 This last is the true measure of the total light-giving 
 power of a lamp. Fig. 30 illustrates the curve of hori- 
 zontal distribution for one of the early lamps, having a 
 flat U-shaped filament. The circle is drawn to show a 
 uniform 16 candle-power, while the irregular curve 
 shows the actual horizontal distribution of light. This 
 particular lamp overran its rating, but its main char- 
 acteristic is that it gave a strong light in one horizontal 
 diameter and a weak one in the diameter at right angles 
 to this. 
 
io6 
 
 THE ART OF ILLUMINATION. 
 
 Such a distribution as this is generally objectionable, 
 and most modern filaments are twisted or looped, so 
 that the horizontal distribution is nearly circular. 
 Fig. 31 shows a similar curve for a recent i6-cp lamp 
 of the type shown in Fig. 28. In the small inner circle 
 is shown the projection of the looped filament as one 
 looks down upon the top of the lamp. Fig. 32 shows 
 
 Horizontal Distribution 
 
 Vertical on SD... Horizontal 
 
 Figs. 31 and 32. Distribution of Light from Looped Filament. 
 
 a similar delineation of the distribution of light in a 
 vertical plane taken in the azimuth shown in Fig. 31, 
 with the socket up. 
 
 The looping of the filament is such that the horizontal 
 distribution is very uniform, while in the vertical down- 
 wards there is a marked diminution of light, and of 
 course in the direction of the socket much of the light is 
 cut off. The total spherical distribution, if one can con- 
 ceive it laid out in space in three dimensions, resembles 
 a very flat apple with a marked depression at the 
 blossom end and a cusp clear in to the center at the 
 stem end. Fig. 33 is an attempt to display this spherical 
 distribution to the eye. 
 
 If the filament were a simple U or the double U of Fig. 
 
THE ELECTRIC INCANDESCENT LAMP. 107 
 
 29, assuming the same total length and temperature of 
 filament, the apple would have still greater diameter, but 
 the depression at the blossom end would be considerably 
 wider and deeper. 
 
 If the filament has several convolutions, as in Fig. 34, 
 this depression is considerably reduced, but there is a 
 
 Fig. 33. Distribution of Light from Incandescent Lamp. 
 
 marked flattening in one horizontal direction, so that 
 the horizontal distribution would somewhat resemble 
 Fig. 30. But the total luminous radiation would be 
 quite unchanged. 
 
 If the lamps were rated by their mean horizontal 
 candle-power the U filament would show abnormally 
 large horizontal illumination for the energy consumed, 
 and would apparently be very efficient, while if one were 
 foolish enough to rate lamps by the light given off the 
 
io8 
 
 THE ART OF 'ILLUMINATION. 
 
 tip alone, Fig. 34 would show great efficiency, the distri- 
 bution in one horizontal diameter having been reduced 
 to fatten the curve at the tip. In reality, however, each 
 one of the three forms of lamp would have exactly the 
 
 Fig. 34. Lamp with Multiple-Looped Filament. 
 
 same efficiency, and in practice there would be little 
 choice between them. 
 
 In the every-day work of illumination incandescent 
 lamps arc installed with their axes in every possible di- 
 rection, the vertical being the rarest, and angles between 
 30 degrees and 60 degrees downwards from the hori- 
 zontal the commonest. 
 
 Bearing in mind this general distribution of the axes 
 and the fact that diffusion goes very far toward oblit- 
 
THE ELECTRIC INCANDESCENT LAMP. 109 
 
 crating differences in the spherical distribution as re- 
 gards general illumination, it is easy to see that the shape 
 of the filament is, for practical purposes of illumination, 
 of little account. In the few cases where directed il- 
 lumination is needed it is best secured by a proper re- 
 flector, which gives far better results than can be ob- 
 tained by juggling with the shape of the filament. 
 
 The thing of importance is to get uniform filaments 
 of first-class durability* and of as good efficiency as pos- 
 sible. The only proper test for efficiency, however, is 
 that based on mean spherical candle-power, since a 
 lamp will give a different apparent efficiency for each 
 direction of measurement, varying from zero in the di- 
 rection of the socket to a maximum in some direction 
 unknown until found. 
 
 Efficiency has most often been taken with respect to 
 the mean horizontal candle-power. But this leads to 
 correct relative results only when comparing lamps hav- 
 ing filaments similarly curved. The mean spherical 
 candle-power is usually from 80 to 85 per cent, of the 
 mean horizontal candle-power, a ratio larger than is 
 found in the case of any other artificial illuminant. 
 
 As regards efficiency, most commercial incandescent 
 lamps require between 3 and 4 watts per mean horizontal 
 candle-power. Now and then lamps are worked at 2.5 
 watts per candle when used with storage batteries, and 
 some special lamps, especially some of those made for 
 voltages above 200, range over 4 watts per candle. As 
 has already been remarked, the efficiency depends upon 
 the temperature at which the carbon filament is worked. 
 And it is in the ability to stand protracted high tem- 
 perature that filaments vary most. 
 
 It is comparatively easy to make a filament which will 
 
no THE ART OF ILLUMINATION. 
 
 stand up well when worked at 4 watts per candle, but 
 to make a good 3-watt per candle filament is a very 
 different proposition. Also, at low voltage, 50 volts 
 for instance, the filament is more substantial than the 
 far slenderer one necessary to give the requisite resist- 
 ance for use at the same candle-power at 100 or 125 
 volts. 
 
 Under protracted use the filament loses substance by 
 slow disintegration and by a process akin to evapora- 
 tion, so that the surface changes its appearance, the re- 
 sistance increases so that less current flows, the efficiency 
 consequently falls off, and the globe shows more or less 
 blackening from an internal deposit of carbon. 
 
 The thinner and hotter the filament the less its en- 
 durance and the sooner it deteriorates or actually breaks 
 down. Modern carbons have by improved methods of 
 manufacture been developed to a point that in the early 
 days of incandescent lighting would have seemed be- 
 yond hope of reach. But the working voltage has 
 steadily risen and constantly increased the difficulties of 
 the manufacturer. 
 
 So-called high efficiency lamps worked at about 3 
 watts per candle power require the temperature of the 
 filament to be carried so high that its life is seriously 
 endangered unless it be of fair diameter; hence such 
 lamps are hard to make for low candle-power or for 
 high voltage, either of which conditions requires a 
 slender filament in the former case to limit the radiant 
 surface, in the latter to get in the needful resistance. 
 An 8-cp 125-volt lamp, or a i6-cp 250- volt lamp pre- 
 sents serious difficulties if the efficiency must be high, 
 while conversely lamps of 24 or 32 candle-power are far 
 more easily made for high voltage. 
 
THE ELECTRIC INCANDESCENT LAMP, in 
 
 The annexed table gives a clear idea of the perform- 
 ance of a modern lamp under various conditions of work- 
 ing. It is from tests made on a i6-cp loo-volt lamp (so- 
 called) by Professor H. J. Weber. The effective radiat- 
 ing surface of the filament in this lamp was o. 1 178 square 
 inch, so that the intrinsic brilliancy was over 250 candle- 
 power per square inch. 
 
 AMPERES 
 
 VOLTS 
 
 WATTS 
 
 CP. 
 
 WATTS PER CP. TEMPERATURE 
 
 0.421 
 
 77-IQ 
 
 32.51 
 
 2.99 
 
 10.87 
 
 I464C. 
 
 0-443 
 
 80.89 
 
 35.85 
 
 4 13 
 
 8.6 7 
 
 1483 
 
 0.467 
 
 84.80 
 
 39-58 
 
 5.6o 
 
 7.07 
 
 1503 
 
 0.490 
 
 88.83 
 
 43-55 
 
 7-41 
 
 5-88 
 
 1522 
 
 0.513 
 
 92.87 
 
 47-70 
 
 9.71 
 
 4.91 
 
 1541 
 
 0.536 
 
 96.71 
 
 51-84 
 
 12.42 
 
 4.18 
 
 1557 
 
 0-559 
 
 100 60 
 
 56.21 
 
 15.76 
 
 3-57 
 
 1574 
 
 0.582 
 
 104.58 
 
 60.90 
 
 19.70 
 
 3-09 
 
 1591 
 
 o 605 
 
 108.60 
 
 65.78 
 
 24.25 
 
 2.71 
 
 1607 
 
 0.629 
 
 112.57 
 
 70.85 
 
 29.41 
 
 2.41 
 
 1621 
 
 The absolute values of the temperatures here given 
 are the least exact part of the table, but the relative 
 values may be trusted to a close approximation. Fig. 35 
 shows in graphical form the relation between the last 
 two columns, showing clearly how conspicuously the 
 efficiency rises with the temperature. At the upper limit 
 given the carbon is too hot to give a long life, although 
 the writer has seen modern lamps worked 12 volts above 
 their rating for several hundred hours before rapid 
 breakage began. Of course the brilliancy had fallen off 
 greatly, however, by that time. 
 
 It is worth noting from the table that for a i6-cp lamp 
 of ordinary voltage the candle-power varies to the ex- 
 tent of quite nearly one candle-power per volt, for 
 moderate changes of voltage from the normal. Weber 
 calls attention to the fact that between 1400 degrees and 
 
112 
 
 THE ART OF ILLUMINATION. 
 
 1650 degrees an increase in temperature of n degrees 
 corresponds very closely to a saving in energy of n per 
 cent, in the production of light. 
 
 If it were possible to carry the temperature still higher 
 without seriously imparing the stability of the filament, 
 lamps of a very high economy could be produced. It is 
 possible to force lamps up to an economy of even 1.5 
 watts per candle temporarily, but they often break al- 
 
 5 6 7 8 9 10 II 
 
 Watts per mean horizontal candle power 
 
 Fig. 35' Variation of Efficiency with Temperature. 
 
 most at once, and even if they hold together they rise to 
 2 or 2.5 watts per candle within a few hours. 
 
 To tell the truth, the temperature corresponding to 
 1.5 watts per candle is dangerously near the boiling 
 point of the material, so near that it is practically hope- 
 less to expect any approximation to such efficiency from 
 carbon filaments, and even at 2.5 watts per candle the 
 life of the lamps is so short that at present prices they 
 cannot be used commercially. 
 
 From such experiments as those tabulated it has been 
 shown that the relation between the luminous intensity 
 and the energy expended in an incandescent lamp may 
 be expressed quite nearly by the following formula: 
 
THE ELECTRIC INCANDESCENT LAMP. 113 
 
 wherein / is the candle-power, W the watts used, and a 
 is a quantity approximately constant for a given type 
 of lamp, but varying slightly from type to type. 
 
 Following the universal rule of incandescent bodies, 
 the radiation from an incandescent lamp varies in color 
 with the temperature, and thus as the voltage changes, 
 or what is about the same thing, as lamps of different 
 efficiencies are used, the color of the light varies very 
 conspicuously. Low efficiency lamps, or lamps in a low 
 stage of incandescence, such as is indicated in the first 
 four lines of the table, burn distinctly red or reddish 
 orange. Then the incandescence passes through the 
 various stages of orange yellow and yellow white until 
 a 3- watt lamp is nearly and a 2.5 watt lamp purely and 
 dazzlingly, white. The color is a good index of the 
 efficiency. 
 
 The sizes of incandescent lamps in fairly common use 
 are 8, 10, 16, 20, 24, and 32 candle-power. The 
 standard in this country is the i6-cp size, a figure bor- 
 rowed from the legal requirements for gas. Some 10 
 candle-power are used here, very few 8 candle-power, 
 and still fewer of candle-powers above 16. Abroad 8-cp 
 lamps are used in great numbers and with excellent re- 
 results. The 2O-cp and 24-cp lamps are found mostly in 
 high voltages, for reasons that will appear shortly. Four 
 and 6-cp lamps are now and then used for decorative 
 purposes or for night-lights, and excellent 5O-cp lamps 
 are available for cases requiring radiants of unusual 
 power. 
 
 Lamps of these various sizes are made usually for volt- 
 ages between 100 and 120 volts, and more rarely for 
 220 to 250 volts, but in the latter case lamps below 16 
 candle-power are almost unknown in America. 
 
ii 4 THE ART OF ILLUMINATION. 
 
 One hundred and ten volts was for some years the 
 standard pressure here, and with this as a basis one may 
 profitably see what are the problems to be met in lamp 
 construction. At this voltage the filament of a i6-cp 
 lamp is 6 or 8 inches long and .008 to .01 inch in 
 diameter, and ordinarily has a resistance when hot of 
 nearly 200 ohms. Now to produce an 8-cp lamp of the 
 same voltage and efficiency the energy consumed must 
 be reduced by one-half, and so also must be the radiating 
 surface. This means that the filament resistance must 
 be doubled, and the radiating surface so adjusted by 
 varying the length, diameter, and specific resistance as 
 to give the required candle-power. 
 
 The latter two factors can be varied during the process 
 of flashing, since the carbon deposited thus is denser 
 and of lower specific resistance than the original 
 squirted core. The net result is a filament consider- 
 ably slenderer than the i6-cp filament and usually 
 of less stability. On the other hand, in making a 
 32-cp lamp the filament may conveniently be made 
 longer, thicker, and more durable. In lamps of higher 
 voltage the filaments must be of much higher resistance, 
 and hence longer and thinner, until at 220 volts the 
 i6-cp lamp must have four times the resistance of its 
 1 10- volt progenitor, and commonly has a total length of 
 filament of 12 to 15 inches. 
 
 In lamps of small candle-power or of high voltage 
 there is some temptation to get resistance by flashing 
 the filaments less thoroughly, to the detriment of dura- 
 bility, since the soft core disintegrates more readily than 
 the hard deposited carbon, which may explain the fre- 
 quent inferiority of such lamps. The greater the candle- 
 power, and the less efficiency required, i. e., the greater 
 
THE ELECTRIC INCANDESCENT LAMP. 115 
 
 the permissible radiating surface, the easier it is to get 
 a strong and durable filament for high voltages. Hence, 
 lamps for 220 to 250 volts are generally of at least 16 
 candle-power, very often of 20 or 24 candle-power, and 
 seldom show an efficiency better than 4 watts per candle- 
 power. 
 
 This forms a serious practical objection to the use of 
 such lamps for general distribution, unless with cheap 
 water-power as the source of energy, ajnd while im- 
 proved methods of manufacture are likely somewhat to 
 better these conditions, yet there are inherent reasons 
 why it should be materially easier to produce durable 
 and efficient incandescent lamps of moderate candle- 
 power and voltage than lamps of extreme properties in 
 either of these directions. 
 
 The life of incandescent lamps practically depends on 
 the temperature at which they are worked, other things 
 being equal. There is a steady vaporization and disin- 
 tegration of the carbon from the moment the lamp is 
 put into service, which ends in a material increase in the 
 resistance of the filament with accompanying decrease 
 of the current, energy, temperature, efficiency, and 
 light. 
 
 If the lamp is started at a low efficiency the tem- 
 perature is relatively low and the decadence of the fila- 
 ment is retarded, while if the lamp is initially of high 
 efficiency the filament under the higher temperature de- 
 teriorates more rapidly and the useful life of the lamp 
 is shortened. 
 
 Under this latter condition the cost of energy to run 
 the lamp is diminished, but at the price of increased ex- 
 pense in lamp renewals. Operating at low efficiency 
 means considerable cost for energy and low cost of the 
 
n6 THE ART OF ILLUMINATION. 
 
 lamp renewals. Between these divergent factors an eco- 
 nomic balance has to be struck. 
 
 It is neither desirable nor economical to operate an 
 incandescent lamp too long, since not only does it de- 
 crease greatly in efficiency, but the actual light is so 
 dimmed that the service becomes poor. If the lighting 
 of a room is planned for the use of i6-cp lamps, and they 
 are used until the candle-power falls to, say, 10, which 
 would be in about 600 hours in an ordinary 3-watt-per- 
 candle lamp, the resulting illumination would be alto- 
 gether unsatisfactory. Quite aside from any considera- 
 tion of efficiency, therefore, it becomes desirable to 
 throw away lamps of which the candle-power has fallen 
 below a certain point. 
 
 Much of the skill in modern lamp manufacture is di- 
 rected to securing the best possible balance between 
 efficiency and useful life, a thing requiring the best ef- 
 forts of the manufacturer. Fig. 36 shows graphically the 
 relation between life, candle-power, and watts per candle 
 derived from tests of high-grade foreign lamps. In com- 
 paring these, like the previous data, with American re- 
 sults, it should be borne in mind that these foreign tests 
 are made, not in terms of the English standard candle, 
 but generally in terms of the Hefner-Alteneck standard, 
 which is somewhat (approximately 10 per cent.) smaller. 
 
 These curves show the results from lamps having an 
 initial efficiency of 2.5, 3.0, and 3.5 watts per candle- 
 power and an initial candle-power of 16. They show 
 plainly the effect of increased temperature on the life 
 of the lamp, and it is unpleasantly evident that in the 
 neighborhood of 3 watts per candle a point is reached 
 at which a further increase of efficiency produces a dis- 
 astrous result upon the life. In other words, that ef- 
 
THE ELECTRIC INCANDESCENT LAMP. 117 
 
 ficiency requires a temperature at which the carbon 
 filament rapidly breaks down. 
 
 And so long as carbon is used as the radiant material 
 there is a strong probability that there can be no very 
 radical improvement in efficiency. Of course, if incan- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 800 
 
 10.00 
 
 200 400 600 
 
 Curves a=watts per c,p. Curves b=c.p, 
 
 Fig. 36. Curves Showing Life, Candle-power and Watts per 
 
 Candle. 
 
 descent lamps were greatly cheapened, it would pay to 
 burn them at a higher efficiency and to replace them 
 oftener. It is quite possible that increased experience 
 and persistent efforts at standardization might lead to 
 this result. 
 
 In production on a large scale the mere manufacture of 
 the lamps can be done very cheaply, probably at a cost 
 not exceeding 7 to 8 cents, but the cost of proper sorting 
 and testing to turn out a uniform high-grade lamp, and 
 the incidental losses from breakage and from lamps of 
 odd and unsalable voltages, raises the total cost of produc- 
 tion very materially. Much of the reduction in the price 
 of incandescent lamps in. the past few years has resulted 
 from better conditions in these latter respects, as well 
 as from the improved methods of manufacture. 
 
n8 THE ART OF ILLUMINATION. 
 
 And it should be pointed out that the difference be- 
 tween good and bad lamps, as practically found upon 
 the market, lies mostly in their different rates of decay 
 of light and efficiency. It is the practice of many of the 
 large lighting companies who renew the lamps for which 
 they furnish current to reject and replace lamps which 
 have fallen to about 80 per cent, of their initial 
 power. 
 
 First-class modern lamps worked in the vicinity of 3 
 watts per candle-power will hold up for 400 to 450 hours 
 before falling below this limit, and at 3.5 or 3.6 watts per 
 candle-power will endure nearly double that time. 
 They are often rated in candle-hours of effective life, and 
 on the showing just noted the recent high efficiency 
 lamp will give a useful life of 6500 to 7000 candle-hours, 
 with an average economy of perhaps 3.25 watts per 
 candle. A medium grade lamp of similar nominal ef- 
 ficiency may not show with a similar consumption of 
 energy more than 250 or 300 hours of effective life say 
 4000 to 4500 candle-hours. 
 
 The economics of the matter appear as follows: The 
 first lamp during its useful life of, say 6500 candle-hours, 
 will consume 21.125 kilowatt-hours, costing at, say, 15 
 cents per kilowatt-hour, $3.17, and adding the lamp at 18 
 cents, the total cost is $3.35, or 0.0515 cent per candle- 
 hour, while the poorer lamp at 4000 candle-hours will 
 use $1.95 worth of energy, and at 18 cents for the lamp, 
 would cost 0.0532 cent per candle-hour. To bring the 
 two lamps to equality of total cost, irrespective of the 
 labor of renewals, the poorer one would have to be pur- 
 chased at 1 1 cents. In other words, poor lamps, if dis- 
 carded when they should be, generally so increase the 
 cost of renewals that it does not pay to use them at any 
 
THE ELECTRIC INCANDESCENT LAMP. 119 
 
 price at which they can be purchased under ordinary 
 circumstances. 
 
 As has already been explained, lamps deteriorate very 
 rapidly if exposed to abnormal voltage, and the higher 
 the temperature at which the lamp is normally worked the 
 more deadly is the effect of increased voltage. It thus 
 comes about that if high efficiency lamps are to be used, 
 very good regulation is necessary. Occasional exposure 
 to a 5 per cent, increase of voltage may easily halve the 
 useful life of a lamp, while, of course, permanent work- 
 ing at such an increase would play havoc with the life, 
 cutting it down to 20 per cent, or less of the normal. 
 Good regulation is therefore of very great importance 
 in incandescent lighting, not only to save the lamps and 
 to improve the service, but to render feasible the use of 
 high efficiency lamps. On the whole, the best average 
 results seem to be obtained in working lamps at 3 to 3.5 
 watts per candle. Those of higher efficiency fail so 
 rapidly that it only pays to use them when energy is very 
 expensive and must be economized to the utmost. The 
 2.5-watt lamp of Fig. 36, for example, has an effective 
 life of not more than 1 50 hours, at an average efficiency 
 of about 2.75 watts per candle. A 2-watt lamp will fall 
 to 80 per cent, of its original candle-power in not far 
 from 30 hours, at an average efficiency of about 2.25 
 watts, while if started as a i.5-watt lamp, in a few hours 
 the filament is reduced to practical uselessness. 
 
 There is seldom any occasion to use lamps requiring 
 more than 3.5 watts per candle-power, save in case of 
 very high voltage installations, where the saving in cost 
 of distribution may offset the cost of the added energy. 
 The difficulty of making durable 25o-volt lamps on ac- 
 count of the extreme thinness of the filament has been 
 
120 THE ART OF ILLUMINATION. 
 
 already referred to, and it is certainly advisable to use 
 in such installations lamps of 20 candle-power or more 
 whenever possible, thus making it practicable to work at 
 better efficiency without increased risk of breakage. 
 Even when power is very cheap there is no object in 
 wasting it, and a little care will generally procure regu- 
 lation good enough to justify the employment of in- 
 candescent lamps of good efficiency. 
 
 Further, in the commercial use of lamps it is necessary 
 for economy that the product should be uniform. It 
 has already been shown that medium grade lamps are 
 characterized by a shorter useful life than first-class 
 lamps. Unfortunately, there are on the market much 
 worse lamps than those described. It is not difficult to 
 find lamps in quantity that are so poor as to fall to 80 per 
 cent, of their initial power in less than 100 hours. A 
 brief computation of the cost of replacement will show 
 that these are dear at any price. Now, if lamps are not 
 carefully sorted, a given lot will contain both good 
 lamps and poor lamps, and will not only show a de- 
 creased average value, but will contain many individual 
 lamps so bad as to give very poor and uneconomical 
 service. Fig. 37 shows what is sometimes known as a 
 " shotgun diagram," illustrating the variations found in 
 carelessly sorted commercial lamps. In this case the 
 specifications called for i6-cp, 3-5-watt per candle-power 
 lamps. The variation permitted was from 14.5 to 17.5 
 mean horizontal candle-power, and from 53 to 59 total 
 watts, which is a liberal allowance, some companies de- 
 manding a decidedly closer adherence to the specified 
 limits. 
 
 The area defined by these limits is marked off in the 
 cut, forming the central " target." The real measure- 
 
THE ELECTRIC INCANDESCENT LAMP. 121 
 
 ments of the lamps tested are then plotted on the dia- 
 gram and the briefest inspection shows the results. In 
 this case only 46 per cent, of the lamps hit the specifica- 
 
 52 53 54 
 
 17 
 
 SI.SW. 
 
 13 
 
 50.1 W. 
 
 Watts 
 55 56 
 
 57 58 59 
 
 
 sow. 
 
 60.7 W. 
 
 62.8 W. 
 
 Fig. 37. Shotgun Diagram. 
 
 tions. All lamps above the upper slanting line are be- 
 low 3.1 watts per candle-power, and hence are likely 
 to give trouble by falling rapidly in brilliancy and break- 
 ing early. Lamps below the lower slanting line are over 
 4 watts per candle-power, hence are undesirably inef- 
 ficient. Moreover, the initial candle-power of the lot 
 varies from 12.2 candle-power to 20.4 candle-power. 
 
122 THE ART OF ILLUMINATION. 
 
 Such a lot will necessarily give poorer service and less 
 satisfactory life, and is, as a matter of dollars and cents, 
 worth much less to the user than if the lamps had been 
 properly sorted at the factory. Filaments cannot be 
 made exactly alike, and the manufacturer has to rely 
 upon intelligent sorting to make use of the product. 
 For example, the topmost lamp of Fig. 37 should have 
 been marked for a lower voltage, at which it would have 
 done well. Nearly all the lot would have properly fallen 
 within commercial specifications for i6-cp lamps at 
 some practicable voltage and rating in watts per candle- 
 power. The imperfect sorting has misplaced many of 
 the lamps and depreciated the whole lot. 
 
 In commercial practice lamps should be carefully 
 sorted to meet the required specifications, and the per- 
 sons who buy lamps should insist upon rigid adherence 
 to the specifications and should, in buying large quan- 
 tities, test them to ensure their correctness. To sum 
 up, it pays to use good lamps of as high efficiency as is 
 compatible with proper life, and to see that one gets 
 them. 
 
 The real efficiency of an incandescent lamp, i. e., the 
 proportion of the total energy supplied which appears as 
 visible luminous energy, is very small, ordinarily from 
 4 to 6 per cent., not over 6.5 per cent, even in a 3-watt 
 per candle-power lamp. This means that in working 
 incandescent lamps from steam-driven plants not over 
 0.5 per cent, of the energy of the coal appears as useful 
 light. This is a sad showing, and one which should spur 
 invention. To get better results, it seems necessary to 
 abandon the carbon filament, at least in any form in 
 which we now know it, and either to turn to some other 
 material for the incandescent body, or to abandon the 
 
THE ELECTRIC INCANDESCENT LAMP. 123 
 
 principle of incandescence altogether and pass to some 
 form of lamp in which the luminosity is not due to the 
 high temperature of a solid radiant. The writer is 
 strongly disposed to think that the ultimate solution 
 lies in the latter alternative, although the former offers 
 hope of very considerable and perhaps revolutionary 
 improvements. 
 
 Within the past few years a large number of attempts 
 have been made at preparing a filament for incandescent 
 lamps of some material far more refractory than pure 
 carbon, and hence better able to endure the high tem- 
 perature necessary for securing high efficiency. A 
 glance at the temperature curve, Fig. 35, shows that a 
 rise of 200 degrees C. or so in the working temperature 
 would produce an efficiency of nearly or quite one candle- 
 power per watt. 
 
 These attempts have been of several kinds. One 
 method has been to incorporate refractory material with 
 the carbon in manufacturing the filaments, thus both in- 
 creasing the resistance of the filaments and giving them a 
 certain proportion of heat-resisting substances. Owing, 
 however, to the fact that such filaments still contain a 
 considerable proportion of carbon which is compara- 
 tively easily vaporized, there is good reason to doubt 
 the efficacy of the process. The carbon, which is the 
 cement, as it were, once disintegrated, the filament 
 would give way, and experience up to date has tended 
 to throw doubt on the success of any such scheme. 
 
 An interesting modification of this method is that pro- 
 posed by Langhans, who forms filaments of carbide of 
 silicon, i. e., employs carbon in chemical combination 
 instead of merely as a species of cement. This process 
 has not been carried to commercial success, but it cer- 
 
124 THE ART OF ILLUMINATION. 
 
 tainly looks more hopeful, on general principles, than the 
 process of incorporation. 
 
 Another line of attack on the problem is that of Auer 
 von Welsbach, who proposed a filament of platinum or 
 similar metal, coated with thoria, the rare earth which 
 is the chief constituent of the Welsbach mantle. This 
 looks mechanically dubious. Still another modification 
 of this idea is the use of a filament mainly of carbon, but 
 with a coherent coating of thoria or the like, a line of 
 investigation which appears worth pursuing. Akin to 
 this is the Nernst lamp, which is at present exciting great 
 interest, although it is barely yet in the commercial 
 stage. The basic fact on which Dr. Nernst's work is 
 founded is that many substances, non-conductors at 
 ordinary temperatures, become fairly good conductors 
 when heated. Thus a tiny pencil of lime, magnesia, or 
 the rare earths, when once heated, will allow a current to 
 pass at commercial voltages sufficient to maintain it at 
 vivid incandescence. From this fundamental fact 
 Nernst has developed a most interesting and promising 
 glow lamp. 
 
 The variation of resistance with temperature in such 
 substances as the rare earths as used by Nernst is truly 
 prodigious. They seem really to pass from insulators to 
 conductors. Even glass, in fact, conducts fairly well at 
 high temperatures, although in all such cases conduction 
 is probably, at least in part, electrolytic in its character, 
 a fact which is of considerable practical moment. As 
 developed by Nernst the filaments when cold have sev- 
 eral hundred times the resistance which they have when 
 hot. Fig. 38 shows graphically from Nernst's tests the 
 way in which the specific resistance falls off as the tem- 
 perature rises. From the somewhat meager data it is 
 
THE ELECTRIC INCANDESCENT LAMP. 125 
 
 of necessity only approximate, but it gives a vivid idea 
 of the extraordinary nature of the phenomenon. Of 
 course, carbon shows a great decrease of resistance when 
 hot, but it is a pretty fair conductor when cold, while 
 the Nernst filament is practically an insulator in that 
 
 1100 
 
 500 
 
 2000 
 Ohms per Cubic Centimeter 
 
 4000 
 
 Fig. 38. Curve of Resistance Variation. 
 
 condition. But the oxides of the Nernst filament are 
 enormously more refractory than carbon, and can not 
 only be carried to far higher incandescence without 
 breaking down, but probably have, at least in some of 
 the combinations used, a rather more efficient distribu- 
 tion of energy in the spectrum than is the case with 
 carbon. 
 
 But being an insulator at ordinary temperatures some 
 means has to be taken to get current through the fila- 
 ment. It has long been known in a general way that 
 magnesia and similar materials conduct at a high tern- 
 
126 
 
 THE ART OF ILLUMINATION. 
 
 perature, and both Le Roux and Jablochkoff had dab- 
 bled with the idea years ago. But Nernst took up the 
 matter anew and in earnest. The lamp which he has 
 produced consists essentially of a thin pencil of mixed 
 oxides, forming the incandescent body. This pencil is 
 much thicker and shorter than a carbon filament as used 
 
 Metdl 
 
 Fig. 39 and 40. Connections of Nernst Lamp. 
 
 in incandescent lamps, being, say, from 1-64 to 1-16 inch 
 in diameter, and 3-4 to i 1-2 inch long. If heated by 
 a match or spirit lamp the filament becomes a conductor, 
 and goes to vivid incandescence. Such artificial heating 
 being somewhat of a nuisance, much of the work spent 
 in developing the Nernst lamp has been in the direction 
 of providing means for artificial lighting. As developed 
 abroad the self-lighting Nernst lamp has taken the form 
 shown in Fig. 39. Rising from the base of the lamp G 
 
THE ELECTRIC INCANDESCENT LAMP. 127 
 
 are two stiff wires, DD, spaced near the ends by a porce- 
 lain disk C. Across the platinum tips of these rods is 
 fastened the glower A, secured at its terminals by con- 
 ducting cement. Coiled in loose turns about the glower 
 is a porcelain spiral B, into the surface of which has 
 been baked a fine platinum wire closely coiled around 
 it. The office of this resistance spiral is to bring the 
 glower to a temperature at which it begins to conduct. 
 At the start A and B are in shunt, but when current gets 
 fairly started through the former it energizes a tiny 
 electro-magnet, F, situated in the base of the lamp, its 
 armature L is attracted and the circuit through B broken, 
 turning the whole current through A. 
 
 At is a very interesting and important feature of the 
 lamp. It is a " ballast " resistance of fine iron wire 
 wound upon a porcelain rod and sealed into a little bulb 
 to prevent oxidation. It is connected in series with the 
 filament. Now iron has a resistance that increases rap- 
 idly with the temperature, and this increase is particu- 
 larly rapid at about 450 to 500 degrees C. This re- 
 sistance coil is designed so that with normal current in 
 the lamp the temperature will rise to the point noted, 
 and its office is to steady the lamp. Without it the 
 Nernst lamp would be terribly sensitive to variations in 
 voltage, but if the voltage rises with this resistance in 
 circuit, its increasing resistance chokes the current. 
 Even with this steadying element the Nernst lamp is still 
 somewhat sensitive to changes of voltage. The glower 
 does not function properly in an exhausted globe, and 
 must be worked in the free air, although a glass shade 
 is provided to protect it from draughts, dust, etc. In 
 point of fact, protection from draughts is at present 
 rather necessary, since the filament is so sensitive to 
 
128 
 
 THE ART OF ILLUMINATION. 
 
 changes' in temperature that it can readily be blown out 
 by the breath. Fig. 40 gives a clear idea of the con- 
 nections of the lamp of Fig. 39, while Fig. 41 shows 
 
 Fig. 41. Nernst Lamps. 
 
 complete at the left, one of the earlier Nernst lamps 
 without the automatic lighting device. 
 
 The foreign manufacturers of these lamps are pro- 
 ducing them of 25, 50, and 100 candle-power, for no- 
 and 22O-volt circuits. The Nernst principle lends itself 
 more readily to powerful high-voltage lamps than to 
 small low voltage ones, and the glower is found to work 
 better on alternating than on continuous-current circuits, 
 
THE ELECTRIC INCANDESCENT LAMP. 129 
 
 apparently for reasons depending on the electrolytic na- 
 ture of the conduction. Like other incandescent lamps, it 
 works the more efficiently as the temperature rises, but 
 owing to its refractory composition the Nernst filament 
 can be pushed to very high efficiency. As the lamps are 
 produced at the present time their initial efficiency is 
 about 1.50 to 1.75 watts per candle, including the energy 
 lost in the steadying resistance (about 10 per cent.), and 
 the useful life is said to be about 300 hours. The fila- 
 ment at the end of this time rises in resistance and falls 
 in efficiency, much like an ordinary incandescent fila- 
 ment, but rather more suddenly. 
 
 The real life of a Nernst lamp, when defined as it 
 should be in terms of, say, a 2O-per-cent. drop in candle- 
 power, is not at the present time known. It has only 
 been put upon the market abroad within the present 
 year, and the owners of the American rights have not 
 yet put lamps out in large commercial quantities, so that 
 really accurate data are entirely lacking as yet. 
 
 The American Nernst lamp, as developed in the 
 Westinghouse laboratory, retains all the general features 
 of the foreign lamps, but is modified in some important 
 details. It has been so designed as to give a considerably 
 longer life at a slightly lower efficiency. The heaters are 
 good-sized simple cylinders of porcelain, instead of 
 spirals, and are placed close to and above the glower. 
 The unit is a single 5O-cp glower, but it is found that 
 from the higher working temperature and better con- 
 servation of heat in a multiple glower lamp a better 
 efficiency is obtained, so that the ordinary sizes are those 
 with two, three, and six glowers, rated respectively at 
 100, 170, and 400 candle-power. This rating is in the 
 direction of maximum intensity. 
 
THE ART OF ILLUMINATION. 
 
 Fig. 42 shows the heaters and glowers of these lamps 
 assembled on a porcelain cap with connection wires 
 which automatically make all the necessary connections 
 when the holder is pushed into its base. Thus far only 
 the single glower lamp is made for no volts, the others 
 being for 220 volts. Fig. 43 shows the general appear- 
 ance of the lamps as fitted for indoor use. 
 
 
 
 MEAN INTENSITY IN H. U. 
 
 WATTS PER MEAN H. U. 
 
 
 
 Spherical 
 
 Lower 
 Hemisphere 
 
 Spherical 
 
 Lower 
 Hemisphere 
 
 
 R 
 
 
 
 I 
 
 
 
 0) 
 
 
 K 
 
 I* 
 
 W 
 
 
 1 
 
 
 
 
 B 
 
 o 
 
 
 
 
 
 tj 
 
 fa 
 
 O 
 
 O 
 
 'ea 
 
 O 
 
 O 
 
 1 
 
 i 
 
 O 
 
 o 
 
 
 
 O , " 
 
 
 o . 
 
 i 
 
 ^ 
 
 u 
 
 j 
 
 IH 
 O 
 
 w 
 
 "5 
 a. 
 
 8 
 
 
 1 
 
 
 
 M 
 
 1 
 
 rt 
 
 M 
 
 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 
 O 
 
 o 
 
 
 
 
 U 
 
 
 f6-Glower 
 
 220 
 
 2.35 
 
 517 
 
 I.O 
 
 149* 
 
 
 
 147 
 
 240* 
 
 
 
 279 
 
 347* 
 
 ! 3-5 
 
 2.15* 
 
 
 
 1.8 5 
 
 A. C. Arc. 
 
 no 
 
 5.29 
 
 417 
 
 .6 
 
 130 
 
 159 
 
 152 
 
 
 
 190 
 
 2543 21 
 
 2.62 2.49 
 
 
 
 2.23 11.48 
 
 D. C. Arc 
 
 no 
 
 4.9 
 
 539 
 
 I.O 
 
 177 
 
 207 
 
 
 
 
 
 272 
 
 
 
 3-03 
 
 2.60 
 
 
 
 
 
 1.98 
 
 
 
 An opal inner globe or heater-case was used in all cases except the four readings 
 marked.* 
 
 * A clear heater-case and sand-blasted spherical globe were used. 
 f Rated at 400 cp. 
 
 The foregoing table gives its performance as com- 
 pared with alternating current and direct current 
 enclosed arc lamps, the intensities being in Hef- 
 ner units. From this it appears that the Nernst lamp is 
 fairly comparable in efficiency with the enclosed arcs, 
 while giving a steadier light decidedly better in color. 
 
 The distribution of light from these lamps is obviously 
 somewhat peculiar. It is specially strong in the lower 
 hemisphere, being designed with downward illumina- 
 tion in mind. The horizontal distribution from a single 
 glower, as determined by M. Hospitalier, is shown in 
 
THE ELECTRIC INCANDESCENT LAMP. 131 
 
 Fig. 44. The glower was horizontal and the measure- 
 ments were taken in the horizontal plane passing 
 through it. The section of the glower appears in the 
 center of the diagram. Broadside on this glower gave 
 about 40 cande-power; when nearly end on, about 10 
 candle-power. 
 
 After about 100 hours' run the inner globe or " heater- 
 case " becomes darkened by a deposit from the glower 
 and its platinum contacts and from the heaters, and has 
 
 Fig. 42. Glowers and Heaters of American Nernst Lamp. 
 
 to be cleansed, so that with respect to care the new lamp 
 resembles arcs rather than incandescents. The effective 
 life of the glowers is said to be about 800 hours, the effi- 
 ciency holding up pretty well until they break. The in- 
 trinsic brilliancy of the glower is very great, 1000 to 1250 
 cp per square inch. Hence the shading of Nernst lamps 
 by diffusing globes or other screens must be very thor- 
 ough, so as to cut down the intolerable brightness of the 
 glower itself. 
 
 The automatic lighting device seems to work well, 
 bringing the lamp up to full brilliancy in not far from 
 
132 
 
 THE ART OF ILLUMINATION. 
 
 half a minute. On continuous current the life of the 
 glower is very greatly reduced, probably to one-third 
 its normal duration, so that at present the device be- 
 longs essentially to alternating current distributions, 
 and the life also tends to increase with the frequency, 
 
 Fig. 43. Types of ..ndoor Lamps. 
 
 so that the very low frequency circuits are somewhat at 
 a disadvantage in using Nernst lamps. 
 
 It seems, however, certain that the Nernst lamp is an 
 important addition to the art within at least a limited 
 sphere of usefulness. 
 
 The glower can be replaced at a moderate cost, ulti- 
 mately below the cost of replacement of incandescent 
 lamps of equivalent candle-power, so that even with 
 a rather short life of the glower the lamp would still 
 be economical in use. While less efficient than the 
 best arc lamps, it compares favorably with enclosed arcs 
 of moderate amperage, and it is just now to be regarded 
 
THE ELECTRIC INCANDESCENT LAMP. 133 
 
 rather as a competitor of the arc than of the glow lamp. 
 However, it would take no great advance to change this 
 condition, and the ease with which Nernst lamps may 
 be made for high voltage is a rather important matter. 
 If one institutes a comparison on the basis of 25o-volt 
 lamps the result is very greatly in favor of the Nernst 
 
 36 / 36 
 
 Fig. 44. Horizontal Distribution from Nernst Lamp. 
 
 filament at any reasonable estimate of its life. As in 
 ordinary lamps so with Nernst lamps, filaments for small 
 candle-power involve unusual difficulties, but at present 
 the i6-cp lamp should be taken as the normal glow 
 lamp, while with the Nernst lamp perhaps 5o-cp may be 
 regarded as the normal unit glower. 
 
 At present one must regard the Nernst lamp as in a 
 tentative condition, and various problems regarding it 
 must be threshed out in particular there should be 
 radical improvement in the automatic lighting device or 
 such evolution of a filament of higher initial conductivity 
 as will obviate further necessity for special lighting de- 
 vices. But a highly efficient and very easily replaced 
 incandescent body is in itself a material advantage over 
 the delicate filament and exhausted globe of the ordi- 
 nary incandescent lamp, and unless the Nernst lamp shall 
 
1 34 THE ART OF ILLUMINATION. 
 
 develop some unexpected limitation, it must be looked 
 upon as a competitor of the incandescent that, although 
 not now serious, may become so at any time, and per- 
 haps to a very material extent. 
 
 Following up the question of higher luminous ef- 
 ficiency than that given by the incandescent lamp, one nat- 
 urally turns to the vacuum tube, in which the illuminant 
 is not a heated solid, but an incandescent gas. It has 
 long been well known that an electric discharge passed 
 through a tube of highly rarefied gas causes the tube 
 to become the seat of very brilliant luminous phenom- 
 ena. The light produced, however, does not form a con- 
 tinuous spectrum, as does an incandescent solid, but 
 gives a spectrum of bright bands or lines. This fact of 
 itself gives some hope of efficiency, for it is the plentiful 
 production of useless wave lengths that renders an ordi- 
 nary incandescent body so inefficient a source of light. 
 If a gas could be found giving a brilliant spectrum of 
 bands, all of useful wave lengths, one might expect that 
 a considerable proportion of the energy applied to the 
 tube would turn up as useful illumination. Or if not 
 a single gas, then a combination of gases might be found 
 such as would answer the purpose. During the past ten 
 years much work has been put in along this line by Mr. 
 Tesla and others, but as yet without the production of a 
 commercial lamp, although very magnificent effects 
 have been produced experimentally. 
 
 The difficulties which have been met are, first, the need 
 under ordinary conditions of rather high voltage in the 
 discharge, the difficulty of obtaining a steady light of 
 good color, and, most of all, the production of anything 
 like a practicable intrinsic brilliancy. The brightness of 
 an ordinary vacuum tube is apt to be greatly overrated, 
 
THE ELECTRIC INCANDESCENT LAMP. 135 
 
 seen as it usually is with the room otherwise in darkness, 
 and a tallow candle will make a pretty bright vacuum 
 tube look pale as a wisp of fog. 
 
 Now, low intrinsic brilliancy is not in itself at all ob- 
 jectionable, but in the matter under discussion it con- 
 notes a radiant of large dimensions. This means that 
 either there must be an enormous multiplicity of small 
 tubes or else a few tubes so large as to involve rather 
 high electromotive force in driving the discharge 
 through them. In large tubes the light is generally very 
 unsteady, and the best effects seem to be gotten from 
 long coiled tubes of small diameter, which are not easy 
 to excite. As to color, there is a strong tendency toward 
 a bluish or greenish hue, which will have to be removed 
 before a practical lamp is produced. It is easy to find 
 gaseous mixtures free from this objection, but perhaps at 
 a considerable loss of efficiency or other disadvantage. 
 The otherwise promising mercury vapor lamp is ex- 
 tremely bad in color. 
 
 The efficiency of the light produced by vacuum tubes 
 has been several times measured. It appears that the 
 luminous efficiency, that is, the proportion of radiant 
 energy from the tube which is of luminous wave lengths, 
 is something like 25 or 30 per cent., five or six times 
 better than in case of the incandescent lamp. 
 
 If the tubes are forced to anything like the intrinsic 
 brilliancy of even the dullest flames, secondary phe- 
 nomena involving heating seem to arise, considerably 
 decreasing the efficiency, so that it seems to be still a 
 long step from our present vacuum tubes to " light with- 
 out heat." This " light without heat " implies radiant 
 energy that is nearly or quite all luminous. But this 
 condition might be fulfilled and the light yet be most 
 
136 THE ART OF ILLUMINATION. 
 
 impractical, as, for example, in a sodium flame, which 
 gives an effect altogether ghastly. Any monochromatic 
 light is utterly destructive of color, but it might be possi- 
 ble so to combine nearly monochromatic lights of the 
 primary red, blue, and green as to obviate this difficulty. 
 Or, it may be eventually possible to excite luminous 
 radiation in gases or even in solids so as to get results 
 quite different from the ordinary spectra of the bodies. 
 The vacuum tube lamp is probably capable of develop- 
 ment into a commercial method of illumination for some 
 purposes, but in any form in which it has yet been sug- 
 gested it must be regarded rather as a stepping-stone 
 on the way toward light without heat than as the thing 
 itself. It is given this place in a discussion of practical 
 illuminants, not on account of its present position, but 
 because the author is very strongly of the opinion that it 
 may advance to some degree of importance at almost 
 any moment, and because it gives promise of an efficiency 
 considerably beyond anything which has hitherto been 
 reached in artificial illuminants. 
 
 But it may be that we must look to the chemist rather 
 than the electrician for the final word as to i. /animation. 
 It is well within the bounds of possibility that some ex- 
 aggerated prosphorescence may be found which will 
 enable us to store solar energy directly for use at night. 
 Or the firefly may give up his secret and teach us how 
 to get light by chemical changes at low temperatures. 
 And the firefly knows. The light emitted by such light- 
 giving insects is unique and most extraordinary in its 
 properties. For so far as can be ascertained the total 
 radiant energy lies within the limits of the visible 
 spectrum, and not only there, but in the most brilliant 
 part thereof. No similar distribution of radiant energy 
 
THE ELECTRIC INCANDESCENT LAMP. 137 
 
 is elsewhere known. The ordinary firefly of this coun- 
 try is typical of the whole class, giving a greenish-white 
 light that, when examined in the spectroscope, shows 
 a brilliant band extending over the yellow and green and 
 fading rapidly as the red and blue are approached. 
 
 Professor S. P. Langley has carefully investigated the 
 radiation from Pyrophorus noctilucus, a West Indian 
 species which attains a length of i 1-2 inch, and of which 
 a half dozen specimens in a bottle give sufficient light 
 for reading. These insects gave spectra bright enough 
 to permit careful investigation, and by comparison with 
 solar light reduced to the same intensity, Langley found 
 the curves shown in Fig. 45. Here B is the light radi- 
 ated by the insect and A solar light. The curves show 
 by the ordinates at each point the relative intensities of 
 the various parts of the spectra. 
 
 It at once appears that the light of Pyrophorus in- 
 cludes much the less range of color, and is much the 
 richer in yellow and green. The maximum intensity 
 is very near the beginning of the clear green (at about 
 wave length 5500), and the light extends only a little 
 way into the red and the blue. Fig. 45, which shows the 
 apparent distribution of light indicated by the two 
 curves, exhibits the narrow limits of the radiation from 
 Pyrophorus very clearly. The spectrum is practically 
 limited by the solar lines C and F, and Langley's most 
 careful experiments showed nothing perceptible outside 
 of these limits, a most remarkable state of affairs, quite 
 standing alone in our knowledge of radiation. 
 
 For equal light Langley found that Pyrophorus ex- 
 pends only about 1-400 of the energy required by a 
 candle or gas flame. This fact gives us a clew to the 
 efficiency of Pyrophorus as a light producer. It appears 
 
i 3 8 
 
 THE ART OF ILLUMINATION. 
 
 to be about five candles per watt, perhaps even a little 
 better fifteen or twenty times the efficiency of an incan- 
 
 PHOTOMETRIO CURVES. 
 FOR EQUAL TOTAL AMOUNTS OF LIGHT. 
 
 A Sun-light. 
 B- Fire-fly light. 
 
 ABSCISSAE. WAVE LENGTHS. 
 ORDINATES. LUMINOUS INTENSITIES. 
 
 Fig. 45. Curves of Firefly Light and Solar Light. 
 
 descent lamp, about four times the efficiency of an arc lamp 
 at its best. It is a startling lesson. The light-giving 
 
THE ELECTRIC INCANDESCENT LAMP. 139 
 
 process of Pyrophonts is apparently the slow oxidation 
 of some substance produced by it. Even if this sub- 
 stance could be reproduced in the laboratory the light 
 would be too nearly monochromatic to be satisfactory as 
 an illuminant, but it presumably is within the range of 
 possibility to obtain a combination of phosphorescent 
 substances which would give light of better color at very 
 high efficiency. 
 
 Certainly the problem is a most fascinating one, and 
 whether the ultimate solution lies in vacuum-tube light- 
 ing or in some form of phosphorescence, one may say 
 with an approach to certainty that all forms of incandes- 
 cence of highly heated solids are too inefficient in giving 
 light to approach the economy desirable in an artificial 
 illuminant. Indeed, a solid substance of great light- 
 giving efficiency, when heated to incandescence, would 
 be somewhat of an anomaly, since it would probably have 
 to possess an enormous specific heat at moderate tempera- 
 tures. What is really needed is some method, chemical 
 or electrical, of passing by the slow vibrations that char- 
 acterize radiant heat and stirring up directly vibrations 
 of the frequency corresponding to light. The vacuum 
 tube gives the nearest approach to a solution of this 
 problem yet devised, but it still leaves much to be de- 
 sired, and there is plenty of work for the investigator. 
 
CHAPTER VII. 
 
 THE ELECTRIC ARC LAMP. 
 
 THE electric arc is the most intense artificial illuminant 
 and the chief commercial source of very powerful light. 
 A full account of it would make a treatise by itself, so 
 that we can here treat only the phases of the subject 
 which bear directly on its place as a practical illuminant. 
 First observed probably by Volta himself, the arc was 
 brought to general notice by Davy in 1808 in the course 
 of his experiments with the great battery of the Royal 
 Institution. If one slowly breaks at any point an -elec- 
 tric circuit carrying considerable current at a fair voltage 
 the current does not cease flowing when the conductor 
 becomes discontinuous, but current follows across the 
 break with the evolution of great heat and a vivid light. 
 If the separation is at the terminals of two carbon rods 
 the light is enormously brilliant, and by proper mechan- 
 ism can be maintained tolerably constant. The passage 
 of the current is accompanied by the production of im- 
 mense heat, and the tips of the carbon rods grow white 
 hot, and serve as the source of light. In an ordinary 
 arc lamp the upper carbon is the positive pole of the 
 circuit, and is fed slowly downward, so as to keep the 
 arc uniform as the carbon is consumed. The main con- 
 sumption of energy appears to be at the tip of this posi- 
 tive carbon, which is by far the most brilliant part of 
 the arc, and at which the carbon fairly boils away into 
 
THE ELECTRIC ARC LAMP. 141 
 
 Fig. 46. The Electric Arc. 
 
 vapor, producing a slight hollow in the center of the 
 upper carbon, known as the " crater." 
 
 The carbon outside the crater takes the shape of a 
 blunt point, while the lower carbon is rather evenly and 
 more sharply pointed, and tends, if the arc is short, 
 to build up accretions of carbon into somewhat of a 
 
142 
 
 THE ART OF ILLUMINATION. 
 
 mushroom shape. Fig. 46 shows the shape of these tips 
 much enlarged, as they would appear in looking at the 
 arc through a very dark glass. Under such circum- 
 stances the light from the arc between the carbon points 
 seems quite insignificant, and it is readily seen that the 
 crater is by far the hottest and most brilliant region. In 
 
 80 
 
 GO 
 
 540 
 
 c 
 
 200 400 COO 
 
 Mean Hemispherical c.p. Lower Hemisphere 
 
 Fig. 47. Relation between Current Density and Intensity. 
 
 point of fact the crater is at a temperature of probably 
 3500 to 4000 degrees F., and gives about 50,000 candle- 
 power per square inch of surface -sometimes even more. 
 It is obvious that the more energy spent in this crater 
 the more heat and light will be evolved, and that the 
 concentration of much energy in a small crater ought to 
 produce a tremendously powerful arc. It is not surpris- 
 ing therefore to find that the larger the current crowded 
 through a small carbon tip, in other words, the higher 
 the current density in the arc, the more intense the 
 luminous effects and the more efficient the arc. Fig. 47 
 
THE ELECTRIC ARC LAMP. 143 
 
 shows this fact graphically, giving the relation between 
 current density and light in an arc maintained at uni- 
 form current and voltage. 
 
 The change in density of current was obtained by 
 varying the diameter of the carbons employed, the 
 smallest being about 5-16 inch in diameter, the largest 
 3-4 inch. The current was 6.29 amperes, and the voltage 
 about 43.5. The efficiency of the arc appears from these 
 experiments to be almost directly proportional to the 
 current density. But if the carbon is too small it wastes 
 away with inconvenient rapidity, while if it be too large 
 the arc does not hold its place steadily and the carbon 
 gets in the way of the light. 
 
 The higher the voltage the longer arc can be success- 
 fully worked, but here again there are serious limita- 
 tions. With too short an arc the carbons are in the way 
 of the light, and the lower carbon tends to build up 
 mushroom growths, which interfere with the formation 
 of a proper arc. In arcs worked in the open air the arc 
 is ordinarily about 3-32 inch long. If the voltage is 
 raised above the 40 to 45 volts at the arc commonly 
 employed for open arcs, the crater temperature seems 
 to fall off and the arc gets bluish in color from the rela- 
 tively larger proportion of light radiated by the glow- 
 ing vapor between the carbon poles. 
 
 So it comes about that commercial arcs worked in the 
 open air generally run at from 45 to 50 volts, and from 
 6 to 10 amperes. The softer and finer the carbons the 
 lower the voltage required to maintain an arc of good 
 efficiency and proper length, so that arcs can be worked 
 successfully at 25 to 35 volts with proper carbons, and 
 with very high efficiency, but at the cost of burning up 
 the carbons rather too rapidly. Abroad, where both 
 
144 THE ART OF ILLUMINATION. 
 
 high-grade carbons and labor are cheaper than 
 in this country, such low voltage arcs are freely used 
 with excellent results, and give a greatly increased effi- 
 ciency. 
 
 Sometimes three are burned in series across no-volt 
 mains, where in American practice one, or at most two 
 arc lamps, would be used in series with a resistance coil, 
 the same amount of energy being used in each case. 
 With proper carbons too, a steady and efficient arc can 
 be produced taking only 3 or 4 amperes, and admirable 
 little arc lamps of such kind are in use on the Continent. 
 The carbons are barely as large as a lead pencil and the 
 whole lamp is proportionately small, but the light is 
 brilliant and uniform. 
 
 The upper carbon burns away about twice as fast as 
 the lower, and the rate of consumption is ordinarily 
 from i to 2 inches per hour, according to the diameter 
 and hardness of the carbons. 
 
 The carbons themselves are generally about 1-2 inch 
 in diameter, and one or both are often cored, i. e., pro- 
 vided with a central core, perhaps 1-16 inch in diameter, 
 of carbon considerably softer than the rest. This tends 
 to hold the arc centrally between the carbons and also 
 steadies it by the greater mass of carbon vapor provided 
 by the softer portion. Generally it is found sufficient 
 to use one cored and one solid carbon in each arc, al- 
 though in this country arcs burning in the open air usu- 
 ally are provided with solid carbons only. 
 
 In American practice such open arcs are very rapidly 
 passing out of use, and are being replaced by the so- 
 called enclosed arcs. During the past three or four 
 years these have gone into use in immense numbers, 
 until at the present time the open arc is very rarely in- 
 
THE ELECTRIC ARC LAMP. 145 
 
 stalled, and illuminating companies are discarding 
 them as rapidly as they find it convenient to purchase 
 equipment for the enclosed arcs. 
 
 The principle of the enclosed arc is very simple. It 
 merely consists in fitting around the lower carbon a thin 
 elongated vessel of refractory glass with a snugly fitting 
 metallic cap through which the upper carbon is fed, the 
 fit being as close as permits of proper feeding. The 
 result is that the oxygen is rapidly burned out of the 
 globe, and the rapid oxidation of the carbon ceases, the 
 heated gas within checking all access of fresh air save 
 for the small amount that works in by diffusion through 
 the crevices. 
 
 The carbon wastes away at the rate of only something 
 like 1-8 inch per hour under favorable circumstances, 
 and the lamp, only requires trimming once in six or 
 eight full nights of burning, instead of each night. For 
 all-night lighting it used to be necessary to employ a 
 double carbon lamp, in which were placed two pairs of 
 carbons, so that when the first pair was consumed the 
 second pair would automatically go into action and fin- 
 ish out the night. The enclosed lamp burns 100 hours 
 or more with a single trimming. Even much longer 
 burning than this has been obtained from a 1 2-inch 
 carbon, such as is customarily used, but one cannot 
 safely reckon on a better performance without very un- 
 usual care. 
 
 Fig. 48 shows a typical enclosed arc lamp, of the de- 
 scription often used on no-volt circuits, both with and 
 without its outer globe and case. The nature of the 
 inner globe is at once apparent, but it should also be 
 noted that the clutch by which the carbon is fed acts, 
 as in many recent lamps, directly upon the carbon itself, 
 
146 
 
 THE ART OF ILLUMINATION. 
 
 thereby saving the extra length of lamp required by the 
 use of a feeding rod attached to the carbon. Finally, 
 at the top of the lamp is seen a coil of spirally wound 
 resistance wire. The purpose of this is to take up the 
 
 Fig. 48. Typical Enclosed Arc Lamp. 
 
 difference between no volts, which is the pressure at 
 the mains, and that voltage which it is desired to use at 
 the arc and in the lamp mechanism. 
 
 Such a resistance evidently involves a considerable 
 waste of energy, but in the enclosed arc the voltage 
 at the arc itself is, of necessity, rather high, 70 to 75 
 volts, so that the waste is less than it would otherwise be. 
 
 It has been found that when burning in an inner globe 
 
THE ELECTRIC ARC LAMP. 147 
 
 without access of air, the lower or negative carbon be- 
 gins to act badly, and to build up a mushroom tip, when 
 the voltage falls below about 65 volts. Hence it is neces- 
 sary to the successful working of the scheme that the 
 arc should be nearly twice as long as when the carbons 
 are burning in open air. This has a double effect, in 
 part beneficial, in part harmful. With the increased 
 length the crater practically disappears, and the light is 
 radiated very freely without being blocked by the car- 
 bons. Hence the distribution of light from the enclosed 
 arc is much better than from an open arc. 
 
 On the other hand, there is no point of the carbon at 
 anything like the temperature of the typical open arc 
 " crater," and the intrinsic efficiency is thereby lowered. 
 Also if the enclosed arc is to take the same energy as a 
 given open arc, the current in the former must be re- 
 duced in proportion to the increased voltage, hence, 
 other things being equal, the current density is lowered, 
 w r hich also lowers the efficiency. 
 
 The compensation is found in the lessened care and 
 the lessened annual cost for carbons. The carbons them- 
 selves have to be of a special grade, and are about two 
 and a half times as expensive as plain solid carbons, but 
 the number used is so small that the total cost is low. 
 There is some extra expense on account of breakage of 
 the inner globes, but the saving in labor and carbons 
 far outweighs this. Moreover, the light, albeit some- 
 what bluish white, is much steadier than that of the 
 ordinary open arc, and the inner globe has material 
 value in diffusing the light, being very often of opal 
 glass, so that the general effect is much less dazzling 
 than that of an open arc, and the light is far better dis- 
 tributed. 
 
148 THE ART OF ILLUMINATION. 
 
 In outdoor lighting the greater proportion of hori- 
 zontal rays from the enclosed arc is of considerable bene- 
 fit, while in buildings the same property increases the 
 useful diffusion of light, as will be presently shown. 
 Of course, when enclosed arcs are operated in series, as 
 in street lighting, the resistance of Fig. 48 is reduced 
 to a trivial amount, or abolished, so that the extra voltage 
 required with the enclosed arc is the only thing to be 
 considered. The enclosed arc used in this way is very 
 materially better as an illuminant than an open arc tak- 
 ing the same current, and experience shows that it may 
 be substituted for an open arc, taking about the same 
 energy, with general improvement to the illumination. 
 
 The weak point of the open arc is its very bad distri- 
 bution of light, which hinders its proper utilization. The 
 fact that most of the light is delivered from the crater 
 in the upper carbon tends to throw the light downward 
 rather than outward, and much of it is intercepted by the 
 lower carbon. Fig. 49 gives from Wybauw's experi- 
 ments the average distribution of light from 26 different 
 arc lamps, representing the principal American and 
 European manufacturers. The radii of the curve give 
 the intensities of the light in various angles in a vertical 
 plane. The distribution of light in space would be nearly 
 represented by revolving this curve about a vertical axis 
 passing through its origin, although at any particular 
 moment the distribution of light from an arc may be far 
 from equal on the two sides. 
 
 The shape of the curve is approximately a long ellipse 
 with its major axis inclined 40 degrees below the hori- 
 zontal. The presence of globes on the lamps may mod- 
 ify this curve somewhat, but in ordinary open arcs it 
 always preserves the general form shown. The small 
 
THE ELECTRIC ARC LAMP. 
 
 149 
 
 portion of the curve above the horizontal plane shows 
 the light derived from the lower carbon and the arc 
 itself, while the major axis of the curve measures the 
 light derived from the crater. The tendency, then, of the 
 open arc is to throw a ring of brilliant light downward 
 
 80 60 
 
 Fig. 49. Distribution of Light from an Open Arc. 
 
 at an angle of 40 degrees below the horizontal, so that 
 within that ring the light is comparatively weak, and 
 without it there is also considerable deficiency. Hence 
 the open arc, if used out of doors, fails to throw a strong 
 light out along the street, while the illumination is daz- 
 zling in a zone near the lamp. 
 
 For the same reason the open arc is at a disadvantage 
 in interior lighting, for the reason that most of the light, 
 being thrown downward, falls upon things and surfaces 
 far less effective for diffusion than the ordinary walls 
 and ceiling. Hence one of the very best ways of using 
 arcs for interior lighting is to make the lower carbon 
 positive instead of the upper, and to cut off all the down- 
 ward light by a reflector placed under the lamp. Then 
 
ISQ THE ART OF ILLUMINATION. 
 
 practically all the light is sent upward and outward to 
 be diffused by the walls and ceiling. 
 
 The enclosed arc, on the other hand, gives a much 
 rounder, fuller curve of distribution, the light being 
 thrown well out toward the horizontal and there being 
 a pretty strong illumination above the horizontal. For 
 the same energy the maximum illumination is little more 
 than half the maximum derived from a n open arc, but 
 the result in distribution is such 10 fully compensate 
 for this difference if one considers the lamps as illumi- 
 nants and not merely as devices for transforming electri- 
 cal into luminous energy. 
 
 Fig. 50 shows a composite distribution curve from ten 
 or a dozen enclosed arc lamps, such as are used on con- 
 stant potential circuits, including various makes. Most 
 of them were lamps taking about 5 amperes, and there- 
 fore using nearly 400 watts at the arc, besides the energy 
 taken up in the resistance and the mechanism. Figs. 49 
 and 50 afford a striking contrast in distribution, and it is 
 at once obvious that the lamps represented by the latter 
 have a great advantage as general illuminants either in- 
 doors or outside. These figures include the inner globe, 
 of course, generally of opal glass, which is of some bene- 
 fit in correcting the bluish tinge which is produced by 
 the long arc. After a few hours' burning a slight film 
 collects on the inner globe, which tends to the same re- 
 sult. For interior lighting, outer globes of opal or 
 ground glass are generally added, so that the color 
 question is partially eliminated. 
 
 As ordinarily employed, enclosed arc lamps take from 
 5 to 7 amperes, although now and then 3 or 4 ampere 
 lamps are used. These smaller sizes are less satisfac- 
 tory in the matter of color of the light, and are not widely 
 
THE ELECTRIC ARC LAMP. 
 
 used. Abroad open arcs taking as little as 2.5 amperes 
 are sometimes used. The carbons in this case are very 
 slender and of particularly fine quality, and these tiny 
 lamps can be made to give an admirable light. 
 
 Outside of America, the enclosed arc is little used, for 
 abroad labor is much cheaper than here, and carbons of 
 
 60* 50" 4'J 
 
 Fig. 50. Distribution of Light from Enclosed Arc. 
 
 a grade costly or quite unattainable here are there rea- 
 sonably cheap, so that the somewhat higher efficiency 
 of the open arc compensates for the extra labor and 
 carbons. Aside from this the bluish tinge of the light 
 from enclosed arcs of small amperage is considered ob- 
 jectionable, and the gain in steadiness so conspicuous in 
 American practice almost or quite disappears when the 
 comparison is made with open arcs taking the carbons 
 available abroad. 
 
 At its best the electric arc has fully three times the 
 efficiency of a first-class incandescent lamp, but this ad- 
 vantage is somewhat reduced by the need of diffusing 
 globes to keep down the dazzling effect of the arc, and 
 
152 THE ART OF ILLUMINATION. 
 
 to correct the distribution of the light. Taking these 
 into account, and also reckoning the energy wasted in 
 the resistances in case of arc lamps worked from con- 
 stant potential circuits, the gain in efficiency is consid- 
 erably reduced, and if one also figures the better illumi- 
 nation obtained by using distributed lights in incandes- 
 cent lighting, the arc lamp has a smaller advantage than 
 is generally supposed. Many experiments bearing on 
 this matter have been made, and a study of the results is 
 highly instructive. 
 
 By far the most complete investigation of the proper- 
 ties of the enclosed type of arc lamps is that recently made 
 by a committee of the National Electric Light Associa- 
 tion. The investigation was upon the arc lamps both for 
 direct and alternating currents, as customarily used on 
 constant-potential circuits. The results, however, are not 
 materially different, so far as distribution of light goes, 
 from those that belong to similar lamps for series circuits. 
 Fig. 50 is the composite curve of distribution obtained 
 by this committee in the tests of direct-current lamps. 
 
 The individual curves vary somewhat, although show- 
 ing the same essential characteristics. Fig. 51 shows a 
 typical example both with the outer globe opalescent, like 
 the inner globe, and also with a clear outer globe. The 
 effect of the former in reducing and also in diffusing the 
 light is very conspicuous. The opalescent globe absorbed 
 a little over 14 per cent, of the light. This absorption is 
 much less than would be given by a ground or milky glass 
 shade, but it serves to cut down the intrinsic brilliancy to 
 a useful degree. A clear globe absorbs about 10 per cent. 
 
 The weak point of such lamps as efficient illuminants 
 lies in the large amount of energy wasted in the lamp 
 mechanism, including the resistance for reducing the 
 
THE ELECTRIC ARC LAMP. 
 
 J 53 
 
 voltage of the mains to that desirable for the enclosed arc. 
 This loss amounts ordinarily to nearly 30 per cent, of the 
 total energy supplied, so that while the arc itself is highly 
 efficient, the lamps as used are wasteful. No one but an 
 American would think of working a 75-volt arc off a 120- 
 
 Fig. 51. Effect of Globes on Enclosed Arc. 
 
 volt circuit and absorbing the difference in an energy- 
 wasting resistance, but the advantages of the enclosed arc 
 are so great in point of steadiness and moderate cost of 
 labor that the practice has been found commercially ad- 
 vantageous, and the open arc has been practically driven 
 from the field for all indoor illumination, and is being 
 rapidly displaced in street lighting. 
 
154 THE ART OF ILLUMINATION. 
 
 Foreign practice tends, as already noted, toward the use 
 of two or even three open arcs in series on constant poten- 
 tial circuits. These can be fitted with diffusing globes to 
 keep the intrinsic brilliancy within bounds, and obviously 
 give a far larger amount of light for the energy consumed 
 than is obtained here with enclosed arcs, but we have 
 neither cheap high-grade carbons nor cheap labor, and as 
 in the last resort the thing which determines current prac- 
 tice must be the total cost of light per candle-hour, it is 
 likely that both methods of lighting are right when judged 
 by their respective conditions. 
 
 At present alternating-current arc lights are being 
 rather widely used, both on constant potential and on 
 constant-current circuits, and such arcs present some very 
 interesting characteristics. Evidently when an arc is 
 formed with an alternating current there is no " positive " 
 and no " negative " carbon, each carbon being positive 
 and negative alternately, and changing from one to the 
 other about 7200 times per minute 120 times per 
 second. 
 
 Under these circumstances no marked crater is formed 
 on either carbon, and the two carbons are consumed at 
 about an equal rate. As a natural result of the intermit- 
 tent supply of energy and the lack of a localized crater, 
 the average carbon temperature is somewhat lower than 
 in case of the direct-current arc, and the real efficiency of 
 the arc as an illuminant is also somewhat lowered. 
 Tests made to determine this difference of efficiency have 
 given somewhat varied results, but it seems probable that 
 for unit energy actually applied to the arc itself the direct- 
 current arc will give somewhere about 25 per cent, more 
 light than the alternating-current arc. But since when 
 working the latter on a constant potential circuit the 
 
THE ELECTRIC ARC LAMP. 155 
 
 surplus voltage can be taken up in a reactive coil, which 
 wastes very little energy, instead of by a dead resistance, 
 which wastes much, the two classes of arcs stand upon a 
 more even footing than these figures indicate. This com- 
 parison assumes enclosed arcs in each case, in accordance 
 with present practice. 
 
 For street lighting, as we shall presently see, the alter- 
 nating arcs have certain advantages of considerable mo- 
 ment with respect to distribution, so that as practical 
 illuminants they are often preferred. 
 
 The chief objection to the alternating-current arc has 
 been the singing noise produced by it. This is partly due 
 to the vibration produced in the lamp mechanism and 
 partly to the pulsations impressed directly on the air by 
 the oscillatory action in the arc itself. The former can be 
 in great measure checked by proper design and manu- 
 facture, but the noise due directly to the arc is much more 
 difficult to suppress. 
 
 Abroad where, for the reason already adduced, open 
 arcs are commonly used, a specially fine, soft carbon is 
 used for the alternating arcs, and the noise is hardly per- 
 ceptible. These soft volatile carbons, particularly when 
 used at a considerable current density, give such a mass 
 of vapor in the arc as to endow it with added stability and 
 to muffle the vibration to a very marked degree. The 
 result is a quiet, steady, brilliant arc of most excellent 
 illuminating power. But in this country such carbons 
 are with difficulty obtainable, and, even if they were to be 
 had at a reasonable price, could not be used in enclosed 
 arc lamps on account of rapid smutting of the inner globe. 
 Hence it is by no means easy to get a quiet alternating 
 arc, and in a quiet interior there is generally a very per- 
 ceptible singing, pitched about a semi-tone below bass C, 
 
156 THE ART OF ILLUMINATION. 
 
 with noticeable harmonics, a kind of chorus not always 
 desirable. 
 
 In selecting alternating-current lamps for indoor work 
 great care should be exercised to get a quiet lamp. Some 
 of the American lamps when fitted with tight outer globes 
 and worked with a rather large current are entirely unob- 
 jectionable, but in many cases there is noise in the mech- 
 anism, or the globe serves as a resonator. With a current 
 of 7 to 7.5 amperes, and a well fitted and non-resonant 
 globe, little trouble is likely to be experienced. Out of 
 doors, of course, a little noise does not matter. 
 
 The chief characteristic of the alternating arc, as re- 
 gards distribution of light, is its tendency to throw its 
 light outward rather than downward like the direct- 
 current arc; in fact, considerable light is thrown above the 
 horizontal, which materially aids diffusion. 
 
 For this reason it is often advantageous to use reflect- 
 ing shades for such lamps, so as to throw the light out 
 nearly horizontally when exterior lighting is being done. 
 Indoors, diffusion answers the same purpose, unless 
 powerful downward light is needed, when the reflector is 
 of service. 
 
 Fig. 52, from the committee report already mentioned, 
 shows the distribution of light from an alternating- 
 current lamp, with an opalescent outer globe, with a 
 clear outer globe, and with no outer globe, and with a por- 
 celain reflecting shade of the form indicated by the dotted 
 lines in the figure. The abolition of the outer globe and 
 the use of the reflector produces a prodigious effect in 
 strengthening the illumination in the lower hemisphere, 
 and this hemispherical illumination is for some purposes 
 a convenient way of reckoning the illumination of the 
 lamp. But a truer test is the spherical candle-power, 
 
THE ELECTRIC ARC LAMP. 157 
 
 since that takes account of all the light delivered by the 
 lamp. Alternating arc lamps seem to work best at a fre- 
 quency of 50 to 60 cycles per second. Above 60 cycles 
 they are apt to become noisy, and below about 40 cycles 
 
 Fig. 52. Distribution from Alternating Enclosed Arc. 
 
 the light flickers to a troublesome extent. The light of 
 the alternating arc is really of a pulsatory character, owing 
 to the alternations. A pencil rapidly moved to and fro 
 in the light of such an arc shows a number of images 
 one for each pulsation, and this effect would be very dis- 
 tressing if one had to view moving objects, like quick 
 
158 
 
 THE ART OF ILLUMINATION. 
 
 running machinery, by such light. A harrowing tale is 
 told of a certain theater in which alternating arcs were 
 installed for some gorgeous spectacular effects, and of the 
 extraordinary centipedal results when the ballet -came on. 
 
 Ths pulsation is somewhat masked when the enclosed 
 arc is used, even with a clear outer globe, and is generally 
 rather inconspicuous when an opal outer globe is used. 
 It is also reduced when a fairly heavy current (7 to 8 
 amperes) is used, and when very soft carbons are em- 
 ployed, as they can be in open arcs. 
 
 
 
 WATTS 
 CONSUMED 
 
 MEAN INTENSITY 
 IN H. U. 
 
 MEAN WATTS 
 
 I 
 
 6 
 
 1 
 
 s 
 
 O 
 
 G 
 
 jchanism 
 
 Spherical 
 
 Lower 
 Hemi- 
 spherical 
 
 Spherical 
 H. U. 
 
 Lower 
 Hemi- 
 spherical 
 
 (H 
 
 sS 
 
 b 
 
 * 4) 
 
 hj 
 
 Q 
 
 O 
 
 g 
 
 
 
 
 
 O 
 
 OO 
 
 Clear 
 
 O s 
 
 So 
 
 Clear 
 
 
 
 
 
 
 
 
 Outer 
 
 
 
 Outer 
 
 
 
 
 
 
 
 235 
 
 332 
 
 
 2-37 
 
 1.66 
 
 i 
 
 5.01 
 
 55i 
 
 401 
 
 150 
 
 172 
 
 256* 
 
 362* 
 
 3.10 
 
 2.18* 
 
 1.52* 
 
 3 
 
 5-08 
 
 559 
 
 406 
 
 252 
 
 i95 
 
 216 
 
 282 
 
 2.85 
 
 2.60 
 
 1.99 
 
 4 
 
 4.76 
 
 524 
 
 381 
 
 i43 
 
 127 
 
 139 
 
 208 
 
 4.12 
 
 3-76 
 
 2.52 
 
 5 
 
 4-i6t 
 
 458 
 
 333 
 
 125 
 
 154 
 
 174 
 
 221 
 
 2.96 
 
 2.63 
 
 2.07 
 
 7 
 
 4-76 
 
 524 
 
 38i 
 
 i43 
 
 203 
 
 333 
 
 317 
 
 2.63 
 
 2.20 
 
 1.65 
 
 9 
 
 4.84 
 
 S3 2 
 
 387 
 
 MS 
 
 182 
 
 226 
 
 28l 
 
 2.83 
 
 2. 3 8 
 
 1.89 
 
 10 
 
 4-99 
 
 549 
 
 399 
 
 150 
 
 202 
 
 242 
 
 309 
 
 2-74 
 
 2.24 
 
 1.77 
 
 12 
 
 4.87 
 
 -536 
 
 390 
 
 146 
 
 I 7 8 
 
 195 
 
 230 
 
 3-5 
 
 2.66 
 
 2-33 
 
 Mean 
 
 4.9 
 
 529 
 
 384 
 
 M4 
 
 I 7 6 
 
 207 
 
 2 7 2 
 
 3-03 
 
 2.60 
 
 1.98 
 
 P. 
 
 
 a 
 
 ^ u a 
 
 O 
 
 -. !- 
 
 - E 
 
 
 
 
 
 
 
 <->0 
 
 ^+j 
 
 cS 
 
 5 
 
 <J< 
 
 ^"o o 
 
 fi 
 
 
 
 
 
 
 
 <s 
 
 oH 
 
 -Hrt 
 
 3 
 
 c 
 
 O ct t-j 
 
 aS 
 
 
 
 
 
 
 
 101 
 
 6. 4 o 
 
 448 
 
 .63 
 
 $40 
 
 .82 
 
 108 
 
 127 
 
 141 
 
 206 
 
 3-52 
 
 3-i7 
 
 2.17 
 
 
 
 
 
 
 
 
 
 203 
 
 236 
 
 
 26 
 
 1.94 
 
 102 
 
 6.79 
 
 4S9 
 
 .61 
 
 37S 
 
 73 
 
 84 
 
 I 4 6 
 
 
 226t 
 
 3.31 
 
 .6ot 
 
 I. 7 2t 
 
 103 
 
 
 424 
 
 .6s 
 
 344 
 
 7 r j 
 
 80 
 
 116 
 
 130 
 
 M7 
 
 3.66 
 
 15 
 
 2.88 
 
 105 
 
 6.20 
 
 414 
 
 .61 
 
 ,82 
 
 .80 
 
 .32 
 
 128 
 
 187 
 
 219 
 
 3-24 
 
 .20 
 
 1.89 
 
 
 
 
 
 
 
 
 
 '53 
 
 169 
 
 
 56 
 
 2.23 
 
 106 
 
 6.12 
 
 378 
 
 56 
 
 208 
 
 .70 
 
 80 
 
 132 
 
 l82t 
 
 284 
 
 2.82 
 
 .i 9 t 
 
 i. 4 8t 
 
 108 
 
 6.48 
 
 4S7 
 
 .64 
 
 383 
 
 .80 
 
 74-5 
 
 133 
 
 175 
 
 211 
 
 3 ' 3 * 
 
 .61 
 
 2.16 
 
 no 
 Mean 
 
 6.18 
 6.29 
 
 339 
 417 
 
 49 
 .60 
 
 276 
 342 
 
 .72 
 
 .76 
 
 63 
 74-5 
 
 140* 
 130 
 
 126 
 
 143 
 I 9 
 
 3-3 1 
 
 2.68 
 2.66 
 
 2-37 
 2.23 
 
 * Condition of no outer globe, f Condition with shade on lamp. 
 NOTE. All marked values not included in the mean. 
 
THE ELECTRIC ARC LAMP. 159 
 
 An interesting comparison of direct-current and alter- 
 nating-current enclosed arcs, as used on constant potential 
 circuits, is found in the foregoing table, from the report 
 already quoted. 
 
 It must be remembered that the results are in Hefner- 
 Alteneck units. This unit is. roughly 0.9 cp, so that the 
 mean results, reduced to a candle-power basis, are, for 
 efficiency when using clear outer globes, as follows : 
 
 Direct-current arc 2.89 watts per cp. 
 
 Alternating arc 2.96 watts per cp. 
 
 These efficiencies are on their face but little better than 
 those obtained from incandescent lamps. There is little 
 doubt that as a matter of fact a given amount of energy 
 applied to 3-watt incandescent lamps will give more use- 
 ful illumination than if used in arcs of the types here 
 shown. The incandescents lose somewhat in efficiency, 
 but gain by the fact of their distribution in smaller units. 
 
 But for many purposes the arcs are preferable on ac- 
 count of their whiter light and the very brilliant illumi- 
 nation that is obtainable near them. 
 
 Both direct and alternating-current enclosed arcs gain 
 by the use of rather large currents, both in steadiness and 
 in efficiency, and moreover give a whiter light. The 
 same is true, for that matter, of open arcs, in which the 
 larger the current the higher the efficiency. Very many 
 experiments on the efficiency of open arcs have been made 
 with moderately concordant results. Their efficiency 
 ranges in direct-current arcs from about 1.25 watts per 
 candle in the smallest to about 0.6 or a little less in the 
 most powerful. Fig. 53 shows a considerable number 
 of results by different experimenters consolidated into a 
 curve giving the relation between current and efficiency, 
 as based on mean spherical candle-power. 
 
i6o 
 
 THE ART OF ILLUMINATION. 
 
 The data for forming a similar curve for alternating 
 arcs are not available, but there is in this case the same 
 sort of relation between current and efficiency as that just 
 shown. There is generally accounted to be from 15 to 
 
 Current in amperes. 
 
 Fig. 53- Relation'between Current and Efficiency. 
 
 25 per cent, difference in absolute efficiency in favor of the 
 continuous-current arc. 
 
 Obviously the open arc has a much greater efficiency 
 than the enclosed form, but the very great intrinsic bril- 
 liancy of the former is from the standpoint of practical 
 illumination a most serious drawback. For all indoor 
 use and for much outdoor use the open arc must be 
 shielded by a diffusing globe, which to be effective should 
 cut off about 25 per cent, of the light. Taking this into 
 account, the working efficiency of the open arc in the sizes 
 generally used is likely to range between 1.25 and 1.50 
 watts per candle-power. It cannot, however, be made a 
 
THE ELECTRIC ARC LAMP. 161 
 
 satisfactory illuminant upon any terms until better car- 
 bons are used than are now available at a moderate price 
 in this country. 
 
 Recently some very interesting experiments have been 
 tried abroad with carbons impregnated with certain 
 metallic salts. These composite carbons have given ex- 
 traordinary efficiency, down to less than 0.5 watt per 
 spherical candle-power, and give a light softer and less 
 brilliantly white than usual. The product has not yet, 
 however, been brought into commercial form, so that it is 
 too early to speak with certainty regarding its merits. 
 Arcs formed between two slender pencils of such material 
 as is used in the Nernst glower have also been tried, and 
 have given an enormous efficiency, even greater than that 
 just mentioned. But the process is yet far from being in 
 commercial shape, so that nothing definite can be judged 
 as to its practical value. The immense intrinsic brilliancy 
 of such an arc would be a serious difficulty with its use as 
 an illuminant. 
 
 We may now form some idea of the relative efficiency 
 of different classes of lights. The annexed table puts the 
 facts in convenient form for reference : 
 
 KIND OF ARC. PERSPH CP. REMARKS. 
 
 Direct current, open ............. . i.o Medium power arc. 
 
 Direct current, shaded. . . . ....... 1.3 Medium power arc. 
 
 Alternating current, open ......... 1.7 Approximate., 
 
 Alternating current, shaded ...... 2.2 Approximate. 
 
 Direct current, enclosed ......... 2.4 No outer globe. 
 
 Direct current, enclosed ........ 2.9 Clear outer globe. 
 
 Direct current, enclosed ......... 3.3 Opal outer globe. 
 
 Alternating current, enclosed ...... 3.0 Clear outer globe. 
 
 Alternating current, enclosed ...... 3.6 Opal outer globe. 
 
 Alternating current, enclosed ...... 2.5 No outer globe. j 
 
 Direct current, enclosed .......... 1.9 Series lamp, approximate. 
 
 In watts per horizontal candle-power the enclosed arcs, 
 particularly the alternating ones, do relatively much bet- 
 
 I 
 
 m 
 
 UNIVERSITY ) 
 
162 THE ART OF ILLUMINATION. 
 
 ter than is indicated in the table. And in comparing arcs 
 with incandescents it must be remembered that the latter, 
 when rated like the above arcs, on mean spherical candle- 
 power and average efficiency while in use, will not do 
 much better than 4 to 4.5 watts per candle. But it is 
 adaptation to the work in hand that must determine the 
 use of one or another illuminant. 
 
CHAPTER VIII. 
 
 SHADES AND REFLECTORS. 
 
 As has already been pointed out, the illuminants in 
 common use leave much to be desired in the distribution 
 of light, and have, for the most part, too great intrinsic 
 brilliancy. The eye may suffer from their use, and even 
 if this does not occur, the illumination derived from them 
 is less useful than if the intrinsic brilliancy were reduced.. 
 
 Hence the frequent use of shades and reflectors in 
 manifold forms. Properly speaking, shades are intended 
 to modify the light by being placed between it and the 
 eye, while reflectors are primarily designed to modify the 
 distribution of the light rather than its intensity. Prac- 
 tically the two classes often merge into each other or are 
 combined in various ways. 
 
 There is, besides, a considerable class of shades of 
 alleged decorative qualities, which neither redistribute the 
 light in any useful manner nor shield the eye to any 
 material degree. Most of them are hopelessly Philistine, 
 and have no aesthetic relation to any known scheme of 
 interior decoration. Figs. 54 and 55, a stalactite and 
 globe respectively, of elaborately cut glass, are excellent 
 examples of things to be shunned. Cut glass is not at its 
 best when viewed by transmitted light, and neither dif- 
 fuses nor distributes the light to any advantage. Such 
 fixtures logically belong over an onyx bar inlaid with 
 silver dollars, and to that class of decoration in general. 
 Almost equally bad are shades that produce a strongly 
 
i6 4 
 
 THE ART OF ILLUMINATION. 
 
 streaked or mottled appearance, like Figs. 56 and 57. 
 These neither stop the glare from a too intense radiant 
 nor render the illumination more practically useful by 
 improving its distribution. These shades happen to be 
 
 Figs. 54 and 55. Cut Glass Stalactite and Globe. 
 
 all of them for incandescent lamps, but they are evil in 
 both principle and application, and would be equally bad 
 in connection with any other kind of illuminant. 
 
 With open gas flames a shade may be of some use as a 
 protection from draughts, but generally its purpose is to 
 
 Figs. 56 and 57. Shades to Avoid. 
 
 improve the illumination, and if it fails of this it has no 
 excuse for being. For artistic reasons it is sometimes de- 
 sirable even to reduce the illumination to a deep mellow 
 glow quite irrespective of economy, and in such case 
 shades may be made ornamental to any degree and of any 
 density required, or lights may be distribute^ for purely 
 
SHADES AND REFLECTORS. 165 
 
 decorative purposes, but gaudy spotted and striped affairs, 
 like those just shown, are useless even for these ends. 
 
 The first requirement of a shade is that it shall actually 
 soften and diffuse the light it shelters. If it does not do 
 this, no amount of ornamentation can make it tolerable 
 from an aesthetic standpoint. Almost any kind of orna- 
 mentation is permissible that does not defeat this well- 
 defined object. Translucent porcelain, ground and 
 etched glass are all available in graceful forms. If per- 
 fectly plain shades, like Fig. 58, seem too severe, the 
 
 Figs. 58, 59, and 60. Shades. 
 
 those finely etched in inconspicuous figures, like Figs. 59 
 and 60, may answer the purpose. The main thing is to 
 conceal the glaring incandescent filament or mantle so 
 that it will not show offensively bright spots. Hence the 
 general objection to cut glass, which, if used at all, should 
 for the display of its intrinsic beauty be so arranged that 
 it can be seen by strong reflected light rather than by that 
 which comes from its interior. 
 
 Thin paper and fabrics may be most effectively em- 
 ployed for shades and can readily be made to harmonize 
 with any style of ornamentation or color scheme that may 
 be in hand. In this respect such materials are far pre- 
 ferable to glass or porcelain, although more perishable and 
 less convenient for permanent use on a large scale. They 
 also entail much loss of light, and are far better suited to 
 domestic illumination than to larger installations. 
 
i66 THE ART OF ILLUMINATION. 
 
 The real proportion of light cut off by such shades has 
 not, to the author's knowledge, ever been accurately 
 measured, and, indeed, by reason of the immense variety 
 in them, it would be almost impossible to average. It is 
 safe to say, however, that it is generally over 50 per cent, 
 although the light is so much softened that the loss is not 
 seriously felt in reading or in other occupations not tax- 
 ing the eyes severely. 
 
 With respect to porcelain and glass shades the propor- 
 tion of light absorbed has been measured many times, and 
 on many different kinds of shades, so that actual, even if 
 diverse, figures are available. The following table gives 
 the general results obtained by several experimenters on 
 the absorption of various kinds of globes, especially with 
 reference to arc lights: 
 
 PER CENT. 
 
 Clear glass 10 
 
 Alabaster glass 15 
 
 Opaline glass 20-40 
 
 Ground glass 25-30 
 
 Opal glass 25-60 
 
 Milky glass 30-60 
 
 The great variations to which these absorptions are 
 subject are evident enough from these figures. They 
 mean, in the rough, that clean, clear glass globes absorb 
 about TO per cent, of the light, and that opalescent and 
 other translucent glasses absorb from 15 to 60 per cent., 
 according to their density. Too much importance should 
 not be attached to this large absorption, since it has al- 
 ready been shown that in most cases, so far as useful effect 
 is concerned, diffusion and the resulting lessening of the 
 intrinsic brilliancy is cheaply bought even at the cost of 
 pretty heavy loss in total luminous radiation. 
 
 The classes of shades commonly used for incandescent 
 
SHADES AND REFLECTORS. 167 
 
 lamps and gas lights have been recently investigated with 
 considerable care by Mr. W. L. Smith, to whom the au- 
 thor is indebted for some very interesting data on this 
 subject. 
 
 The experiments covered more than twenty varieties of 
 shades and reflectors, and both the absorption and the 
 redistribution of light were investigated. One group of 
 results obtained from 6-inch spherical globes, intended to 
 diffuse the light somewhat without changing its distribu- 
 tion, was as follows, giving figures comparable with 
 those just quoted : 
 
 PER CENT. 
 
 Ground glass 24.4 
 
 Prismatic glass 20. 7 
 
 Opal glass 32.2 
 
 Opaline glass 23.0 
 
 The prismatic globe in question was of clear glass, but 
 with prismatic longitudinal grooves, while the opal and 
 opaline globes were of medium density only. 
 
 Etched glass, like Figs. 59 and 60, has considerably 
 more absorption than any of the above, the etching being 
 optically equivalent to coarse and dense grinding. Their 
 diffusion is less homogeneous than that given by ordi- 
 nary grinding, so that they may fairly be said to be un- 
 desirable where efficiency has to be seriously considered. 
 
 A plain, slender canary stalactite behaved like the globes 
 as respects distribution, and showed just the same absorp- 
 tion as the ground glass globe, i. e., 24.4 per cent., but 
 permitted an offensively brilliant view of the filament 
 within. 
 
 Another group of tests had to do with reflecting shades 
 designed to throw light downward, in some cases giving a 
 certain amount of transmitted light, in others being really 
 opaque. The characteristics of some common forms of 
 
i68 THE ART OF ILLUMINATION. 
 
 such shades are plainly shown by the curves of light dis- 
 tribution made with the shades in place. Figs. 61 and 
 62 show two thoroughly typical examples of these shades. 
 Fig. 6 1 is the ordinary enameled tin 8-inch shade, green 
 
 Fig. 61. Conical Shade. Fig. 62. Fluted Cone. 
 
 on the outside and brilliant white within, a form too often 
 used over desks. Fig. 62 is almost as common, being a 
 fluted porcelain 6-inch shade, used in about the same way 
 as Fig. 61. Figs. 63 and 64 give the respective vertical 
 distributions produced by these two shades, the outer 
 
 Fig. 63. Distribution from Fig. 64. Distribution from 
 
 Conical Shade. Fluted Cone. 
 
 circles showing for reference the nominal i6-cp rating. 
 The porcelain not only gives a more uniform reflection 
 downwards, but transmits some useful light outwards. 
 The case as between it and the tin shade of Figs. 61 and 
 63, which gives a strong but narrow cone of light down- 
 ward, may be tabulated as follows : 
 
SHADES AND REFLECTORS. 169 
 
 8-INCH TIN 6-INCH FLUTED 
 ENAMELED. PORCELAIN. 
 
 Mean spherical candle-power 8.12 9.89 
 
 Maximum candle-power 29.49 18.15 
 
 Horizontal candle-power o.oo 5.26 
 
 Absorption, per cent 28.1 12.4 
 
 The absorption is, of course, based, as elsewhere, on the 
 mean spherical candle-power. Of these two shades the 
 porcelain one is considerably the better for practical pur- 
 poses. Although it gives a somewhat smaller maximum 
 candle-power directly below the lamp, it gives a much 
 
 Fig. 65. Shallow Cone. Fig. 66. McCreary Shade. 
 
 larger well-lighted area, and is for every reason to be pre- 
 ferred. The unaltered vertical distribution of an incan- 
 descent lamp is given in the curve shown in Chapter VI., 
 and that curve was from the same lamp used in testing 
 these shades. 
 
 It should be noted that the relations of these two forms 
 would not be materially altered if they were of appro- 
 priate size and were applied to Welsbach burners, the dis- 
 tribution of light from which bears a rather striking re- 
 semblance to that from an incandescent lamp. The tin 
 shade gives too much the effect of a bright spot to be 
 really useful for most purposes. If such a concentrated 
 beam is desired it is far better obtained by other and more 
 perfect methods. 
 
170 
 
 THE ART OF ILLUMINATION. 
 
 Figs. 65 and 66 show two other forms of reflecting 
 shade in somewhat common use, the former designed to 
 give the light a general downward direction, the latter 
 to produce a strong and uniform downward beam. Fig. 
 65 is a 6-inch fluted porcelain shallow cone, while Fig. 66 
 is the well-known and excellent McCreary shade, 7-inch. 
 They are intended for widely different purposes, which 
 come out clearly in the curves of distribution, Figs. 67 
 and 68. 
 
 The flat porcelain cone, Fig. 67, merely gathers a con- 
 siderable amount of light that would ordinarily be thrown 
 
 VERTICAL OK Of HORIZOJJIAL 
 
 Figs. 67 and 68. Curves of Distribution. 
 
 upward, and scatters it outwards and downwards. It 
 has a generally good effect in conserving the (light, and 
 whether applied to an incandescent lamp or a Welsbach 
 deflects downward a good amount of useful illumination. 
 The McCreary shade, on the other hand, is deliberately 
 intended to give a rather, concentrated beam, softened, 
 however, by the ground glass bottom of the shade. As 
 Fig. 68 shows, it accomplishes this result quite effectively, 
 giving a powerful and uniform downward beam. The 
 
SHADES AND REFLECTORS. 171 
 
 annexed table shows in a striking manner the difference in 
 the two cases : 
 
 FLAT PORCE- 
 LAIN CONE. McCREARY. 
 
 Mean spherical candle-power 9.84 7.50 
 
 Maximum candle-power 15.72 42.72 
 
 Horizontal candle-power 13-94 2.29 
 
 Absorption, per cent 12.8 33.5 
 
 The small absorption in the first instance is merely due 
 to the fact that the shade is not reached by any con- 
 siderable portion of the light, while the large absorption 
 in the later case only indicates that nearly the whole body 
 of light is gathered by reflection, and sent out through a 
 diffusing screen. 
 
 The porcelain cone is irremediably ugly, but a less 
 offensive shade having the same general properties may 
 often be put to a useful purpose. The McCreary shade 
 is purely utilitarian, but neat, and does its work well in 
 producing a strong, directed illumination a bit too con- 
 centrated, perhaps, for ordinary desk work, but most use- 
 ful for work requiring unusually bright light. Of fancy 
 shades modified in various ways there are a myriad, 
 usually less good than the examples here shown. 
 
 In cases where concentration of light downwards along 
 the axis of the lamp is desirable, rather efficient results are 
 attained by combining lamp and reflector, that is, by shap- 
 ing the bulb of the lamp itself so that when the part of it 
 nearest the socket is silvered on the outside it shall form 
 an effective reflector of proper shape. Obviously when 
 the lamp burns out or dim the whole combination becomes 
 useless, in which respect the device is less economical than 
 an ordinary lamp in a carefully designed reflecting shade 
 like the McCreary. On the other hand, the reflector 
 lamps are, on the whole, somewhat more efficient during 
 
172 THE ART OF ILLUMINATION. 
 
 their useful life, and for general purposes of illumination 
 are much less obtrusive. 
 
 In such lamps the bulb, instead of being pear-shaped, 
 is spherical or spheroidal, with the upper hemisphere sil- 
 vered, the silvering being protected by a coat of lacquer. 
 The filament usually has several convolutions of rather 
 small radius, so as to bring as large a proportion of the 
 incandescent filament as possible near to the center of the 
 bulb. A filament so disposed throws an unusual propor- 
 tion of the light upwards and downwards when the lamp 
 is mounted with its axis vertical, but, of course, at the 
 expense of the horizontal illumination. 
 
 It has sometimes been proposed to use a filament so 
 shaped in an ordinary bulb for the purpose of throwing a 
 strong light downwards. Evidently, however, such a 
 lamp throws just as much light upwards toward the 
 opaque socket and the ceiling as downwards toward the 
 tip. Hence there is certainly no gain in general illumi- 
 nation, and no important gain in the downward illumina- 
 tion unless the lamp is mounted with a reflector. Fig. 69 
 shows in outline a nominal 5O-cp. spherical bulb lamp with 
 a silvered upper hemisphere, and Fig. 70 the curve of light 
 distribution in a vertical plane. The maximum down- 
 wards is about 75 candle-power, while a fair amount of 
 light is thrown out laterally up to 30 degrees from the 
 horizontal. The dotted curve shows the distribution with 
 the silvered bulb, the solid curve the distribution after the 
 silvering had been removed. 
 
 Fig. 71 shows a common type of reflecting bulb lamp. 
 As usually employed, the lower part of the bulb is ground 
 so as to soften and diffuse the light. The general charac- 
 teristics of reflectors and their field of usefulness have 
 already been discussed, so that it is here hardly necessary 
 
SHADES AND REFLECTORS. 
 
 '73 
 
 to say more than to reiterate that whenever a downward 
 light is desired reflector lamps may furnish a most useful 
 means of getting it. Their weak point, as generally 
 made, is a tendency to produce a spotted effect through 
 too great concentration of the light along the central axis. 
 If a projector effect is the one thing desired silvered bulb 
 lamps are not the best means of getting it, and for their 
 
 Figs. 69 and 70. Reflector Lamp and Its Distribution. 
 
 greater usefulness as illuminants they should be so con- 
 structed as to give a nearly uniform hemispherical dis- 
 tribution. Most of the silvered bulb lamps on the market 
 fail to do this. And it should be noted that since the 
 whole lamp is lost when the filament breaks, there is a 
 strong temptation to fit such lamps with a low efficiency 
 filament in order to give a longer life. A properly de- 
 signed lamp of this class planned for hemispherical dis- 
 tribution could not fail to be of much use in general 
 illumination, and could be produced at a very small extra 
 cost above standard lamps. 
 
174, THE ART OF ILLUMINATION. 
 
 For various illuminants shades require to be somewhat 
 modified in form, and an enormous variety of shades and 
 reflectors are on the market, of which those here described 
 may merely serve as samples. Shading the radiant, 
 whatever it may be, is a simple matter, and so is the use 
 
 Fig. 71. Reflector Lamp. 
 
 of a pure reflector (best made of silvered glass) to direct 
 the light in any particular direction. But the commonest 
 fault of powjerful radiants, as we have already seen, is too 
 great intrinsic brilliancy, which calls for diffusion, and 
 good diffusion without great loss of light is difficult of 
 attainment, particularly if at the same time there is need 
 of redistributing the light so as to strengthen the illumina- 
 tion in any particular direction. 
 
 By far the most successful solution of this troublesome 
 
SHADES AND REFLECTORS. 175 
 
 problem is found in the so-called holophane globes, de- 
 vised a few years ago by MM. Blondel and Psaroudaki, 
 and now in somewhat extensive use both here and abroad. 
 The general principle employed by these physicists was 
 to construct a shade of glass so grooved horizontally as 
 to form the whole shade of annular prisms. These are 
 not formed as in a lighthouse lens, to act entirely by re- 
 fraction, because in the attempt to bend the rays through 
 a large angle by refraction alone there is a large loss by 
 total reflection. 
 
 The prisms of the holophane globe are relieved, as it 
 were, at certain points, so that rays which need to be but 
 little deflected are merely refracted into the proper direc- 
 tion, while those that must be greatly bent to insure the 
 proper direction are affected by total reflection. This 
 combination of refracting and reflecting prisms in the 
 same structure accomplished the efficient redistribution 
 of the light in a very perfect manner. The diffusion re- 
 mained to be effected, and the means adopted was to form 
 the interior of the globe into a series of rather fine, deep, 
 rounded, longitudinal grooves. 
 
 The total result is a great reduction of the intrinsic bril- 
 liancy, coupled with almost any sort of distribution re- 
 quired, the total loss of light meanwhile being less than 
 in any other known form of diffusing shade or reflector. 
 Fig. 72 shows in detail, considerably magnified, the 
 structure of the holophane prisms and the combination of 
 refraction and reflection that is their characteristic fea- 
 ture. Here the ray A is merely refracted in the ordinary 
 way, emerging with a strong downward deflection from 
 the prism face in the direction A 1 . Ray B B l is totally 
 reflected at the face & 1 , and then refracted outwards at b. 
 C is strongly refracted and emerges from the surface c, 
 
I 7 6 
 
 THE ART OF ILLUMINATION. 
 
 while D D 1 is refracted at entrance, totally reflected at d 1 , 
 and again refracted at emergence from d. 
 
 The net result is to keep in this particular form of prism 
 surface nearly all the rays turned downward below the 
 horizontal. Obviously other prismatic forms might be 
 
 Fig. 72. Section of Holophane Globe. 
 
 employed, which would give a very different final distri- 
 bution, but the principles involved are the same. 
 
 Fig. 73 shows, likewise on a greatly enlarged scale, the 
 interior fluting which accomplishes the necessary diffu- 
 sion of light. The ray .a is here split up into* a reflected 
 component, afterwards refracted b, e, i, g and a 
 purely refracted component, b, c, d. The shape of the 
 flutings is such as by this means to secure very excellent 
 diffusion at a very small total loss of light. The inner 
 and outer groovings being at right angles produce a 
 somewhat tessellated appearance, but aside from this the 
 surface is quite uniformly illuminated. 
 
 These holophane globes are made for all kinds of 
 
SHADES AND REFLECTORS. 177 
 
 radiants, but are most commonly applied to Welsbach 
 gas burners and to incandescent electric lights. Evi- 
 dently the shape of both grooves and globe must vary 
 with the purpose for which the shade is desired, which 
 results in a very large number of forms from which a 
 selection may be made for almost any variety of illumina- 
 tion. 
 
 It should be noted that these holophane shades both 
 diffuse and redistribute the light in a very thorough man- 
 ner. Speaking generally, they are of three distinct 
 classes. The first is laid out according to the general 
 principles of Fig. 72, and is intended to direct most of the 
 
 Fig- 73. Diffusing Curves of Holophane. 
 
 light downwards, serving the same end as a reflector, but 
 giving at the same time the needful diffusion without the 
 use of a ground or frosted globe. The general results are 
 strikingly shown in Fig. 74, which gives a very graphic 
 idea of what such a globe actually does. 
 
 The second class of globes has for its purpose a fairly 
 uniform distribution of the light, mainly below the hori- 
 zontal, and it is intended for ordinary indoor lighting, 
 where a particularly strong light in any one direction is 
 quite needless. Its effect is shown in Fig. 75. The third 
 general form of holophane globe is designed for the 
 
178 THE ART OF ILLUMINATION. 
 
 especial purpose of throwing a strong light out in a nearly 
 horizontal direction, and is shaped so as thus to redis- 
 tribute the light, putting it where it is most useful for 
 such work as street lighting, large interiors, and the like. 
 The effect produced is admirably shown by Fig. 76. The 
 
 Fig. 74. Holophane, Downward Distribution. 
 
 shapes of globes shown in these last three figures are those 
 intended for mantle burners. 
 
 In general, the device enables a good degree of diffu- 
 sion to be secured together with almost any peculiarity of 
 distribution that could be wanted, and with a degree of 
 efficiency that is unequaled by any known system of 
 shades or reflectors, unless it be the Fresnel lighthouse 
 lenses. 
 
 One does not generally get such a combination of good 
 qualities without certain disadvantages that must be taken 
 in partial compensation. In the holophane system the 
 
SHADES AND REFLECTORS. 179 
 
 weak point is dirt. The doubly grooved surface makes 
 an excellent dust catcher, and a layer of dust can easily 
 be accumulated quite sufficient to cut down the efficiency 
 very seriously. And, moreover, a hasty dab w r ith a rag 
 does not clean a holophane globe; it must be gone over 
 carefully and thoroughly. When kept clean, the globes 
 actually will do just what is claimed for them, and are not 
 
 Fig. 75. Holophane, General Distribution. 
 
 at all a merely theoretical development excellent only on 
 paper; but they must be kept clean, and should not be used 
 where they cannot or will not receive proper attention. 
 
 This is probably the chief reason, aside from the extra 
 cost, why such globes have not been more extensively 
 used for street lighting, to which their power of redis- 
 tributing the light in the most useful direction admirably 
 fits them. The results obtained in tests of these globes 
 are so striking as to merit examination in some detail. 
 
 In spite of the trouble from dust, the holophane globes 
 have come into considerable use for street lighting in some 
 European cities, notably Munich, where several thousand 
 have been used on Welsbach street lamps for several years 
 
i8o THE ART OF ILLUMINATION. 
 
 past. The net results is reported to be exceedingly good, 
 although the amount of labor involved must be, from an 
 American standpoint, very considerable. Breakage in 
 this case is reported at about 10 per cent, per annum. 
 
 If this device could be successfully applied to arc lamps 
 for street lighting, a very valuable redistribution of the 
 
 Fig. 76. Holophane, Outward Distribution. 
 
 light might be effected, but certain obstacles seem to be 
 interposed on account of the shifting of the arc as the 
 carbons are consumed. With a focusing form of lamp 
 this trouble would be averted, but such lamps have never 
 yet come into considerable use in this country. With en- 
 closed arcs, however, it should be possible to use these 
 globes with a very fair degree of success. 
 
 Tests of holophane globes on incandescent lamps show 
 in a thoroughly typical manner the effect of their peculiar 
 structure in diffusing and redistributing the light. For 
 example, Fig. 77 exhibits the distribution produced by a 
 holophane of the " stalactite " shape designed to throw 
 the light downward. It does this very effectively, giving 
 less of a spotted effect than any of the reflecting devices, 
 and at the same time diffusing the light from the bare fila- 
 ment very thoroughly. 
 
SHADES AND REFLECTORS. 181 
 
 The light absorbed by this stalactite was only 14.6 per 
 cent., which is considerably less than would be lost by any 
 equally effective device for diffusion alone, to say noth- 
 ing of the matter of redistribution. And this figure for 
 absorption is probably increased by the deflection of so 
 much light downwards, since similar shades designed to 
 
 Figs. 77 and 78. Distribution Curves. 
 
 throw more light laterally give materially smaller absorp- 
 tion. 
 
 From a considerable number of tests the holophane 
 shades appear to average about 12 per cent, absorption 
 when clean. That is, they actually transmit very nearly 
 as much light as clear glass globes that have no value in 
 redistributing or diffusing the light. As to their use- 
 fulness in indoor work there is little doubt. 
 
 For street lighting they are capable of immensely im- 
 proving the distribution of the light, but the matter of 
 keeping them clean is serious. Their greatest chance for 
 practical usefulness would be in connection with arc lamps 
 
i8a THE ART OF ILLUMINATION. 
 
 if the details of such an application should be thoroughly 
 worked out. 
 
 Fig. 78 shows the distribution curve from a holophane 
 stalactite designed to produce a more general distribution 
 than Fig. 77, the absorption in this instance being only 
 10.8 per cent. The principle employed in these shades 
 would readily lend itself to the distribution of light in 
 special directions for special purposes, but the variety of 
 work in which light has to be directed is so great that 
 special problems are best treated by the use of reflectors 
 which are comparatively cheap and can readily be adapted 
 to any required form. 
 
CHAPTER IX. 
 
 DOMESTIC ILLUMINATION. 
 
 THE lighting of houses is a most interesting and gen- 
 erally neglected branch of illumination. Artificial light 
 has been distinctly a luxury until within comparatively 
 recent times, and in domestic lighting there has not 
 been the same pressure of commercial necessity which 
 has resulted in the general efforts to illuminate other 
 buildings. Indeed, until within half a century there was 
 very little effort at really good illumination in the home, 
 everyone depending on portable lights which could be 
 brought directly to bear upon the work in hand, gas, 
 which provides fixed radiant points, being confined to 
 large cities, and in these to houses of the better class. 
 Even at the present time very little pains is taken to ar- 
 range the lighting in a systematic and efficient manner. 
 
 The comparatively small areas to be lighted in dwell- 
 ings, the small need for extremely intense light, and the 
 very discontinuous character of the need for any light at 
 all, render domestic lighting rather a problem by itself. 
 Of ordinary illuminants all may be freely used for such 
 work, save arc lamps and very powerful gas lamps, such 
 as the large regenerative burners and the most powerful 
 incandescent mantles. 
 
 Arcs are of very unnecessary power, hence most un- 
 economical, and are generally so unsteady as to be most 
 trying to the eyes. In the home, as a general thing, one 
 does not keep the eyes fixed in any definite direction, as 
 
 183 
 
i8 4 THE ART OF ILLUMINATION. 
 
 one would if working steadily by artificial light, so that 
 far more than usual care must be taken to avoid intense 
 and glaring lights. Therefore, arcs are most objection- 
 able, and the gas lights of high candle-power equally so, 
 particularly as the latter throw out a prodigious amount 
 of heat and burn out the oxygen of the air very rapidly. 
 
 As to other illuminants, the main point is to choose 
 those of low intrinsic brilliancy, or to keep down the in- 
 trinsic brilliancy by adroit and thorough shading. Any- 
 thing over two or three candle-power per square inch it 
 is well to avoid as needlessly trying to the eyes without 
 any compensating advantage save economy, which can 
 better be secured in other ways. 
 
 Aside from the physiological side of the matter, very 
 bright lights seldom give good artistic results or show 
 an interior at anything like its true value. Of the com- 
 mon illuminants, gas and incandescent lamps are those 
 generally most useful, while petroleum lamps and 
 candles are even now auxiliaries by no means to be de- 
 spised. Professor Elihu Thomson once very shrewdly 
 remarked to the writer that if electric lights had been in 
 use for centuries and the candle had been just invented, 
 it would be hailed as one of the greatest blessings of the 
 century, on the ground that it is absolutely self-con- 
 tained, always ready for use, and perfectly mobile. 
 
 Now, gas and incandescents, while possessing many 
 virtues, lack that of mobility. They are practically fixed 
 where the builder or contractor found it most con- 
 venient to install them, for while tubes or wires can be 
 led from the fixtures to any points desired, these strag- 
 gling adjuncts are sometimes out of order, often in the 
 way, and always unsightly. Besides, the outlets are 
 often for structural reasons in inconvenient locations, 
 
DOMESTIC ILLUMINATION. 185 
 
 and their positions need to be chosen very carefully if 
 artistic effects are at all to be considered; so that while 
 these lights are the ordinary basis of illumination wher- 
 ever they are available, lamps and candles, which can be 
 put where they are wanted and not necessarily where 
 some irresponsible workman chose to locate them, are 
 often most useful additions to our resources. 
 
 In domestic, as in other varieties of interior illumina- 
 tion, two courses are open to the designer of the illumi- 
 nation. In the first place, he can plan to have the whole 
 space to be lighted brought uniformly or with some ap- 
 proximation to uniformity, above a certain brilliancy 
 more or less approximating the effect of a room receiv- 
 ing daylight through its windows. Or, throwing aside 
 any purpose to simulate daylight in intensity or dis- 
 tribution, he can put artificial light simply where it is 
 needed, merely furnishing such a groundwork of general 
 illumination as will serve the ends of art and con- 
 venience. 
 
 While the first method is for purely utilitarian pur- 
 poses sometimes necessary, it is always uneconomical 
 and generally most inartistic in its results. Its sin 
 against economy is in furnishing a great deal of light 
 which is not really needed, while in so doing it usually 
 sends light in directions where it deadens shadows, blurs 
 contrasts, and illuminates objects on all sides but the 
 right one. The second method is the one uniformly to 
 be chosen for domestic lighting, from every point of 
 view. 
 
 In electric lighting the most strenuous efforts are con- 
 stantly being made to improve the efficiency of the in- 
 candescent lamps by a few per cent., and an assured gain 
 of even 10 per cent, would be heralded by such a fanfare 
 
186 THE ART OF ILLUMINATION. 
 
 of advertising as has not been heard since the early days 
 of the art. Yet in lighting generally, and domestic 
 lighting in particular, a little skill and tact in using the 
 lights we now have can effect an economy far greater 
 than all the material improvements of the last twenty 
 years. The fundamental rule of putting light where it 
 is most useful, and concentrating it only where it is 
 needed, is one too often forgotten or unknown. If 
 borne in mind, it not only reduces the cost of illumina- 
 tion, but improves its effect. 
 
 In applying this rule in practice, one of the first things 
 which forces itself upon the attention is the fact that the 
 conditions can seldom be met by the consistent use of 
 lights of one uniform intensity, or one uniform char- 
 acteristic as regards the distribution of the light around 
 the radiant. Even one kind of illuminant is sometimes 
 an embarrassing condition. Both the kind and quantity 
 of the illumination must be adjusted to the actual re- 
 quirements, if real efficiency is to be secured. 
 
 As has already been shown, the effective illumination 
 depends upon two factors the actual power of the 
 radiant in candles or other units, and the nature of the 
 surroundings, which determine the character and 
 amount of the diffuse reflection which re-enforces the 
 direct light. If the radiant in a closed space furnishes 
 a certain quantity of light, L, then the strength of the 
 illumination produced at any point within the space will 
 depend, if the walls are non-reflecting, simply on the 
 amount of light received from the radiant, in accordance 
 with the law of inverse squares. If the walls reflect, then 
 the total illumination at any point will be that received 
 directly, L. and in addition a certain amount k L (where 
 k is the coefficient of reflection), once reflected, a further 
 
DOMESTIC ILLUMINATION. 187 
 
 amount k 2 L twice reflected, and so forth. The total 
 illuminative effect will then be: 
 
 Z(i + + ^ + 3 +. . . &) 
 
 As k is obviously always less than unity, this series is 
 convergent upon the limiting value -M t _ /, li which 
 
 expresses the relative effect of the walls in re-enforcing 
 the light directly received from the radiant. 
 
 It is clear from the values of k already given for vari- 
 ous surfaces, that such assistance may be of very great 
 practical importance. A simple experiment showing the 
 value of the light diffusely reflected is to read at some 
 little distance from the radiant in a room having light 
 walls, and then to cut off the direct rays by a screen 
 close to the radiant and just large enough to shade the 
 book. If the conditions are favorable, the amount of 
 diffused illumination will be somewhat startling. A 
 repetition of the experiment in a room with dark walls 
 will exhibit the reverse condition in a most striking 
 manner. 
 
 In practice the interior finish of dwelling houses is 
 highly heterogeneous, the walls being tinted and broken 
 with doors and hangings, the ceiling being often of an- 
 other color, and the floors covered with colored rugs or 
 carpets, and generally provided with furniture at least 
 as dark as the walls. The floor is in point of fact the 
 least important surface from the standpoint of illumina- 
 tion, for it not only carries the furniture, but from its 
 position cannot diffuse light in any useful direction. So 
 far as it is concerned, only the small terms in k 2 and 
 higher powers enter the general equation, since the illumi- 
 nation diffused from below is not of any practical account 
 whatever. 
 
i88 THE ART OF ILLUMINATION. 
 
 A good idea of the practical amount of help received 
 from diffusion may be gained by computing the effect 
 for various values of k. The following table shows the 
 results for values of k between .05 and .95. 
 
 t \ 
 
 (*-*) 
 
 -95 ............................................ 20. 
 
 .90 .......................... ................. 10. 
 
 .85 ............................................ 6.66 
 
 -.80 ............................................. 5.00 
 
 75 ............................................. 4-00 
 
 .70 ... ......................................... 3-33 
 
 .65 ........................................... 2.85 
 
 .60 ................................... , ........ 2.50 
 
 .55 ............. ................... .......... 2.22 
 
 .50 ............................................ 2.00 
 
 .81 
 
 .66 
 
 53 
 .42 
 
 33 
 
 25 
 
 .11 
 
 05 
 
 These values show the great difference between good 
 and poor diffusing surfaces in their practical effect. 
 Reference to the table already given shows that ordi- 
 nary wall surfaces give values of k ranging from about 
 .60 down to .10 or less. These are likely to be reduced 
 by the gradual absorption of dust at the surface, but it 
 is quite within bounds to say that the effective illumina- 
 tion in a room may be nearly or quite doubled by the 
 light diffused from the walls. 
 
 On the contrary, the ceiling is a very important con- 
 sideration, for the light diffused downward is highly 
 valuable. Paneled or vaulted ceilings are notorious in 
 their bad effect upon the illumination. If used at all, 
 
DOMESTIC ILLUMINATION. 189 
 
 they should be employed with full knowledge of the 
 fact that they quite effectively nullify all attempts at 
 brilliant general illumination, and when considerations 
 of harmony permit, ceilings ought to be very lightly 
 tinted. 
 
 As to the walls themselves, wainscoting and dark soft- 
 finished papers absorb light very strongly, and render 
 lighting difficult, while the white-painted wood and 
 light papers freely used in Colonial houses produce 
 exactly the reverse effect. The character of interior 
 finish, being determined by the contemporaneous 
 fashion, can of course seldom be really subordinated to 
 the matter of illumination, which affects only personal 
 comfort; but in planning a scheme of decoration it is 
 necessary to bear in mind that the darker the general 
 effect the more light should be provided. 
 
 The outlets for gas and electricity provided for and 
 quite adequate to light a brightly-finished house, will 
 prove entirely insufficient if a scheme of decoration in 
 dark colors be afterward carried out, so that it is the part 
 of wisdom to arrange the original outlets to meet the 
 worst probable conditions for lighting. This will gen- 
 erally mean arranging for about double the minimum 
 amount of illumination wanted on the hypothesis of 
 strong diffusion from the walls. 
 
 If conditions demand or fashion dictates any attempt 
 at very bright illumination, a sort of simulated daylight, 
 all matters relating to diffusion are of very serious im- 
 port. Fortunately, such is not the usual case. Where 
 the main purpose is that already strongly urged, of 
 merely furnishing such illumination as is necessary for 
 practical or artistic purposes, there need be no effort at 
 uniform intensity of light or at making dark corners 
 
1 90 THE ART OF ILLUMINATION. 
 
 brilliant; and, while the aid of favorable diffusion is still 
 important in reducing the total amount of artificial light 
 furnished, it no longer so completely controls the situa- 
 tion. 
 
 With the data now at hand we can form a fairly defi- 
 nite idea of the quantity of light which must generally 
 be provided. One can get at the approximate facts by 
 considering the amount of light that must be furnished 
 in a room of given size to bring the general illumina- 
 tion up to a certain value. The particular value as- 
 sumed must depend upon the purpose for which the 
 room is to be lighted. For instance, since i candle-foot 
 is an amount which enables one to read rather easily, let 
 us assume that we are to furnish in a room 20 ft. square 
 and, say, 10 ft. high, a minimum of i candle- foot. 
 
 To start with, we must make some assumption as to 
 the amount gained by diffusion from ceiling and walls. 
 This, in a concrete case, we can make an educated guess 
 at from the data already given. In general, Wybauw 
 found that in moderate-sized rooms the diffusion in- 
 creased the effective value of the radiant 50 per cent., 
 which, as it agrees pretty closely with our own values, 
 taking into account a light ceiling, we will use for the 
 present purpose. 
 
 Let the assumed radiant be at r, Fig. 79, and at a 
 height of 6' 6" above the floor. Now draw an imagi- 
 nary plane a b at a height of 2! 6" above the floor, and 
 take this as the surface to be illuminated. If r is in the 
 center of the room, the greatest distance from r to a cor- 
 ner of the plane a b will be \/ '216 ft. = 14.7 ft. Each 
 candle-power at r must be reduced proportionately, so 
 that i candle at r would give 1-216 candle-foot at the 
 point in question. According to our hypothesis, diffu- 
 
DOMESTIC ILLUMINATION. 191 
 
 sion aids by 50 per cent., so that instead of requiring 216 
 candle-power to give i candle-foot in the remotest cor- 
 ner, the real amount would be 144 candle-power, which 
 would be handily furnished by a cluster of nine i6-cp 
 incandescent lamps. The result would be a room quite 
 brilliantly lighted, for, except very near the walls, the 
 illumination would be much in excess of i candle-foot, 
 rising to 4 or 5 candle-feet upon the plane of lighting 
 under and near the lights. 
 
 Such an arrangement of the lights is, however, un- 
 economical in the extreme, since the distant corners are 
 
 4 
 
 Fig. 79- Vertical Section of Room. 
 
 illuminated at a very great disadvantage. Fig. 80 shows 
 the advantage gained by a rearrangement. Here the 
 room is divided by imaginary lines into four ro-ft. 
 squares, and in the center of each of these is a light 6' 6" 
 above the floor, as before. Now, if a corner of the plane 
 of lighting, as E, receives i candle-foot, the requirements 
 are fulfilled. But E is distant from D just about 8 ft., 
 from C and B almost exactly 16 ft. and from A less than 
 22 ft. It, therefore, receives, neglecting A, for each 
 candle-power at D 1-64 candle-foot, ajid for each at C 
 
I 9 2 
 
 THE ART OF ILLUMINATION. 
 
 and B a total of 1-128 candle-foot, or, allowing for diffu- 
 sion, 1-43 and 1-87, respectively (nearly), so that it at 
 once becomes evident that four 32-cp lamps are more 
 than sufficient to do the work. 
 
 Taking A into account, four 25~cp lamps would al- 
 most suffice, but obviously the maximum illumination 
 is perceptibly lowered. It would be a maximum at the 
 center, and for 32-cp lamps would there amount to 2 
 candle-feet. A still further subdivision would lead to 
 
 1 
 
 -20- 
 
 At, 
 
 D 
 
 X \ 
 
 
 
 Fig. 80. Floor Plan. Fig. 8 1. Floor Plan. 
 
 still better distribution from .the point of view of 
 economy, and, indeed, something can still be gained by 
 a further redistribution of the light. For, with lights 
 arranged as in Fig. 81, at the center and on the circle 
 inscribed in the room in question, five 2O-cp lamps 
 would very closely fulfill the conditions, reducing the 
 total amount of light required to meet the assumed con- 
 dition from 144 to 100 candle-power in all. 
 
 Obviously, with a fixed minimum illumination and no 
 other requirement, the conditions of economy will be 
 most nearly met by a nearly uniform distribution of the 
 minimum intensity required. There is, however, a 
 limit to practical subdivision in limited areas, such as 
 
DOMESTIC ILLUMINATION. 193 
 
 rooms. In the case of large buildings, as we shall pres- 
 ently see, one can easily figure out the illumination on 
 the basis just taken, but in domestic lighting we have 
 to deal with a very limited number of radiants, at least 
 in considering gas and electricity. 
 
 By far the best results are attained by providing a 
 very moderate general illumination and then superpos- 
 ing upon it strong local illumination for special pur- 
 poses. For example, in most rooms better practical re- 
 sults than those of Fig. 81 would be reached by follow- 
 ing the same arrangement, but using four i6-cp or even 
 four 8-cp lamps and one 32-cp lamp, the latter being 
 placed near the point where the strongest illumination 
 is required. The result would be to give the extreme 
 corners all the light they really need, and to provide 
 plenty of light where it is of most practical value. 
 
 The same rules apply to the use of gas or other 
 illuminants, always bearing in mind that the total 
 amount of light required is strongly affected by the hue 
 of the walls, and that the principal radiant should be 
 placed where it will do the most good. Illumination 
 thus regulated is both safer physiologically and far more 
 efficient in use of the material than any attempt at uni- 
 form distribution over the entire area. 
 
 One's choice of illuminants must obviously be gov- 
 erned by the question of availability. Incandescent 
 electric lamps easily hold the first place when economy 
 is not the first consideration, by reason of their being 
 quite steady, giving out little heat, and in no way vitiat- 
 ing the atmosphere. They should always, however, be 
 furnished with ground bulbs, or so shaded as to reduce 
 their otherwise very high intrinsic brilliancy. 
 
 Next in order of desirability unquestionably comes 
 
i 9 4 THE ART OF ILLUMINATION. 
 
 gas. Used with the incandescent mantle burner, it is 
 the cheapest known illuminant for domestic purposes, 
 but in this form is too bright for anything except the 
 principal radiant. Mantle burners should always be 
 shaded, both to reduce the intrinsic brilliancy and to 
 modify the greenish cast of the light which otherwise is 
 highly objectionable. Ordinary gas jets, if the pressure 
 be fairly steady, give a good subordinate illumination. 
 
 Lamps and candles have strong merits for particular 
 pr.i-poses, but are inferior for general work. The former 
 are often used with good effect to furnish the principal 
 radiant, which may be re-enforced by small gas jets. 
 Candles, on the other hand, are extremely useful for 
 partial and subsidiary illumination, since they are the 
 only available source of small intensity unless one goes 
 to considerable trouble in wiring for tiny electric bulbs, 
 which are better adapted to purely decorative purposes 
 than to the regular work of illumination. 
 
 From this general basis of facts we can now take up 
 the practical and concrete side of domestic lighting. 
 
 As to the distribution of the lights required for in- 
 terior illumination, one must be guided by the intensity 
 which is necessary. The examples already given show 
 the general character of the problem. The laws upon 
 which the solution depend may be formulated as fol- 
 lows : If we write L for the required or existing inten- 
 sity of illumination in candle-feet at any point, C for the 
 candle-power of the radiant, and d for the distance in feet 
 from that radiant, then: 
 
 If the point in question receives light from more than 
 
DOMESTIC ILLUMINATION. 195 
 
 one radiant, the illuminative effects must be summed, 
 and, if the radiants are equal, 
 
 C = 
 
 L is of course in candle-feet and C in candle-power. 
 In these expressions no account is taken of the varying 
 angles of incidence of the light received from the sev- 
 eral radiants. In principle, L = , where i is the 
 
 angle of incidence; in other words, the illumination de- 
 creases as it becomes oblique. 
 
 In certain cases account must be taken of this fact, 
 but since, as a rule, objects to be lighted are oblique to 
 the plane of illumination, and cos i is small only in case 
 of rather distant lights, of which the entire effect is 
 small, and since the diffused light cannot be reckoned 
 with, having no determinate direction, the question of 
 obliquity, particularly when the radiants are numerous 
 and well distributed, has seldom to be dealt with. It is 
 rendered the more uncertain by the notorious inequality 
 of the distribution of the light from ordinary illuminants, 
 and it must be remembered that the whole aspect of the 
 matter is changed by the use of reflectors. 
 
 In ordinary interior illumination one constantly meets 
 limitations imposed by structural or artistic considera- 
 tions. For example, we have already seen that the 
 arrangement shown in Fig. 81 was highly desirable far 
 economic reasons. The five lamps dangling by cords 
 
196 THE ART OF ILLUMINATION. 
 
 or rods, however ornamental, from the ceiling of a room 
 20 ft. square, might be tolerated in an office, but would 
 be quite inadmissable in a drawing room. For domestic 
 lighting one is practically confined to chandeliers, side 
 lights, or ceiling lights. The latter have been consider- 
 ably used of late, sometimes with beautiful effects; some- 
 times unw r isely. 
 
 To examine the effect of ceiling lights on the situation, 
 refer to Fig. 82, which shows the same room as Fig. 79. 
 
 IT 
 
 
 i 
 
 
 'T 
 
 
 v| 1 
 
 
 if 
 
 i i 
 
 111 
 
 
 i 
 i 
 
 i 
 
 
 Fig. 82. Location of Ceiling Lights. 
 
 Assuming the same general conditions, let us find 
 the illumination at a point p in the plane of illumina- 
 tion when given by a light r in the old position, and a 
 ceiling light /, 6 ins. below the ceiling. The light 
 
 being of 16 candle-power, the light at p is L -- = .39 
 
 4 1 
 
 candle-foot, when the lamp is at r, or Z = = .21 
 
 74 
 
 when the lamp is at r', close to the ceiling, neglecting 
 diffused light. 
 
 In a room very bright with white paint or paper, hav- 
 ing, for example, k = .60 and ( - [ \ = 2.50, the total 
 
DOMESTIC ILLUMINATION. 197 
 
 illumination would be .39 + .97 = 1.36, and since the 
 diffusion does not materially change with the position 
 of the light, the illumination in the second case would 
 be, roughly, .21 + .97 = 1.18; in other words, the 
 change in position of the light would make but a small 
 change in the intensity of the illumination. 
 
 There is evidently some error made in assuming that 
 diffusion increases the illumination by a certain ratio, 
 and Wybauw's hypothesis of replacing the diffused light 
 by an imaginary radiant directly above the real radiant 
 involves the same error. It is probably nearer the 
 truth to assume, in case of an apartment having several 
 radiants, that the total illumination at any point is that 
 due to the lights severally, plus a uniform illumination, 
 due to diffusion and proportional to k and C. 
 
 The practical upshot of the matter, however one may 
 theorize on the rather hazy data, is that shifting the 
 lights in a room from their usual height to the ceiling 
 does not affect the illumination seriously if the walls and 
 ceiling diffuse strongly, while if they are dark the 
 change is decidedly unfavorable. This does not, how- 
 ever, imply that ceiling lights should not be used in 
 dark-finished rooms, although it is very plain that if they 
 are so used the lamps should be provided with reflectors, 
 or themselves form reflectors, as in some lamps recently 
 introduced. 
 
 If the walls have a very low coefficient of diffusion it 
 is obvious that all light falling upon them is nearly 
 wasted, at least from the standpoint of illumination, and 
 therefore the economic procedure is to deflect this light 
 so that instead of falling upon the walls it shall be di- 
 rected upon the plane of illumination, which is chosen 
 to represent the average height from the floor at which 
 
198 THE ART OF ILLUMINATION. 
 
 are the things to be illuminated. If reflectors or their 
 equivalents are skillfully applied, the radiants, for the 
 purpose in hand, are nearly or quite doubled in intensity, 
 so that there is a good opportunity for efficient lighting. 
 But these reflecting media must be used with caution 
 to avoid the appearance of beams giving definite bright 
 areas, and by far the best results may be obtained by 
 using ground or frosted bulbs in such cases. So far as 
 economy of light is concerned, reflectors can be advan- 
 tageously used wherever the effective reflection exceeds 
 the total diffusion coefficient of the walls. For example, 
 with a hemispherical reflector having a coefficient of 
 reflection of .70, the hemispherical intensity of the radiant 
 is 1.70 C, assuming a spherical distribution of the light. 
 This value corresponds, so far as the plane of illumination 
 is concerned, with a diffusion of k = .40, which signifies 
 that, except in very light-finished rooms, the radiant is 
 used more efficiently by employing a reflector than by 
 trusting to the really very serviceable diffusion from the 
 walls. 
 
 But if the reflector aperture be as great as a hemi- 
 sphere, there is still some material aid to be gained by 
 diffusion. In the case already discussed in Fig. 81, if 
 reflector lamps were used, five i6-cp lamps would meet 
 the requirements, and would fall but a trifle below the 
 requirements even if used as ceiling lamps. 
 
 It is safe to say that by the use of reflector lamps the 
 work of effective lighting from the ceiling is made fairly 
 easy, if the ceilings are of ordinary height. Without 
 reflectors it is a method greatly lacking in economy. 
 
 The use of side lights close to the wall, or on short 
 brackets, is preferable to lighting from the ceiling when 
 the latter is high, or when, as often happens, strong 
 
DOMESTIC ILLUMINATION. 
 
 199 
 
 local illumination is needed. Reflector lamps may here 
 again be used with very great effect if the walls are at 
 all dark in tone. Fig. 83 gives in diagram the simplest 
 arrangement of such lamps. We may assume their 
 height as a trifle less than in the case of the suspended 
 lights, say, 3 ft. above the plane of illumination, and 
 that they are equipped with reflectors giving a hemi- 
 spherical distribution of light. In Fig. 83 the positions 
 
 -20 
 
 IQ: 
 
 Fig. 83. Side Lights. 
 
 of the lamps are indicated by black dots, as before. It 
 is evident that the corners will be the points of mini- 
 mum illumination, and that in the central part of the 
 room the lighting will be rather weak, although, on the 
 whole, the distribution of light will be good. With 
 help from diffusion to the extent assumed in the last 
 example, four 2O-cp reflector lamps would do the work, 
 while with dark walls the case would call for four 32-cp 
 lamps. 
 
 Now, summarizing our tentative arrangements of 
 light, it appears that to illuminate a room 20 ft. square 
 and 10 ft. high on the basis of a minimum of i candle- 
 foot, will require from 80 to 144 effective candle-power, 
 
200 THE ART OF ILLUMINATION. 
 
 according to the arrangement of the lights, if the finish 
 is light, and half as much again, at least, if the finish is 
 dark. The floor space being 400 sq. ft., it appears that 
 the illumination is on the basis of about 3 to 5 sq. ft. per 
 effective candle-power. The former figure will give 
 good illumination under all ordinary conditions; the lat- 
 ter demands a combination of light finish and very skill- 
 fully arranged lights. 
 
 For very brilliant effects, no more than 2 sq. ft. per 
 candle should be allowed, while if economy is an object, 
 i-cp to 4 sq. ft. will furnish a very good groundwork of 
 illumination, to be strengthened locally by a drop-light 
 or reading lamp. The intensity thus deduced we may 
 compare to advantage with the results obtained by vari- 
 ous investigators, reducing them all to such terms as will 
 apply to the assumed room which we have had under 
 discussion. 
 
 Just deduced i-cp per 3 sq. ft. 
 
 Uppenborn i-cp per 3.6 sq. ft. 
 
 Piazzoli i-cp per 3. 5 sq. ft. 
 
 Fontaine i-cp per 7.0 sq. ft. (approximation). 
 
 In very high rooms the illumination just indicated 
 must be materially increased, owing to the usual neces- 
 sity for placing the lamps rather higher than in the case 
 just given, and on account of the lessened aid received 
 from diffuse reflection. The amount of this increase is 
 rather uncertain, but in very high rooms it would be 
 wise to allow certainly i-cp for every 2 sq. ft., and some- 
 times, as in ballrooms and other special cases requiring 
 the most brilliant lighting, as much as i-cp per square 
 foot. 
 
 On the other hand, in most domestic lighting, the 
 amount of lighting needed may be reduced by a little 
 
DOMESTIC ILLUMINATION. 201 
 
 tact. Ordinary living rooms, such as parlors, libraries, 
 and the like, do not require to be uniformly and brightly 
 lighted in most cases. It is sufficient if there is ample 
 light throughout the main portion of the room. 
 
 A groundwork illumination of 0.5 candle-foot over the 
 whole room, plus a working illumination of i to 1.5 
 candle-foot in addition over a part of the room, gives an 
 excellent result. This is something the result that 
 would be reached in Fig. Si by using a 32-cp central 
 lamp and four lo-cp lamps for the rest of the room. 
 Dining rooms need ample light upon the table, but do 
 not in the least require illumination of equal power in 
 the remote corners. Sleeping and dressing rooms do 
 not require strong light so much as well-placed light. 
 A bedroom of the dimensions we have been discussing 
 could be very effectively lighted with three or four i6-cp 
 lamps, provided they were placed where they would do 
 the most good. 
 
 To go into detail a little, perhaps the most important 
 rule for domestic lighting is never to use, indoors, 
 an incandescent or other brilliant light, unshaded. 
 Ground or frosted bulbs are particularly good when in- 
 candescents are used, and opal shades, or holophane 
 globes, which also reduce the intrinsic brilliancy, are 
 available with almost any kind of radiant. Ornamental 
 shades of tinted glass or of fabrics are exceedingly use- 
 ful now and then, when arranged to harmonize with 
 their surroundings. 
 
 In incandescent lighting the lamps may be placed in 
 any position. With gas or other flame radiants ceiling 
 lights are not practicable. As to the intensity of the 
 individual radiants, considerable latitude may be given. 
 In many instances, incandescents or gas or other lights 
 
202 THE ART OF ILLUMINATION. 
 
 of as low as 8 to lo-cp are convenient, while for stronger 
 illumination radiants of 15 to 2O-cp reduce the cost of 
 installation, and for special purposes lights of 30 to 
 5o-cp, incandescents or incandescent gas lamps, are 
 most useful. To get a clear view of the application of 
 the principles here laid down it will be well to take up 
 in some detail the illumination of a typical modern house 
 in its various particulars. 
 
 Beginning at the porch, the light here is of purely 
 utilitarian value. One 32-cp incandescent or its equiva- 
 lent in gas would generally be sufficient, enclosed in an 
 inoffensive antique iron lantern. Fig. 84 shows a fine 
 specimen of eighteenth century ironwork. 
 
 Hall. Assumed dimensions, 15 ft. x 20 ft., finished in 
 some combination like ebony and old yellow. Gen- 
 erally the staircase forbids the effective use of a chande- 
 lier, and lights can best be put upon wrought-iron side- 
 brackets. The lighting required for the 300 sq. ft. is 
 not strong, and four i6-cp or eight 8-cp units, arranged 
 on two or four brackets, would give all the illumination 
 ever required. Fig. 85, an antique two-light iron bracket, 
 will give a useful hint. Lanterns are often used here, 
 but they generally are in the way. 
 
 Library. Assumed, 20 ft. x 20 ft., in mahogany and 
 dull green. The form of the room and the presence of 
 bookcases complicate the illumination. The bookcases, 
 unless so much space is absolutely necessary, should not 
 be carried to the ceiling. The conditions are severe. 
 With incandescents very good results could be reached 
 by, say, twelve 8-cp ground-bulb, reflector lamps, 
 worked into the frieze, and a reading lamp of not less 
 than 32-cp, as a drop-light, preferably with a tinted holo- 
 phane or other globe. With gas, or with high bookcases, 
 
DOMESTIC ILLUMINATION. 
 
 203 
 
 old brass side-brackets on each side of the fireplace or else- 
 where opposite the cases, carrying in all the equivalent 
 
 Fig. 84. Iron Porch Lantern. 
 
 of eight i6-cp lamps, and a mantle burner, well shaded, 
 as a reading lamp, would answer. In fact, very good 
 work could be gotten from two shaded mantle burners 
 as side lights. 
 
 Reception Room. Assumed, 15 ft. x 15 ft., cream and 
 rose, or similar light finish. Strong light is not needed 
 here, and an ornate gilt brass chandelier, carrying four 
 
204 
 
 THE ART OF ILLUMINATION. 
 
 8-cp or lo-cp lamps or their equivalent should prove 
 ample. 
 
 Music Room. Assumed, 20 ft. x 25 ft., in white and 
 gold or the like. For musical purposes two 32-cp lights 
 
 -Tl 
 
 Fig. 85. Iron Wall Bracket. 
 
 in holophane globes, carried as piano lamps, shaded, and 
 for general illumination about twelve 8-cp lights, carried 
 in groups on elaborate gilt bronze brackets or sconces. 
 The arrangement, of course, hinges on the position of 
 windows, etc., and since such a room is often used as a 
 
DOMESTIC ILLUMINATION. 
 
 205 
 
 Fig. 86. Gilt Bronze Bracket. 
 
 ballroom, in case of electric lighting provision should 
 be made for replacing the 8-cp by 32-cp lamps. With 
 
2o6 THE ART OF ILLUMINATION. 
 
 Fig. 87. Gilt Bronze Bracket. 
 
 gas, the fixtures should be planned so as to provide ad- 
 ditional lamps. Figs. 86 and 87 show two examples of 
 
DOMESTIC ILLUMINATION. 
 
 207 
 
 fine eighteenth century bras de cheminee well adapted to 
 cases like the present. 
 
 Dining Room. Assumed, 15 ft. x 20 ft., in dark an- 
 tique oak or mahogany and tapestries or other dark wall 
 
 Fig. 88. Wrought-Iron Bracket. 
 
 finish. Here ceiling or frieze lamps are in place, one or 
 the other, according to the nature of the finish. Eight 
 8-cp reflector lamps, ground, worked into the decora- 
 tion, would give a good groundwork, backed up by, say, 
 
2o8 THE ART OF ILLUMINATION. 
 
 six more 8-cp or lo-cp ground-bulb lamps, on wrought- 
 iron brackets, of which Fig. 88 gives an excellent an- 
 tique specimen, flanking the mantle, or, for a yet better 
 artistic effect, by shaded candelabra upon the table itself. 
 Using gas, one would almost be driven to an elabora- 
 tion of the side brackets, or to a chandelier, too often an 
 abomination, and always difficult to make artistic in 
 such a place. 
 
 Billiard Room. Assumed, 15 ft. x 20 ft. Dull reds 
 and greens in finish. Lighting here must be utilitarian. 
 It requires four 32-cp lamps bearing directly upon the 
 table. Incandescents or mantle burners in holophane 
 globes, or with slightly translucent reflectors, answer 
 the purpose well. 
 
 SLv Bedrooms. Assumed, 15 ft. x 15 ft., finished in 
 cream or other light paint and with rather light walls. 
 In the smaller rooms, two i6-cp lights bearing upon the 
 dressing table are ample, and in the larger rooms these 
 with an additional bracket, carrying another i6-cp lamp, 
 are all that would be required. 
 
 Two Dressing Rooms. Assumed, 10 ft. x 15 ft., in 
 light finish, like the chambers. Two i6-cp lamps, bear- 
 ing on the dressing table, will do the work well. 
 Brackets here and in chambers should generally be of 
 gilt brass. 
 
 Three Bathrooms. Assumed, 8 ft. x. 10 ft., in white 
 and Delft blue or the like. One i6-cp light, carried on 
 bracket, is sufficient. 
 
 Three Servants' Rooms. Assumed, 10 ft. x 15 ft. 
 Light finish. One i6-cp lamp, bracketed, near dressing 
 table. 
 
 Kitchen. Assumed, 15 ft. x 15 ft. Light wood and 
 paint. Two i6-cp lamps. 
 
DOMESTIC ILLUMINATION. 
 
 209 
 
 Pantry. Assumed, 10 ft. x 15 ft. One i6-cp. 
 
 Back hall, laundry, and cellar would be lighted with 
 8-cp lamps, in all to the number of about ten. Upstairs 
 halls, three i6-cp. 
 
 This programme is merely intended as a hint about 
 the requirements, and while it is laid out for a fairly 
 large house, containing twenty rooms and three baths, 
 its details will furnish suggestions applicable to* many 
 places. In closing, it is worth mentioning that where 
 incandescents are available, an 8-cp lamp of the reflector 
 variety should be placed in the ceiling of every large 
 closet, and controlled by a switch from the room or by 
 an automatic switch, turning it on when the door is fully 
 opened. 
 
 The lighting just described may be summarized as 
 follows : 
 
 ROOM. 
 
 8-CP. 
 
 i6-cp. 
 
 32-CP. 
 
 SQ. FT. 
 PER CP. 
 
 REMARKS. 
 
 Hall 
 
 8 
 
 
 
 4.7 
 
 
 
 12 
 
 
 I 
 
 3.1 
 
 8-cp reflector lamps 
 
 Reception room 
 
 4 
 
 
 
 7.0 
 
 
 Music room . . . 
 
 12 
 
 
 2 
 
 3.O 
 
 
 Dining room 
 
 14 
 
 
 
 2.7 
 
 Eight reflector lamps 
 
 
 
 
 4 
 
 2.3 
 
 32-cp with reflectors 
 
 Porch 
 
 
 
 I 
 
 
 
 Bedrooms (6) 
 Dressing rooms (2) 
 Servants' rooms (3). . . . 
 Bathrooms (3) 
 
 
 14 
 
 4 
 3 
 
 5 
 
 
 7.0 
 4-7 
 9.4 
 5O 
 
 
 Kitchen ) 
 
 
 q 
 
 
 
 
 Pantry j" 
 Halls ) 
 
 IO 
 
 3 
 
 
 
 
 Cellar J" 
 
 Closets (4) 
 
 A 
 
 
 
 
 Reflector lamps 
 
 
 
 
 
 
 
 Total 
 
 64 
 
 30 
 
 8 
 
 
 
 
 
 
 
 
 
210 THE ART OF ILLUMINATION. 
 
 The noticeable thing about this table is the large 
 number of 8-cp lamps. These are for the purpose of 
 giving good distribution of light in the rooms where it is 
 most necessary. The total is equivalent to 78 i6-cp 
 lamps, by no means a large installation for a house of 
 this size. In using gas, mantle burners should be used 
 where 32-cp lamps are indicated. These should always 
 be given pinkish or yellowish shades, to kill the green- 
 ish tinge of the light. Pink glass shades, or, better, 
 holophane globes, are useful, or very diaphanous orna- 
 mental fabric shades, lightly dyed with erythrosine, 
 aurine, or saffronine. The former is rather fugitive, al- 
 though perhaps the best in tint. In a room with red walls 
 of almost any shade, the diffused light partially corrects 
 the greenish tint of the radiant, but the light itself is too 
 bright to go without shading in any event. Mantle 
 burners greatly economize the use of gas, and when 
 properly shaded may be advantageously used almost 
 anywhere, since they use just about the same amount of 
 gas as ordinary burners and give about three times as 
 much light. They are much too powerful to give the 
 best artistic results, however, unless very cautiously 
 used. In applying them to a case such as that we have 
 just been considering, they should be regarded as 
 equivalent to two i6-cp incandescents, for, while really 
 somewhat brighter than this suggestion would indicate, 
 a single radiant is less effective than two, each of half 
 the given power. 
 
CHAPTER X. 
 
 LIGHTING LARGE INTERIORS. 
 
 THIS branch of illumination differs from ordinary 
 domestic lighting in several important particulars. In 
 the first place, the aid received from diffusion from the 
 walls is much less than in the case of smaller rooms, as 
 has already been indicated. The experiments of Fon- 
 taine indicate that within moderate limits the light re- 
 quired is determined by the volume of the space to be 
 illuminated, rather than by the floor space. That is, in 
 a given room, doubling the height of the ceiling should 
 double the light required for proper illumination. 
 
 Since, however, the only physical effect of the in- 
 creased height is to increase the mean distances of the dif- 
 fusing surfaces from the radiants and hence slightly to 
 
 diminish . the significant ratio I -;), the change 
 
 could, in point of fact, alter only that part of the total 
 illumination due to diffused light, provided that with in- 
 creased height of ceiling the radiants are not themselves 
 raised. Hence only in the case of walls capable of very 
 brilliant diffusion can the variation due to increased 
 dimensions alone approach the magnitude indicated by 
 Fontaine's empirical rule, which, however, possesses the 
 merit of causing one to err in the right direction and to 
 give ample illumination. 
 
 In large and high rooms there is a strong tendency 
 to increase the height of the radiants above the plane of 
 
212 THE ART OF ILLUMINATION. 
 
 illumination, especially in case of using chandeliers, 
 and this is perhaps an important factor in the rule afore- 
 said. Obviously in increasing the distance of the 
 radiants one decreases the direct illumination approxi- 
 mately in the ratio of the inverse squares of the dis- 
 tances, and does not materially improve the diffusion. 
 
 Therefore the illumination falls off seriously. In a 
 large and high hall lights arranged in the ceiling or as 
 a frieze, while often giving admirable effects, are quite 
 uneconomical, and should be used, if at all, with a full 
 appreciation of this fact. If for artistic or other reasons 
 the lights must be placed high, reflector lamps, or their 
 equivalent, are strongly to be recommended. 
 
 In large buildings, too, the quantity of light required 
 is subject to enormous variation, according to the pur- 
 poses to which the building is devoted, and whether the 
 whole interior must for artistic reasons be illuminated. 
 In a ballroom an effect of great brilliancy is generally 
 aimed at, while a room of equal size used as a factory 
 needs strong illumination only where it will facilitate the 
 work. 
 
 Again, in very large rooms the power of the indi- 
 vidual radiants can advantageously be increased, and 
 some sources of light inadmissible in domestic lighting 
 such as arc lamps and large regenerative gas burners, 
 or to be used only with caution, like mantle gas burners 
 may be used very freely. 
 
 But in large buildings, as elsewhere, the fundamental 
 purpose of the lighting is to produce a certain intensity 
 at the plane of illumination, which in such work should 
 be assumed about three feet above the floor. The abso- 
 lute illumination required may vary greatly, over a 
 range, in fact, as great as from half a candle-foot to two 
 
LIGHTING LARGE INTERIORS. 
 
 213 
 
 candle-feet or even more, but the lighting may properly 
 be calculated from an assumed value, just as in the case 
 already discussed. 
 
 For purposes of discussion, we may first consider a 
 hall 100 ft. long by 30 ft. high by 50 ft. wide. The plane 
 of illumination will then have an area of 5000 sq. ft., and 
 the total volume is 150,000 cu. ft. And for simplicity 
 
 -- 41- 
 
 *- -15 1 -! 
 
 
 r ->' 
 
 Fig. 89. Plan of Hall. 
 
 we will assume i candle-foot as the minimum intensity 
 to be permitted in any part of the space. Fig. 89 shows 
 the plan of this assumed space. We will first take up 
 the case of suspended radiants, which is the most usual 
 method of treating such a problem. 
 
 Obviously in a room of the shape given a single 
 radiant is out of the question, on the ground of econ- 
 omy, since in meeting the requirement of a given mini- 
 mum of illumination the most economical arrangement 
 is that which exceeds this minimum at the fewest points 
 possible. Two radiants give a possible solution, and are 
 worth a trial. Clearly they must be located on the 
 major axis of the room A B; but since a corner, as E, is 
 the most unfavorable spot to light, the radiants must be 
 placed well toward the ends of the room. We will as- 
 
2i 4 THE ART OF ILLUMINATION. 
 
 sume their height as 15 ft. above the floor, and 12 ft. 
 above the plane of illumination. 
 
 Now the best position of a given radiant a is easily 
 determined it is such that, calling the projections of 
 the points E and C upon the plane of illumination E 1 
 and C 1 , a C 1 = a E 1 ^2 , approximately. To fulfill this 
 condition Aa : =Bb = i5 f very nearly, and the two 
 radiants are at once located. In this case d~ = 994, 
 and since C = L d 2 , C should be practically 1000 candle- 
 power. Allowing ( ITT) = T '^' eacn f * ne radiants 
 
 should be of about 666 candle-power, a requirement 
 which could be practically met by a nominal 2OOO-cp 
 open arc, if its glare were not so forbidding. 
 
 Using incandescents, 42 of 16 candle-power would be 
 required in each group, which should be increased to 
 about 60 if ground bulbs in a chandelier were to be 
 used, since lamps so mounted interfere with each other's 
 effectiveness to a certain extent. Reducing these 
 figures to square feet per candle-power, it appears that 
 the assumed conditions are satisfied by allowing as a 
 maximum about 3.75 sq. ft. per candle-power, or with 
 allowance for properly softening the light, 2.6 sq. ft. 
 per candle-power. 
 
 Lighting such a space from two points only is usually 
 by no means the best way, and a much better effect 
 would be secured by using six radiants. The same rea- 
 soning which led us to place a and b near the ends of the 
 major axis of the room indicates a similar shifting in the 
 case of six lights. From symmetry, two should be on 
 the minor axis DOC, and as regards the projections of 
 C and on the plane of illumination, the best position 
 for a radiant, located in the same horizontal plane as 
 
LIGHTING LARGE INTERIORS. 215 
 
 before, is at a 1 , about 6' from 0, with fr 1 at a correspond- 
 ing point on the other side of 0. Now for the lateral 
 pairs of lights. One of them may be approximately 
 located with reference to E 1 , and the projection of the 
 middle point of the line to a 1 , much as a 1 itself was 
 located. This leads to a position c 1 , 4.1' from a 1 and 9' 
 from the wall. Forming now the equation 
 
 C = ^ -, <F = 306, ^ = 1906, 
 
 and the sum of the other terms is little greater than the 
 term in d*. Simplifying thus, the candle-power of each 
 radiant comes out very nearly 235, without allowance 
 for diffusion on the one hand or for ground bulbs and 
 incidental losses on the other. 
 
 Setting these off against each other, it appears that 
 the conditions call for 15, i6-cp lamps in each of the six 
 groups, a total of 90 as against 120 in the previous ar- 
 rangement. The total rated candle-power is then 1440, 
 or i candle-power for every 3.5 sq. ft. 
 
 It is interesting to check this computation, based en- 
 tirely on an assumed minimum illumination of i candle- 
 foot, with the result of experiment. For large rooms, 
 ranging from about 1000 to 5000 sq. ft. in area, Uppen- 
 born's careful investigations show that for good illumi- 
 nation 3 to 3.5 sq. ft. per candle-power is the amount 
 required in practice. In most cases these large spaces 
 are finished in light color, so that in spite of the high 
 ceilings they are scarcely more difficult to light than 
 ordinary dwellings. The absolute brilliancy required is 
 determined by the purpose of the illumination, and the 
 proper arrangement of the lights depends largely on 
 
2i6 THE ART OF ILLUMINATION. 
 
 architectural considerations. Oftentimes frieze and 
 ceiling lights are used in halls, and their application to 
 the case in hand is worth considering. 
 
 If arranged as a frieze, the lamps would be equally 
 spaced around the walls, at about 5 ft. below the ceiling, 
 bringing them 22 ft. above the plane of illumination. 
 For simplicity we will assume the use of 90 i6-cp re- 
 flector lamps, or their equivalent. Each gives approxi- 
 mately 27 candle-power in its hemisphere of illumina- 
 tion. These lamps would be spaced a little more than 
 3 ft. apart, giving 30 on each side of the hall and 1 5 on 
 each end. Now, taking for examination the corner E l , 
 which is as unfavorable a locality as any, and roughly 
 running up the illumination at this point, it falls a 
 little short of i candle-foot, but a diffusion factor of 1.25 
 would carry it just about to the required amount. With 
 lightly ground bulbs, which are far preferable to the 
 clear ones in such a case, an increase to 36 lamps on 
 each side and 18 o<n each end would be desirable, and 40 
 and 20 on sides and ends respectively would do still 
 better. 
 
 With the original 90 lamps the total rated hemi- 
 spherical candle-power would be 2430, which is at the 
 rate of 2.06 sq. ft. per candle-power. 
 
 Lighting from the ceiling would lead to a slightly 
 worse result, and it is safe to say that an increase of 30 
 to 50 per cent, in the total candle-power of the radiants 
 is required in changing to frieze or ceiling lighting from 
 pendent or side lights. Lights so arranged, however, 
 can give a very valuable groundwork of illumination 
 when re-enforced by lights more favorably placed. 
 They have the advantage of being unobtrusive and of 
 producing a generally brilliant effect, but give, if rsed 
 
LIGHTING LARGE INTERIORS. 217 
 
 to the exclusion of everything else, an illumination pain- 
 fully lacking in chiar-oscuro, and light directed almost 
 entirely downwards is, moreover, somewhat trying, like 
 a stage scene in the absence of footlights. 
 
 As has been already explained, the illumination at any 
 particular point should have a predominant direction, 
 else the effect on the eyes is apt to be serious, A room 
 lighted by brilliantly phosphorescent wall paper, for ex- 
 ample, would produce a most disagreeable effect unless 
 the luminosity were confined to one side, or, in general, 
 to limited portions of wall. 
 
 Something of the same objection appertains to ceil- 
 ing or frieze lighting when pushed to an extreme. In 
 the room under discussion, the best general effect would 
 probably be produced by combining pendent or brack- 
 eted lights with about an equal amount of illumination 
 from frieze or ceiling lights. If the room were to be 
 used for purposes like manufacturing, lighting from 
 rather powerful incandescents, in part with reflectors, 
 placed at a convenient height above the machines, would 
 be the most efficient procedure. 
 
 Where merely rough work is being done, arcs may be 
 effectively used, always, however, shaded by ground or 
 similar globes. These are distinctly cheaper, because 
 more efficient, than incandescents, but their light lacks 
 the steadiness desirable for work requiring close atten- 
 tion. Six 35O-watt arcs would give, in the room shown 
 in Fig. 89, very good illumination when placed in ap- 
 proximately the positions deduced for the six chande- 
 liers, with a total expenditure of 2100 watts as against 
 about 4500 watts required by the clustered incandes- 
 cents, and, say, 3600 watts required by about 36 pendent 
 32-cp lamps. In many cases less light than this would 
 
2i8 THE ART OF ILLUMINATION. 
 
 be required, and the total amount of energy could be 
 correspondingly reduced, but about the above ratios 
 would hold good. 
 
 From Fig. 89 it appears that in using arcs, about 2000 
 to 2500 sq. ft. may be assigned to each 5OO-watt arc, and 
 1000 to 1500 sq. ft to each 35O-watt arc. It should be 
 remembered that the enclosed arcs with inner globes 
 are somewhat less efficient than this, although greatly to 
 be preferred by reason of their steadiness, and that alter- 
 nating arcs are slightly less efficient than continuous- 
 current arcs. 
 
 Arcs do their best work when placed fairly high and 
 used in cases where protracted close attention on the 
 part of the workmen is not necessary. They are some- 
 what preferable to incandescents, too, when colored ob- 
 jects are to be illuminated. 
 
 In workshops where special objects are to be illumi- 
 nated, arcs are at a great disadvantage with respect to 
 the distribution of light, since their relatively small 
 number forbids placing them in the most advantageous 
 positions with respect to all the machines. They have, 
 in short, the disadvantage of being radiants too power- 
 ful for the best distribution. It is thus found that in 
 practical illumination arcs are considerably less efficient 
 than their actual candle-power would indicate. The 
 effect of the bright radiant upon the eyes, the rather 
 dense shadows and the slanting light at a distance from 
 the arc, unite to produce results that cannot be predi- 
 cated from photometric measurements alone. 
 
 For example, a 35O-watt open arc is, in point of mean 
 spherical candle-power, closely equivalent to ten 32-cp 
 incandescent lamps; but in an actual installation in-doors 
 there are few cases in which the arc could not be replaced 
 
LIGHTING LARGE INTERIORS. 219 
 
 by six such incandescents without detriment to the illumi- 
 nation. These interesting questions will be the object 
 of some future attention, but the obvious continuation 
 of the present problem is the adaptation of gas lighting 
 to the case in hand. 
 
 If mere illumination is the object to be attained, there 
 is little doubt that mantle burners should invariably be 
 used in rooms of the size considered. As already inti- 
 mated, each such burner of the ordinary size is equiva- 
 lent to about two i6-cp distributed incandescents. If 
 the lamps are grouped in each case, the mantle burner 
 must be given a rather better rating, being equivalent 
 to between 2.5 and 3 such incandescents. Properly 
 shaded, the mantle burner is a very economical and 
 effective illuminant. Were it not for the very objection- 
 able color of the unshaded light, it would be much more 
 extensively used than it is at present. 
 
 For lighting large areas, like the one we have been 
 considering, it is very well adapted, but if the lights are 
 placed high it is necessary not only so to shade them as 
 to correct the color, but they must in addition be fur- 
 nished with such shades or reflectors as will throw the 
 light downward ; for it must be remembered that mantle 
 burners must be placed with the mantle in a substan- 
 tially vertical position, and give the maximum intensity 
 of light a little above rather than below the horizontal 
 plane, while incandescent lamps, which we have been 
 chiefly considering, throw the light in more nearly a 
 spherical distribution, although really considerably de- 
 parting from it. Reflectors or holophane globes used 
 with the mantle burners will correct this faulty distribu- 
 tion and enable them to be used more effectively in the case 
 in hand. 
 
220 THE ART OF ILLUMINATION. 
 
 In rooms lower than that already considered it is de- 
 sirable to increase the number of radiants considerably, 
 to avoid too oblique illumination at the more distant 
 part of the field of each light. 
 
 With higher rooms, on the contrary, one can concen- 
 
 4 
 
 FIG. 90. Vertical Section of Hall. 
 
 trate the radiants more advantageously, and has con- 
 siderable more liberty of action in placing the lights. 
 
 Fig. 90 is intended to illustrate the conditions which 
 exist in a very high room of fairly large area. It shows 
 in vertical section a room supposed to be 50 ft. square 
 and 50 ft. high, the plane of illumination, a b, being 3 ft. 
 from the floor. We have here 2500 sq. ft. of floor 
 surface. At the ordinary rate of 3 sq. ft. per candle, 
 
LIGHTING LARGE INTERIORS. 221 
 
 this would demand 833 candle-power, or practically 52 
 i6-cp lamps, or, with a coefficient of diffusion of 1.50, 
 about 36 such lamps. 
 
 But the previous calculations having been made for a 
 room only one-half this height, and with lamps placed 
 considerably below the ceiling, it is clear that the greatly 
 increased height in the present case will lead to some- 
 what different conditions unless the lamps are to be 
 dropped very far below the ceiling so low as to pro- 
 duce a decidedly unpleasing effect. Lamps placed, for 
 example, in the plane c d, corresponding to frieze lamps 
 in the previous instance, are too low to look well, while 
 they would, on the basis just given, furnish the room 
 with satisfactory illumination. If placed on side 
 brackets at or below the plane c d, they would work well 
 on the floor, but would produce the effect of the ceiling 
 fading into dimness unless the ceiling itself had an ex- 
 tremely light finish. 
 
 Such a room, therefore, while very easy to light 
 thoroughly, is very difficult to light both thoroughly and 
 with good artistic results. Rooms of such dimensions 
 are seldom used for manufacturing purposes, these 
 shapes occurring more frequently in rooms for public 
 uses of various kinds. 
 
 Witho-ut going into detailed computation which the 
 reader can readily make for himself in the light of pre- 
 vious work on Fig. 89, it is safe to say that by far the 
 best general effects would be produced by placing per- 
 haps one-third of the total candle-power in 8-cp reflector 
 lamps as a frieze, 8 or 10 ft. below the ceiling, in the line 
 e, f, or thereabouts, and putting the remainder on brack- 
 ets, in groups of three to six, a little below the plane c d. 
 Such an arrangement obviously loses somewhat in the 
 
222 THE ART OF ILLUMINATION. 
 
 efficient disposition of light, on account of the great 
 height of the lamps in the frieze, which can be depended 
 on only for a rather faint groundwork of illumination on 
 the plane of illumination a, b. If, for example, the total 
 installation consists of 600 candle-po<wer, of which 200 
 is in the frieze, the mean distance of the frieze lamps 
 from a point, say, in the middle of the floor, would be in 
 the vicinity of 45 ft. 
 
 Consequently, allowing for the effect of the reflectors 
 of the frieze lamps, and for what each can do by diffu- 
 sion, it is safe to say that the frieze lamps would give an 
 illumination of not over one-fifth candle-foot on the 
 plane of illumination. Hence, something like eight- 
 tenths candle-foot would have to be furnished by the 
 lights upon brackets. The amount of light furnished 
 by these would therefore have to be about eight-tenths 
 of the total illumination, as determined by lights placed 
 in the relative position shown by Fig. 89, that is, the 
 ceiling lights of one-third the total candle-power really 
 would be furnishing not over one-fifth of the total light, 
 which means that for lights placed as just indicated, 
 the total candle-power installed should be increased 
 somewhere from 25 to 33 per cent., or rather more, as 
 the bracket lights cannot be conveniently placed in fav- 
 orable situations. 
 
 Hence in a room so illuminated it would not be safe 
 to allow more than 2 to 2^2 sq. ft. of floor space per 
 candle-power, and generally nearer the former figure 
 than the latter. To attempt the lighting of such a room 
 by frieze or ceiling lights, as ordinarily placed, would be 
 wasteful. If economy is not an important factor in de- 
 signing the illumination, at least half the lights may be 
 placed in the frieze with a distinct gain in artistic effect. 
 
LIGHTING LARGE INTERIORS. 223 
 
 In such case the total installation should be fully 50 per 
 cent, greater than the minimum required. We shall 
 see, however, that there are effective methods of getting 
 a strong groundwork illumination from above without 
 resorting to either of these methods. 
 
 To follow up the effect of raising the lights in a high 
 room still further, it is well to note that the critical point 
 is the amount of available diffusion. If one were deal- 
 ing with a room lined with black velvet, or with translu- 
 cent walls, in which there is only a very minute amount 
 of diffused light, raising the lights would diminish the 
 illumination almost exactly according to the law of in- 
 verse squares. 
 
 Writing now K for the coefficient of diffusion denoted 
 
 by the fraction ( _ ^ I , and recurring to the formulae 
 
 previously given for illumination, we have at once K C 
 = L d 2 , and for fixed values of C and L, d = P ^/ K, 
 where P is a constant. Hence we may conclude that 
 for any desired value of the illumination with a fixed 
 amount of lights available, the height to which these 
 lights can be raised and still produce the required effect 
 is approximately proportional to the square root of the co- 
 efficient of diffusion. 
 
 The moral of this is tolerably obvious. If one deals 
 with a dome finished, let us say, in white and gold, it 
 may be permissible to place a large part of the lights 
 fairly high up, while in a church with a vaulted roof in 
 dark oak, lights placed high are nearly useless for pur- 
 poses of illumination. In such a case lights placed at 
 the level of the roof beams and unprovided with re- 
 flectors have barely more than a decorative value, and 
 should be treated essentially as a decorative feature, use- 
 
224 THE ART OF ILLUMINATION. 
 
 ful for bringing out the details of the architectural 
 design. 
 
 Any real illumination must be accomplished by lamps 
 with reflectors or by lamps placed down nearer the plane 
 of illumination. In these dark interiors reflector lamps 
 can be used to especial advantage, since the coefficient 
 of diffusion is so small that the lessened diffusion due to 
 the partially directed beams from reflector lamps is of 
 trivial consequence. In fact, there are few cases in 
 which reflectors cannot be used to advantage in rooms 
 having very high roofs. 
 
 Churches are generally badly lighted, and are, in fact, 
 rather difficult of treatment, if of any considerable size. 
 They are seldom brilliant in interior finish, usually have 
 rather high vaulted roofs, and require good reading illu- 
 mination. The few cases in which their form approxi- 
 mates to Fig. 89 may easily be treated as there indicated, 
 but such is not the usual condition. Fig. QI gives a 
 roughly typical church floor plan as regards the main body 
 of the building. The total floor space is shown as 5000 
 sq. ft. in the nave and choir" combined, and 800 sq. ft. in 
 each transept. The walls are assumed to be 30 ft. high in 
 the clear, with a Gothic roof above. Now the total area to 
 be lighted is 6600 sq. ft., and the value of K is low, not 
 safely to be taken as exceeding 1.20. The peculiarities of 
 the building, as a problem in lighting, lie in the high walls 
 and the absence of any ceiling, both of which complicate 
 matters. 
 
 As to the nature of the radiants, when electric lights 
 are available, one must depend almost entirely upon in- 
 candescents. Arc lamps are not to be considered for 
 artistic reasons, save perhaps in indirect lighting of the 
 choir. If only gas is available, mantle burners suitably 
 
LIGHTING LARGE INTERIORS. 
 
 225 
 
 and thoroughly shaded had better be the main reliance, 
 as ordinary gas flames are seldom steady in such a place. 
 In either case avoid chandeliers as you would shun 
 poison. A huge circle of lights pendent from a Gothic 
 
 Fig. 91. 
 
 roof is about as bad technically and artistically as any- 
 thing that could be imagined. 
 
 As to the amount of light needed, it would be ad- 
 visable to allow no more than 2.5 sq. ft. per candle- 
 power, which, taking K at 1.20, would call for 2200 net 
 
226 THE ART OF ILLUMINATION. 
 
 candle-power. In point of fact, in using electricity, not 
 less than 150 i6-cp lamps should be used, and even this 
 number, on account of the trying conditions, would 
 have to be very deftly arranged to give the required re- 
 sult. For the best effect they should be chiefly reflector 
 lamps, assigned about as follows: 90 to the nave, 20 to 
 the choir, and 20 to each transept. As to position, the 
 most efficient method would be to put them in groups 
 of six or eight on brackets between the windows, at half 
 to two-thirds the height of the wall, with possibly larger 
 groups massed at the four corners of the crossing. 
 With still more lights available very beautiful results 
 could be attained by adding lights at the capitals, and, 
 in some cases, along the tie-beams, or on the brackets 
 from which the pendent-posts rise. These latter ar- 
 rangements are very effective, but not economical, and 
 if used should be installed on the basis of about i candle- 
 power per 2 sq. ft. of floor surface. All incandescent 
 lamps used without diffusing shades should have ground 
 bulbs. 
 
 In lighting with gas, brackets are about the only 
 thing feasible, since the flames must point upward, and 
 few capitals would fail to look overloaded with ade- 
 quately shaded burners. Mantle burners, of course, do the 
 work most efficiently, but used alone the effect is certain 
 to be grimly utilitarian, and especially around the choir 
 small ordinary jets may be used to very great advantage. 
 The mantle burners should be as unobtrusive as pos- 
 sible in such a case, even if they do the main work of 
 the illumination. 
 
 Only the barest hints can be given for the detail of 
 church lighting, as so much depends on the archi- 
 tectural peculiarities and on the scheme of decoration, 
 
LIGHTING LARGE INTERIORS. 227 
 
 but the foregoing indicates the general principles to be 
 followed. The most important thing is to give a rather 
 brilliant illumination without the individual radiants 
 obtruding themselves unpleasantly on the eyes of the 
 congregation. 
 
 Large public buildings are generally easier to light 
 than churches, since they are, as regards the shape of 
 the several rooms, comparatively simple and are seldom 
 dark in finish. Many rooms may be illuminated along 
 the lines already laid down, but, on the whole, powerful 
 radiants, such as arc lights, may be more freely used 
 here than elsewhere, thereby effecting a very considerable 
 economy. 
 
 In high corridors and high halls without galleries arc 
 lights can be used with very excellent results. They 
 should invariably be shielded by ground or opal globes, 
 and, if hung very high, as is sometimes desirable, to keep 
 them out of the ordinary field of vision, should be pro- 
 vided with reflectors. They should be numerous 
 enough to suppress the shadows that ordinarily ,exist 
 under the lamps. In the absence of such shadows the 
 modern enclosed arcs have a very material advantage. 
 
 Rooms lighted by arc lamps ought to be of light 
 finish, since the lamps must be placed rather high to 
 keep them, even shaded, from glaring unpleasantly, and 
 they give a strong nearly horizontal beam which, in lack of 
 good diffusing surfaces, is for the most part wasted. 
 Reflectors deep enough to turn this downward would 
 usually be most unsightly and would give an unpleasant 
 searchlight effect, which should be avoided. 
 
 Never let the eye rest simultaneously on arc and in- 
 candescent lamps indoors or out, since the latter seem 
 very dim and yellowish in such company, and will never 
 
228 THE ART OF ILLUMINATION. 
 
 be credited with anything- like their real brilliancy. 
 Similar reasoning applies to the use of mantle burners 
 and ordinary gas jets in the same room. When so used 
 the former should be well shaded and unobtrusively 
 placed, and the latter massed and generally unshaded or 
 lightly shaded, so as not to seem of relatively very small 
 intrinsic brilliancy. 
 
 Sometimes in large interiors the powerful regener- 
 ative burners may find a place. They give an excellent 
 downward illumination, which is occasionally very 
 useful. 
 
 Theaters present some very interesting problems in 
 illumination on account of their peculiar shape and the 
 difficulty of lighting the interior with sufficient bril- 
 liancy without making the radiants altogether too con- 
 spicuous. They are, as a rule, far more brightly lighted 
 than other interiors, but seldom judiciously. The 
 usual fault is to place the lights so that they shine di- 
 rectly in the eyes of a considerable part of the audience. 
 The auditorium is commonly very high in proportion to 
 its area, and plentifully supplied with galleries. Fig. 
 92 shows the typical elevation, the floor plan being gen- 
 erally only slightly oblong. The galleries, of course, 
 sweep around the sides, narrowing as they near the 
 proscenium boxes. Not infrequently a fourth gallery is 
 added. 
 
 During the acts no very considerable amount of light 
 is needed, but between them it is generally desirable to 
 produce an effect of great brilliancy. The main floor is 
 far below the roof, and the shelving galleries render it 
 difficult to light the spaces between them. The general 
 fittings are usually light, but the dull hue of the floor and 
 galleries when occupied kills much of the diffusion. 
 
LIGHTING LARGE INTERIORS. 
 
 229 
 
 The actual floor space to be dealt with as a problem 
 in illumination includes the galleries, and hence greatly 
 exceeds the area of the main floor. Assuming the 
 width in Fig. 92 to be 50 ft., the nominal area in front of 
 the footlights is 3000 sq. ft. The total gallery area is 
 
 1 
 
 Fig. 92. Elevation of Theater. 
 
 usually from i to 1.5 times the floor space, so that the 
 entire space to be lighted would be at least 6000 sq. ft., 
 half of it being located so that it can get little advantage 
 from the illumination of the main space above the floor. 
 The space behind A, and the galleries B, C, and to a less 
 extent D, have to be treated almost as separate rooms, 
 
2 3 o THE ART OF ILLUMINATION. 
 
 particularly when, as sometimes happens, the galleries 
 are rather lower than shown in Fig. 92. 
 
 This is the main reason for the apparently abnormal 
 amount of light that is needed in theaters. The fact is 
 that there is really a very great area to light, and it is 
 so placed that it cannot readily be treated as a whole. 
 The following table shows the approximate amount oi 
 illumination furnished in a number of prominent Conti- 
 nental theaters. 
 
 If in Fig. 92 we allow, on account of the high ceiling 
 and conditions unfavorable for diffusion, 2 to 2.5 sq. ft. 
 per candle-power, and take account of the real total floor 
 space, including the galleries, we reach just about the 
 figures given, which are based on the floor plan only. 
 And in practice 3600 candle-power would probably do 
 the work well, although, since this only allows ordi- 
 nary good reading illumination, more light is neces- 
 sary to give the really brilliant effect which is usually 
 desired. Nearly 5000 candle-power would be required 
 to show off the house effectively. 
 
 THEATER. SQ. FT. PER CP. CP. PER SQ. FT. 
 
 Opera, Paris. 0.78 1.28 
 
 Opera, Paris, as ballroom 0.38 2.63 
 
 Odeon, Paris 1.52 0.66 
 
 Gaiete, Paris 1.14 0.87 
 
 Palais-Royal, Paris 0.51 1.96 
 
 Renaissance, Paris 0.52 1.92 
 
 La Scala, Milan 1.07 093 
 
 Massimo, Palermo (ordinary) 0.86 1.16 
 
 Massimo, Palermo (en fete) .. 0.53 1.88 
 
 As to the location of the lights and their character, 
 the body of the house can be usefully lighted by lamps 
 ranged along the galleries at a b c. If these are placed 
 below the edges of the galleries they will glare directly 
 into the eyes of the spectators, so that it is better to illu- 
 minate the gallery spaces from the rear and above, at 
 
LIGHTING LARGE INTERIORS. 231 
 
 a' b r c. The radiants may well be provided with re- 
 flectors, as the diffusion amounts to little, and all lamps 
 on and under the galleries should have ground globes. 
 These lights may be re-enforced to great advantage by 
 ceiling reflector lamps, best sunk in the ceiling deep 
 enough to make them inoffensive from the galleries. 
 These, with some ornamental lighting about the stage 
 and boxes, should give a capital result. The main point 
 is to light the interior brightly without thrusting bright 
 radiants into the field of vision. 
 
 A very beautiful example of theater lighting is shown 
 in the frontispiece, a photograph from the stage of the 
 Metropolitan Opera House, decorated for the performance 
 in honor of Prince Henry of Prussia. The temporary 
 festoons from the ceiling are highly decorative, but better 
 suited to temporary than to permanent use, since they 
 shine directly into the eyes of the occupants of the gal- 
 leries. In this instance the curtain was brilliantly studded 
 with temporary incandescents, and the whole interior was 
 elaborately decorated. 
 
 A useful form of ceiling lighting, applicable to many 
 very high interiors, is arranged by replacing the lamps 
 at d, Fig. 92, by opal glass skylights of rather large di- 
 mensions, and placing above them arc lamps with re- 
 flectors. The skylight surfaces should be flat or slightly 
 projecting rather than recessed, and the reflectors 
 should be planned so that each may throw a cone of 
 light subtending an angle equivalent to the whole floor 
 plan. 
 
 By thus superposing the indirect illumination from a 
 group of lamps the general steadiness of the light is 
 greatly increased. In thus using arcs care should be 
 taken to have the diffusing skylights faintly tinted so as 
 
232 THE ART OF ILLUMINATION. 
 
 to lessen the color contrast between the powerful ceiling 
 lights and the incandescents used elsewhere in the 
 house. It is a considerable advantage thus to place 
 lights above the ceiling, as it avoids the serious heating 
 effect due to massing incandescents near the ceiling of 
 a generally overheated room. 
 
 On account of this heating the use of gas in theaters 
 is highly undesirable, and has been almost completely 
 abandoned. In lack of anything better, fair results 
 could be reached by mantle burners placed somewhat as 
 shown in Fig. 92, and very thoroughly shaded by holo- 
 phane or other diffusing globes, much of the illumination 
 being located above the ceiling in the form of mantle or 
 regenerative burners. 
 
 Large and high spaces cannot often be lighted very 
 efficiently, as the conditions ordinarily preclude placing 
 powerful radiants near the plane of illumination. The 
 natural riposte is to use highly efficient radiants, and with 
 them to employ reflectors freely. Hence the form of 
 ceiling illumination just explained. 
 
 In very large interiors without high galleries, arc 
 lighting may be very effectively used, provided the arcs 
 are well shaded. It is wise to group them so that no 
 single arc shall entirely dominate the illumination at 
 any particular point. It is better to lose a little in uni- 
 formity of the total illumination throughout the area 
 than to take the chances of flickering, which is not en- 
 tirely suppressed even in the best arc lamps. 
 
 In a big space arcs can be treated much like incandes- 
 cents in a small space, but the detail of the work varies 
 so much that only very general suggestions can be 
 given. Often temporary illumination has to be under- 
 taken, and must be fitted to the case in hand. One of 
 
LIGHTING LARGE INTERIORS. 233 
 
 the most beautiful examples of such work that ever fell 
 under the author's notice was the illumination of Madi- 
 son Square Garden for a chrysanthemum and orchid 
 show a few years since. The feature of this work was 
 the very extensive use of both arc and incandescent 
 lamps enclosed in Chinese lanterns. The huge lanterns 
 containing the arcs were very striking, and the whole 
 effect was most harmonious, while the illumination was 
 thoroughly good. It is mentioned here merely as a 
 clever bit of temporary lighting treated to suit the par- 
 ticular occasion. 
 
 In this lighting of large interiors the smaller arcs 
 worked on constant potential circuits are very useful, 
 although not very efficient. Those taking 5 to 6 am- 
 peres give excellent service, and fair results can be 
 obtained with lamps working down even to 4 amperes. 
 Such arcs are equivalent to from 10 to 15 i6-cp lamps in 
 practical effect, and give a greater candle-power per 
 watt. 
 
 Incandescent lamps of the Nernst type, if reduced to 
 a practical form, may be utilized in a similar way, in 
 forming a good basis of illumination where the total 
 amount of light is considerable. In other words, when 
 one is dealing with very large enclosed spaces the light- 
 ing is simplified and made more efficient by utilizing the 
 more powerful radiants. 
 
 Large incandescent lamps giving from 50 to 100 
 candle-power, or even more, would be very valuable but 
 for their high cost and generally rather inadequate life 
 and efficiency. Those of 50 candle-power are pretty 
 satisfactory, in spite of their share of these drawbacks, 
 but the larger sizes are much less advantageous. 
 
 In certain cases, particularly railway stations and 
 
234 THE ART OF ILLUMINATION. 
 
 other buildings likely to be rather smoky, arcs have to 
 be the main reliance, since the globes of incandescents 
 grow dim so quickly that cleaning them is an almost 
 interminable job. Hence it is best to use compara- 
 tively few powerful radiants. The arcs should be 
 carried rather high, say 20 to 25 ft. above the ground or 
 floor. Assuming 0.5 candle-foot as the minimum, and 
 taking into account the illumination due to adjacent 
 lamps, each arc can be counted on to illuminate over a 
 distance at which it gives 0.25 candle-foot. For close 
 detail reference must be made to the actual illumination 
 curves of the type of lamp used, and the general prob- 
 lem is analogous to street lighting, but for lamps 25 ft. 
 above the floor the approximate distance between arcs 
 of the commoner kinds, to give the required illumina- 
 tion, may be derived from the following table: 
 
 APPROXIMATE DISTANCE 
 
 KIND OF ARC. WATTS BETWEEN SQ. FT. 
 
 PER ARC. ARCS. PER ARC. 
 
 Direct current, open, 6.6 amperes 330 80 6,400 
 
 Direct current, open, 9.6 amperes 480 105 11,025 
 
 D. C. enclosed, 6.6 amperes 480 90 8,100 
 
 Alternating enclosed, 6.6 amperes 425 75 5.625 
 
 The alternating arc would do relatively better if 
 placed, say, 15 ft. high, since its light is thrown more 
 nearly horizontally, and since all the arcs are assumed in 
 this table to have clear outer globes, the open arcs, if 
 given pale opal globes, which they should have to lessen 
 the unpleasant glare, will be about on a par with the en- 
 closed arcs. All arcs in enclosed places should have at 
 least one opal globe, and when used where, as in railway 
 stations, diffuse reflection is of small amount, should be 
 provided with reflectors to utilize the light that would 
 otherwise be wasted. This would somewhat improve 
 
LIGHTING LARGE INTERIORS. 235 
 
 the figures given above, and it is quite safe to say that 
 under ordinary circumstances, with properly placed 
 arcs, one arc taking about 6.5 amperes is good for nearly 
 10,000 sq. ft. of floor space at the assumed intensity of 
 illumination. More lights are often desired locally in 
 places where considerably more than 0.50 candle-foot is 
 required, as in the central part of a passenger platform, 
 but they seldom would have to be placed nearer than 60 
 ft. apart, unless the traffic is exceptionally great. 
 
 Certain classes of interiors require, on account of the 
 uses to which they are put, especial adaptations of the 
 radiants, either in kind, amount, or position. One of the 
 commonest demands is for an illumination of unusual 
 brilliancy and steadiness in situations like reading 
 rooms, draughting rooms, schools, weave shops, and 
 such like places, where the eyes are under steady, if not 
 severe strain. Ordinary good reading illumination, 
 such as we have been considering, must be considerably 
 strengthened to meet these requirements. Simple in- 
 crease in the number or power of the radiants sometimes 
 meets the conditions, if such increase can be had with- 
 out thrusting too powerful lights into the field of vision. 
 
 It may be necessary to furnish i candle-power for each 
 2 sq. ft. of area, or, in extreme cases, i candle-power 
 per square foot. One of the most useful schemes for 
 supplying such large amounts of light is the use of the 
 inverted arc in connection with a very light interior 
 finish. 
 
 The ordinary continuous-current arc, in virtue of the 
 brilliant crater of the positive carbon, throws its light 
 downward; but if the current be reversed so as to form 
 the bright crater on the lower carbon, most of the light 
 is thrown upward toward the ceiling, and is there dif- 
 
236 THE ART OF ILLUMINATION. 
 
 fused. If, as usual, these arcs are arranged with in- 
 verted conical reflectors of enameled tin or the like, all 
 the direct rays are cut off and the entire illumination is 
 by the diffused rays. The result is a very soft and uni- 
 form light, white in color, and of any required brilliancy. 
 Fig. 93 shows in diagram the principle of this device. 
 In case a white ceiling is not available, large white dif- 
 fusing screens over the lamps, of enameled tin or even 
 of tightly stretched white cloth or paper, answer the pur- 
 pose. Indeed, this was the original form of the device 
 as shown by Jaspar at the Paris Exposition of 1881. 
 
 With reference to Fig. 93, it is sufficient to note that 
 the conical reflector should be rather shallow, just deep 
 enough to throw the light wholly on the ceiling and 
 upper walls, but shallow enough for two neighbor- 
 ing lights, as shown, to distribute light over each 
 other's fields, which improves the average steadiness of 
 the illumination. The arcs need no diffusing globes, a 
 clear globe being sufficient, and open arcs may be freely 
 used, to the material improvement of the luminous effi- 
 ciency, never very high in this form of lighting. 
 
 The heights of the arcs should depend somewhat on 
 circumstances regarding the appearance and the purpose 
 of the lights, but will generally be half to three- fourths the 
 height of the room. The reflectors may be from 3 ft. 
 to 6 ft. in diameter, and may have an angle at the apex 
 of 1 20 degrees to 140 degrees. Only in case of having 
 to throw the light on special screens rather than on the 
 natural ceiling should the reflectors have less aperture 
 than just indicated. They then become of the nature of 
 projectors, and the angle at the apex may be 90 degrees 
 or so. 
 
 As to the efficiency of such illumination, one may 
 
LIGHTING LARGE INTERIORS. 237 
 
 roughly assume i watt per spherical candle-power for 
 powerful open continuous-current arcs, and may reckon 
 on a loss of about one-half in the process of diffuse re- 
 flection. The diffuse illumination may then be taken as 
 being in candle-power about 0.5 to 0.6 the number of 
 watts expended, not including artificial resistance. 
 Thus, a continuous-current arc, taking 9 amperes to 10 
 amperes at about 50 volts, utilized in this manner will 
 
 Fig. 93. Lighting by Inverted Arcs. 
 
 illuminate 250 sq. ft. to 300 sq. ft. on the basis of i sq. ft. 
 per candle-power, or 500 sq. ft. to 600 sq. ft. at 2 sq. ft. 
 per candle-power. 
 
 It must be noted that if enclosed arcs are used in this 
 way, materially less light is obtained, as is well known. 
 Even with both outer and inner globes clear one cannot 
 count on much better than 2 watts per mean spherical 
 candle-power, although occasional results of 1.5 watts 
 or a little below are attained. Alternating lamps re- 
 quire, of course, still more energy, and with enclosed 
 
238 THE ART OF ILLUMINATION. 
 
 arcs in general one would hardly find it advisable to 
 allow, when using ceiling diffusion, more than half to 
 three-fifths of the area per watt just indicated for open 
 arcs. Enclosed arcs have no marked crater, which 
 operates somewhat against their effectiveness in this class 
 of lighting. 
 
 These figures are necessarily only approximate, but 
 while enclosed arcs have some conspicuous virtues, high 
 efficiency as respects mean spherical candle-power is not 
 one of them. In all this lighting by diffusion the diffus- 
 ing surfaces must be kept clean, else there will be much 
 loss of light. Under even the best conditions one does 
 not do very much better than 2 watts per candle power, 
 and lack of care or bad engineering may easily trans- 
 form this into 3 or 4 watts per candle-power, which is no 
 better efficiency than incandescents would give. 
 
 The chief advantage of this diffused lighting is that it 
 enables one to secure very brilliant illumination with 
 white light, without trying the eyes with intense 
 radiants. 
 
 This illumination has, however, one curious failing, in 
 that as ordinarily installed it is slwdoidess , and the light 
 has no determinate direction. For certain kinds of 
 work this is a very trying peculiarity, severely felt by ( 
 the eyes. It may be remedied in various ways, of which 
 perhaps the simplest is the lateral displacement of the 
 lamps shown in Fig. 94. 
 
 This gives a predominant direction to the light, some- 
 thing akin to the effect produced by a row of windows 
 along the side of the room, and is probably as near an 
 approach to artificial daylight as can be attained by 
 simple means. It is not unlike in principle the " arti- 
 ficial moon " used in the reading room at Columbia 
 
LIGHTING LARGE INTERIORS. 239 
 
 University, consisting of a great white ball intensely 
 illuminated by arcs backed by projectors. 
 
 In using the arrangement of Fig. 94, about the same 
 relative number of arcs is required as in Fig. 93, but 
 they are placed in one row instead of two. The uni- 
 lateral effect could be greatly enhanced by a diffusive 
 screen a b, Fig. 94, running along back of the arcs. Its 
 
 Fig. 94. Unilateral Illumination. 
 
 angle with the ceiling evidently should depend on the 
 shape of the room. 
 
 Unilateral illumination, whether diffused or not, is 
 often desirable from a hygienic standpoint. In many 
 cases well-shaded arcs may replace the diffused lighting 
 just described, though such direct lighting is generally 
 rather less steady. But it must be remembered that an 
 arc having both inner and outer globes opalescent is 
 scarcely, if at all, more efficient than incandescent lamps, 
 assuming both to be worked off constant potential 
 mains; hence, unless the whiteness of the arc light is 
 
240 THE ART OF ILLUMINATION. 
 
 essential, incandescents, being steadier, are very often 
 preferable. 
 
 In factories where colored fabrics are woven, and in 
 shops where they are sold, white illumination is a mat- 
 ter of great importance, and arcs are especially useful. 
 In the mills the necessary illumination depends largely 
 on the color of the fabrics. It should, as a matter of 
 experience, range from 2 sq. ft. per candle-power to i 
 sq. ft. per candle-power in passing from white to dark 
 and fine goods. The candle-power noted here is actu- 
 ally mean spherical, or hemispherical, if reflectors are 
 used, taken from the real performance of the arc well 
 shaded. This qualification means practically 300 sq. ft. 
 to 400 sq. ft. for each arc of 450 watts to 500 watts in 
 the extreme case, and 600 sq. ft. to 800 sq. ft. for white 
 and light-colored goods. Shops where such goods 
 must be sold by artificial light should be lighted on very 
 nearly the same basis. For brilliant illumination, where 
 color distinctions must be accurately preserved, the arc 
 at the present time stands pre-eminent, and should gen- 
 erally be used, although Nernst lamgs and acetylene 
 flames have a similar advantage. It must be remembered, 
 however, that enclosed arcs are distinctly bluish unless the 
 current is pushed up nearly to the limit of endurance of the 
 inner globes, and hence when used in situations where 
 color is important, should have shades tinted to correct 
 this idiosyncrasy. The common opalescent inner globe 
 is entirely insufficient for the purpose. 
 
 Where arc lights are not available, and it is desired to 
 furnish approximately white light, there is difficulty in 
 meeting the requirement. Mantle gas burners with ex- 
 treme care in selecting tinted shades to correct the gen- 
 erally greenish cast of the light may be made to give 
 
LIGHTING LARGE INTERIORS. 241 
 
 fair results, but are considerably inferior to arc lights. 
 Incandescent lamps fail to meet the requirement, and 
 perhaps the closest approximation to the arc in the matter 
 of color is to be found in the acetylene flame or in the 
 Nernst lamp. 
 
 In the lighting of workshops for various purposes, no 
 such brilliant illumination as has been mentioned with 
 reference to textile factories is required. The most 
 economical scheme of illumination is to furnish general 
 illumination in moderate amount, and to re-enforce it, in 
 points where brilliant light is needed, by extra lights at 
 these places. So far as the general illumination is con- 
 sidered, i candle-power to about 4 sq. ft. or 5 sq. ft. is 
 ample. The extra lights should be put bearing as di- 
 rectly as possible on the work in hand, and should fur- 
 nish illumination at that work to the extent of from i 
 candle-foot to 2 or 3 candle feet, according to the needs 
 of the work. 
 
 It should not be forgotten that good illumination in 
 a workshop tends materially to increase the quantity and 
 improve the quality of the work turned out. 
 
 In most instances the color of the light within the 
 range of ordinary illuminants is not a matter of consider- 
 able importance, but the light must always be reasonably 
 steady. Hence the incandescent lamp and the mantle 
 burner for gas are by far the most valuable sources of 
 light generally to be found. Ordinary bat-wing gas 
 burners are probably the worst in point of steadiness, 
 although a badly adjusted electric arc is a close second. 
 
 Where very powerful radiants are desired, the large 
 regenerative gas burners give a very brilliant and steady 
 light. They throw out, however, a great deal of heat, 
 which is sometimes objectionable, and are less econom- 
 
242 THE ART OF ILLUMINATION. 
 
 ical of gas than the mantle burner. If the Nernst in- 
 candescent lamp is brought, as it promises to be, into 
 commercial usefulness, it will prove exceedingly valua- 
 ble for the illumination of large enclosed spaces, by 
 reason of its considerable power and the whiteness and 
 steadiness of its light. At the present time it is not far 
 enough past the experimental stage to be a serious com- 
 petitor of other illuminants. 
 
 A very special branch of illumination is the lighting of 
 immense enclosed spaces, such as are found in exposi- 
 tion buildings. This work is on such a large scale that 
 it almost partakes of the nature of outdoor lighting, with 
 which it is very intimately connected as a practical prob- 
 lem. The amount of light required in single enclosed 
 spaces of colossal dimensions, like exposition halls, varies 
 considerably according to the practical use to which the 
 space is to be put. As a rule, the most brilliant and 
 useful illumination in these large spaces is secured by 
 the use of arc lights to the exclusion of other illumi- 
 nants. In a building covering one or several acres, and 
 perhaps 100 ft. or more in height, incandescents of 
 ordinary powers look lost; and if the roof is not to 
 fade away into darkness, a very large number of lights 
 must be required to bring it into prominence, placed so 
 high from the floor as to be of little service for the gen- 
 eral illumination. 
 
 Moreover, such buildings have generally a very large 
 amount of glazed side and roof space, furnishing the 
 ordinary daylight illumination. Consequently the walls 
 and ceiling diffuse very little light. With arc lights the 
 power of the individual radiants bears some respectable 
 proportion to the size of the space to be illuminated. 
 The luminous efficiency is increased, and by sufficient 
 
LIGHTING LARGE INTERIORS. 243 
 
 massing of lights with reflectors, even the highest halls 
 can be admirably lighted. The work can, of course, be 
 beautifully done with incandescents if enough are avail- 
 able, but at considerably lessened economy. 
 
 The amount of light required per square foot of floor 
 space is very considerable, owing to the height and bad 
 diffusing properties of the building, and for the best re- 
 sults i actual candle-power should be furnished for each 
 2 sq. ft. to 3 sq. ft., according to conditions. 
 
 Incandescent lamps have a very high decorative value 
 in connection with such work, but to be used effectively 
 must be massed somewhere near the plane of illumina- 
 tion, lights in and about the roof being practically only 
 for decorative purposes. Used in sufficient numbers, 
 however, they give, in virtue of their complete sub- 
 division of the illumination, a better artistic result than 
 can be obtained with arcs. 
 
 The subject of exposition illumination is so large and 
 so special in its character as to be hardly appropriate to 
 the scope of the present work. 
 
CHAPTER XL 
 
 STREET AND EXTERIOR ILLUMINATION. 
 
 A SPECIAL and very important department of illumina- 
 tion has to do with streets and other outdoor spaces. 
 It involves not a few unusual difficulties, for there is un- 
 limited space to deal with, as well as an indefinite variety 
 of natural and artificial obstructions. Save in narrow 
 streets bordered by high buildings, one gains little or 
 nothing from the diffusion that is so important a factor 
 in interior lighting, and in many instances the streets are 
 so thickly shaded by trees that the problem of adequate 
 lighting is very difficult. 
 
 Light above the horizontal plane is of comparatively 
 little value, \vhile the ground should receive a strong and 
 even illumination. In the comparative freedom from re- 
 flecting and diffusing surfaces there is, however, one 
 small gain, for this is the case in which the law of inverse 
 squares holds good. Of course, there is some diffusion 
 from the street when the ground is snow covered, a 
 considerable amount but, broadly, one can compute 
 the illumination with fair accuracy. 
 
 In computing the illumination, however, two radically 
 different methods have come into use. In one of these 
 the radiant in street lighting is supposed to illuminate 
 a geometrical plane, of which each element receives 
 illumination depending, not only on its distance from the 
 radiant, but on the obliquity of the rays which strike it. 
 The other method assumes that in reckoning the illumi- 
 
 244 
 
EXTERIOR ILLUMINATION. 245 
 
 x 
 
 nation at any point the element of surface to be con- 
 sidered is not in the plane of the gro\ind, but in a plane 
 perpendicular to the direction of the ray. 
 
 The first method would determine the legibility of a bit 
 of newspaper lying fiat on the pavement, the second method 
 the visibility of a stone projecting above it. The distinc- 
 tion might at first appear like that between tweedle-dum 
 and tweedle-dee, but in fact it makes an absurdly great 
 difference in the theoretical illumination. Fig. 95 
 shows the conditions which arise in the two cases. As- 
 sume a light L of 1000 uniform spherical candle-power 
 at the top of a pole 25 ft. high, and then calculate the 
 
 Fig. 95. Illumination at a Surface. 
 
 illumination at a surface S, 100 ft. from the foot of the 
 pole, by the law of inverse squares on the two hy- 
 potheses. In the first place, taking account of the 
 obliquity, calling the illumination /, we have 
 
 L sin a ,, .. 
 
 / = a g = 0.023 candle feet. 
 
 But considering S, the surface, as on a projecting cob- 
 blestone at the same point, 
 
 / = , a ^ 0.094 candle feet. 
 And the worst of the discrepancy is that it is greatest at 
 
246 THE ART OF ILLUMINATION. 
 
 considerable distances from the radiant, where the light 
 is feeble at best. 
 
 Now in point of fact, the object of lighting streets is 
 neither to enable one to read a page laid flat on the pave- 
 ment nor to observe a surface perpendicular to the ray. 
 But the intensity and distance of the radiants is deter- 
 mined by the minimum illumination allowable midway 
 between lights, and not by the brightness of the regions 
 near the poles, and in these comparatively dimly lighted 
 spaces the things which must be observed and for which 
 the users of streets keep their eyes open are things not 
 in the plane of the pavement, but above it. For ob- 
 serving these the obliquity of the light as regards the 
 pavement does not much matter; in fact, at equal inten- 
 sities oblique light gives stronger contrasts than per- 
 pendicular light. 
 
 Under these circumstances, however, one is justified 
 in reckoning upon the illumination received from one 
 direction only. A printed page laid on the pavement at 
 the point S receives useful light from both the neighbor- 
 ing radiants, and so, if 5* is halfway between two lamp- 
 posts, it will really receive on the first hypotheses 0.046 
 candle-foot. But a projecting stone or the face of a 
 wayfarer is illuminated so far as a given viewpoint is con- 
 cerned by only one of the adjacent radiants. 
 
 For these reasons, in the ensuing discussion, the illu- 
 mination at a point 6" will be assumed to be that which 
 falls on either hemisphere, let us say, of a billiard ball at 
 S. It will then be reckoned by the second formula, 
 
 /== #> + </ 
 
 but only one radiant, or the radiants on one side only, 
 will be considered as useful. 
 
EXTERIOR ILLUMINATION. 247 
 
 It must not be understood from the preceding state- 
 ment as to the importance of the minimum illumination 
 that the lighting of a street should be specified in terms 
 of this minimum only. For such a proceeding leads at 
 once to a reductio ad absurdum. And this is particularly 
 the case if the illumination on the plane of the pavement 
 is considered. For assuming a sidewalk lighted by 
 common candles placed 6 ft. apart and 6 ft. high, and 
 reckoning only the first four candles on each side of a 
 point on the pavement, the illumination based on the 
 
 formula 
 
 L sin a 
 
 ~ tf + </ a 
 
 amounts to no less than 0.052 candle-foot as against 
 0.046 candle-foot from a pair of powerful arc lights 
 placed 200 ft. apart. In other words, the street on this 
 hypothesis would be better lighted by the candles giv- 
 ing less than one-sixtieth the actual amount of light. A 
 very slight effort of the imagination will picture the really 
 vivid contrast between the two conditions. 
 
 In determining the conditions with reference to the 
 minimum illumination, therefore, one must also take into 
 account the real amount of light furnished the street as 
 a whole. We do not judge the lighting of a street by 
 the darkest spots so much as by the general effect. A 
 large number of small lights give an impression of din- 
 giness unless the aggregate candle-power be large. A 
 few large lights, on the other hand, give in places a bril- 
 liant effect, although the minimum illumination may be 
 rather small. 
 
 If artificial illuminants gave a spherical distribution 
 of light the computation of street lighting would be 
 easy, but as has already been seen, they do not. What 
 
248 THE ART OF ILLUMINATION. 
 
 is worse, the nominal brilliancy is generally not found in 
 practice. 
 
 For years open arc lamps- have been classified as 2000- 
 cp or " full arc," and as i2OO-cp, or " half-arc " lamps, 
 but these alleged candle-powers are never obtained even 
 in the direction of maximum illumination. The former 
 are lamps taking about 9.5 to 10 amperes, and 450 to 
 480 watts, the latter 6.5 to 7 amperes and 325 to 350 
 watts. Their actual maximum intensities are, respect- 
 ively, about 1 200 cp. and 700 cp., located at about 45 
 degrees below the horizontal plane. Reduced to mean 
 spherical measures their ordinary intensities are about 
 600 and 300 cp., respectively. In the horizontal plane 
 these intensities fall to about 350 cp and 200 cp. 
 
 The result of this distribution is a zone of very bril- 
 liant light in a circle having its radius equal to about the 
 height of the light, comparatively weak light within it, 
 and rather feeble light at greater distances where the rays 
 are more nearly horizontal. Fig. 96 shows this illumi- 
 nated ring with startling distinctness. The dark area 
 within is exaggerated by the effect of a dirty globe, the 
 globes on open arc lamps being accessible to dust and 
 hence difficult to keep clean. 
 
 More conspicuous even than the dark space is the nar- 
 rowness of the area of brilliant illumination, and the 
 rapid fading away into darkness of such details as fhe 
 picket fence merely across the sidewalk from the pole by 
 which is carried the mast-arm for the arc lamp. This 
 was a so-called 2OOO-cp arc, not kept in the best order, 
 to be sure, but still no worse than can easily be found in 
 any town where there are many open arc lamps. 
 
 Obviously one cannot compute the illumination from 
 such a lamp on the theory that the distribution is even 
 
EXTERIOR ILLUMINATION. 249 
 
 approximately spherical. In order to calculate the 
 effect it is necessary, instead of assuming L in the 
 formulas to be constant, to treat it as a variable, and to 
 use its real value at each angle below the horizontal in 
 
 Fig. 96. Shadow below Open Arc. 
 
 computing the illuminating effect at various points. In 
 other words, the illumination at distant points must be 
 made with reference to the luminous distribution curve 
 of the lamps. 
 
 Fig. 97 gives a set of distribution curves for the forms 
 of arc lamps in most frequent use. 
 
 Curve A is from a series alternating-current enclosed- 
 arc lamp taking 425 watts with a current value of 6.6 
 amperes. It was fitted with the usual opalescent inner 
 
2 5 
 
 THE ART OF ILLUMINATION. 
 
 globe and a clear outer globe, and was used with a solid 
 upper and cored lower carbon. 
 
 Curve B is from the ordinary open continuous-cur- 
 rent arc, taking 330 watts at 6.6 amperes, and including 
 
 Fig. 97. Distribution of Light from Arcs. 
 
 the usual clear glass globe. This is the arc nominally 
 rated at 1200 cp. 
 
 Curve C is from the 9.6-ampere 48o-watt arc usually 
 
EXTERIOR ILLUMINATION. 251 
 
 classed as 2000 cp. Like the preceding, it is a con- 
 tinuous current arc with clear glass globe. 
 
 Curve D is from a continuous-current series arc 
 lamp taking 6.6 amperes and 480 watts. It was pro- 
 vided with an opalescent inner and a clear outer globe, 
 and was worked with both carbons solid. 
 
 Lamps A and D were provided with reflecting shades 
 to turn outwards and downwards the light ordinarily 
 thrown upwards. Lamps B and C could not be thus 
 treated for the reason that so little light is thrown 
 above the horizontal that a reflector is practically 
 useless. 
 
 All these curves are from tests made on commercial 
 arc lamps of recent manufacture at the same period and 
 by the same experimenters in the laboratory of one of 
 the large electrical companies, which manufactures 
 lamps of each of these types. As the tests were at the 
 time made not for publication, but for the private use 
 of the company's engineers, and under uniform condi- 
 tions, the author believes them to be more reliable, par- 
 ticularly as regards the comparative results from the 
 several lamps, than even the average of tests made by 
 different observers on lamps under diverse conditions. 
 Such curves are of necessity only approximate, because 
 of the variations due to differing adjustments and to the 
 peculiarities of different lamps. 
 
 NOW T looking at curve C, for example, we may repeat 
 the computation made with respect to Fig. 95, using the 
 real value of the candle-power instead of the assumed 
 value. 
 
 Taking the arcs as 25 ft. above the ground, and com- 
 puting the illumination at 100 ft. from the pole, the re- 
 quired ray is depressed 14 degrees below the horizontal 
 
252 THE ART OF ILLUMINATION. 
 
 plane. The corresponding candle-power from curve C 
 is about 620, so that putting this value for L 
 
 / = ' = 0.058 candle-foot, 
 
 instead of 0.094 candle-foot as in the previous example. 
 If we take d = 150 ft., the result is still more unsatis- 
 factory, for / = 0.0433 with the assumed 1000 cp, but 
 since in this case a = 9 degs. 30 min., L is really only 
 490, whence / = 0.0168 candle-foot. 
 
 It therefore becomes evident that with the distribu- 
 tion curve of C it is very easy to get good illumination 
 not far from the lamp, but that at distances from the 
 lamp of four or five times the height of the lamp the 
 illumination is very deficient. A glance at curve B 
 shows a distribution curve of very similar shape, and, in 
 fact, all open arcs have the common weakness of giving 
 more light than is necessary near the pole and too little 
 far away from it. The distribution would be much im- 
 proved if the curve could be swung upward about 20 
 degs. toward the horizontal. 
 
 If there were any convenient way of getting a distribu- 
 tion that would give uniform illumination up to a rea- 
 sonable distance it would be extremely useful. The 
 polar equation for such a distribution curve would be 
 
 ut 
 
 J-i r~s 
 
 Fig. 98 shows this curve plotted for h = 25 and 
 / = o.i candle-foot. The theoretical curve obviously 
 becomes asymptotic, which no practical curve could ever 
 do under finite conditions, but it is not outside the 
 bounds of possibility to construct a reflecting and re- 
 fracting system after the pattern of the holophane globe, 
 
EXTERIOR ILLUMINATION. 
 
 253 
 
 which should send out at angles not far from the hori- 
 zontal most of the light which is now wasted in need- 
 lessly brilliant illumination near the light. 
 
 Unhappily the arc slowly changes place in common 
 lamps as the carbons are consumed, so as to interfere 
 
 Fig. 98. Ideal Distribution Curve. 
 
 with the use of such a device. But it would be quite 
 feasible with a focusing lamp, or with Nernst or similar 
 lamps, although globes of the kind in question are not 
 easy to keep clean, and would have to be kept dust-tight 
 to obtain the best results. 
 
 Now, when we remember that the curves of Fig. 97 
 represent the lamps in their respective best working 
 
254 THE ART OF ILLUMINATION. 
 
 conditions, and not when the globes are ill-cared for or 
 the arc abnormal in length and position, it becomes evi- 
 dent that if any attention is to be paid to minimum 
 illumination, even the most powerful commercial arcs 
 cannot be very widely spaced. At 400 ft. spacing the 
 midway point receives from an ordinary i2OO-cp arc 25 
 ft. high just about o.oi candle-foot, which, for most 
 practical purposes, is no light at all. Only by raising 
 the lamp high enough to take advantage of the light at 
 angles further from the horizontal can adequate light 
 at such a distance be obtained, and this is an impractica- 
 ble expedient on account of the cost and trouble and the 
 interference of trees and other obstacles. For with such 
 arcs the maximum illumination is obtained when the 
 height of the lamp is about seven-tenths the distance to the 
 point to be lighted. The tower system of lighting, used 
 extensively in this country fifteen years ago, but now prac- 
 tically abandoned, was the result of this consideration. 
 
 The important thing to be decided as a basis for all 
 computations on street illumination is the amount of 
 light required. There is little general agreement on 
 this important matter. We may get an idea of the mag- 
 nitudes to be employed by remembering that moonlight 
 in the latitude of the Northern States is generally from 
 o.oi to 0.03 candle-foot. This intensity is of considerable 
 service at the maximum limit, but of little use at the 
 minimum. When anything like adequate illumination 
 is to be furnished the minimum should not be less than 
 0.03 candle-foot. With this minimum derived from arc 
 lamps the street, as a whole, will be brilliantly lighted. 
 Under ordinary circumstances only principal streets 
 would be lighted to this extent, and elsewhere the mini- 
 mum mig-ht fall somewhat lower. 
 
EXTERIOR ILLUMINATION. 255 
 
 To determine the real illumination produced at any 
 distance by a particular radiant, it is necessary to take 
 the assumed height and the distribution curve for the 
 radiant, and then to compute the illumination at that 
 distance, using the real candle-power corresponding to 
 the direction of the ray considered. To save labor it is 
 convenient to plot the illumination at various distances 
 in the form of a curve, thus enabling the illumination in 
 candle-feet to be read directly. 
 
 Fig. 99 shows a set of curves thus computed from the 
 four types of arc lamps, of which distribution curves 
 were shown in Fig. 97. The first important lesson to 
 be drawn from them is that none of the lamps shown 
 gives really useful illumination at a distance of more 
 than 150 ft., and that lamps A and B should not be 
 spaced more than 225 ft. apart if the minimum illumina- 
 tion is to be about 0.03 candle-foot. Lamps C and D 
 might be spaced at 300 ft., but not much further with- 
 out producing a conspicuous dark belt. 
 
 A and D, the enclosed arcs, give relatively better illu- 
 mination at considerable distances than the open arcs. 
 It should be noted that the height of all lamps is as- 
 sumed to be 25 ft., and that the curves begin at 50 ft. 
 from the lamp, the space within that distance being 
 relatively well lighted. 
 
 Comparing the several types of lamps, it at once ap- 
 pears that for street lighting the 6.6-ampere enclosed 
 continuous current arc is fully equal to the 9.6-ampere 
 open arc (so-called 2OOO-cp), while the 6.6-ampere alter- 
 nating arc is rather better than the 6.6 ampere con- 
 tinuous-current open arc (so-called i2OO-cp), but is 
 materially less effective than the other two. In the au- 
 thor's opinion, it would correspond to a nominal 1500 
 
CANDLE FEET I* 
 
EXTERIOR ILLUMINATION. 257 
 
 cp as respects the open arcs rated in the old-fashioned 
 way. 
 
 But theory and experience unite in indicating that 
 effective illumination on streets is not measured exactly 
 by the process just explained, useful though it may be. 
 There is a physiological as well as a physical factor in 
 illumination, for in the presence of lights of great in- 
 trinsic brilliancy the iris closes and the image on the 
 retina grows faint, like the image in a camera when the 
 lens is stopped down. 
 
 Thus it is difficult to see beyond a brilliant light, and 
 when the eye is exposed to the intense glare of an open 
 arc it does not recover promptly enough in passing on 
 to get the full value of the relatively feeble light at a dis- 
 tance from the lamp. This effect and the more uniform 
 distribution of light account for the well-established fact 
 that enclosed arcs, for both alternating and continuous 
 currents, give more satisfactory lighting than would 
 seem to be warranted by the computed intensity of the 
 illumination. Besides, the enclosed arcs are generally 
 steadier, which also tends greatly to improve matters. 
 
 The same advantages would be secured by using open 
 arcs with properly designed diffusing globes, but these 
 last really require the employment of a focusing lamp in 
 which both carbons are fed into the arc at rates propor- 
 tional to their rates of consumption, so as to hold the 
 arc in a fixed position. Such lamps are more compli- 
 cated and less fitted to outdoor use than ordinary lamps, 
 and have not come into use for this purpose in this 
 country. 
 
 The advantages in uniformity of distribution and in 
 absence of shadowed areas, gained by the use of en- 
 closed arcs, are very conspicuous. Fig. 100 shows the 
 
258 THE ART OF ILLUMINATION. 
 
 illumination from an ordinary open arc in excellent 
 operative condition, aided by a globe ground below to 
 diffuse the light and to lessen the shadows beneath the 
 arc. In spite of this the illumination is far from uniform, 
 and the side rods of the lamp, on account of the small 
 luminous area of the arc, throw dense lateral shadows. 
 Fig. 101 shows the result of replacing this particular 
 lamp by an enclosed arc. The shadows completely dis- 
 
 Fig. ioo. Open Arc at Its Best. 
 
 appear and the more powerful rays near the horizontal 
 are shown both by the better illumination along the 
 street and by the glare that evidently entered the 
 camera. These results are very striking, and amply 
 justify the present tendency to replace open- by enclosed 
 arcs, quite irrespective of the lessened cost of carbons 
 and of trimming in the latter case. 
 
EXTERIOR ILLUMINATION. 
 
 2 59 
 
 Enclosed continuous-current arcs can be operated for 
 about 100 hours with one pair of carbons; in other 
 words, they have to be trimmed only about once a week 
 in all-night street lighting, while the ordinary open arcs 
 require trimming daily under like circumstances. The 
 resulting saving in labor and in carbons is variously esti- 
 mated, and changes somewhat with local conditions, but 
 the weight of the evidence indicates a net saving of about 
 $10 per year per lamp, the same energy being used in 
 each case. 
 
 Of late a strong tendency has developed toward the 
 
 Fig. 101. Light from Enclosed Arc. 
 
 use of alternating-current lamps, worked in series like 
 the others through the aid of constant-current trans- 
 formers or automatic regulators, which take the alter- 
 nating current at constant voltage and deliver a current 
 constant in amount, but varying in voltage according to 
 the load to be carried. To discuss these interesting 
 
260 THE ART OF ILLUMINATION. 
 
 mechanisms in detail is without the purpose of this 
 volume. Suffice it to say, that they do their work ex- 
 tremely well, although at the cost of certain inconven- 
 iences, less on the whole than those attending the use of 
 ordinary arc generators. 
 
 As has already been noted, the alternating arcs de- 
 mand a little more energy relatively than continuous- 
 current arcs, but as a rule this loss is fully compensated 
 by the more economical distribution of current rendered 
 possible. The enclosed alternating arcs require slightly 
 more expensive carbons, and rather more frequent trim- 
 ming than the enclosed continuous-current arcs. As a 
 rule one pair of carbons will last seventy-five or eighty 
 hours, and the street lamps must be trimmed once in 
 five or six days, but a considerable saving in carbons and 
 labor is still effected. It probably amounts in average 
 cases to about $8 per year per lamp. 
 
 The smaller intrinsic brilliancy of enclosed arcs en- 
 ables some gain to be made by placing them lower than 
 25 ft., perhaps no more than 1 8 or 20 ft., which means 
 a still further gain in effective lighting. As between 
 continuous current and alternating-current arcs taking 
 the same current, the former are distinctly more power- 
 ful; but either at 6.6 amperes can replace the open arcs 
 either of 1200 or 2000 nominal candle-power, lamp for 
 lamp, and give about equally satisfactory illumination. 
 In new installations the alternating lamps may advan- 
 tageously be spaced a little closer than the continuous- 
 current enclosed arcs, as the curves of Fig. 99 in- 
 dicate. 
 
 There is no question that where plenty of light is 
 wanted in a street comparatively clear of trees, well dis- 
 tributed arc lights give by far the best results yet at- 
 
EXTERIOR ILLUMINATION. 261 
 
 tained. But where, as in many residence streets in the 
 smaller cities, the whole roadway is well shaded by trees, 
 arc lamps as ordinarily installed find their usefulness 
 greatly limited by shadows. If the foliage does not 
 come too low good results can be obtained by putting 
 the arcs on cross suspensions over the center of the 
 street at a height of not over 1 8 to 20 ft. In this case 
 the enclosed or otherwise shaded arcs have an immense 
 advantage, as they can thus be swung low without seri- 
 ously dazzling the eyes, and throw shadows far less in- 
 tense than the open arcs. 
 
 But there are streets so well shaded that even arcs so 
 placed are at a disadvantage, and there are also many 
 cases in which there is no real need of a brilliant illumi- 
 nation, but merely enough light is desired to make the 
 way fairly clear. Economy also sometimes dictates 
 caution in the expenditures for street lighting, and in 
 such cases incandescent electric lamps or gas lamps are 
 capable of doing good service at comparatively moder- 
 ate cost. The incandescent lamps used for such service 
 are nearly always operated in series, either on the same 
 circuits as arc lamps or on separate series circuits .by 
 themselves. 
 
 In the former case the lamps are generally 50, 65, or 
 100 candle-power, made to take a constant current cor- 
 responding to that used for arcs, and it should be noted 
 that such lamps are costly in the matter of renewals and 
 difficult to operate satisfactorily. In the latter case the 
 incandescents are generally of 16, 25, or 32 candle- 
 power, worked in series upon an alternating-current cir- 
 cuit of 1000 or 2000 volts, taking 2 to 4 amperes. The 
 running conditions in either case are rather severe, ow- 
 ing to the likelihood of fluctuations in current, and in- 
 
262 
 
 THE ART OF ILLUMINATION. 
 
 candescent lamps intended for such service are rarely of 
 higher initial efficiency than 3.5 to 4 watts per candle. 
 
 In the case of gas lighting, the present tendency is to 
 use mantle burners giving initially 50 to 100 candle- 
 power. Their color is comparatively inoffensive in 
 street lighting, and they can be run at very low cost, but 
 outdoor conditions seem to tend to very rapid deteriora- 
 tion of the mantles, so that in practice it is difficult to 
 find a street in which most of the mantles have not long 
 
 0.2 
 
 0.1 
 
 1PJ 
 
 20' 30' 40' 50' 60' 
 
 Fig. 102. Illumination from Incandescent Lamps. 
 
 outlived their usefulness. Incandescent electric lamps 
 deteriorate fast enough, but even in street service they 
 hold their brilliancy much better than the mantle burn- 
 ers. The moral is that in using either of these illumi- 
 nants, a very liberal allowance should be made for falling 
 off in candle-power. 
 
 As in case of arcs, the illumination should be com- 
 puted from the distance, height, and light-distribution 
 curve of the radiant. For incandescent lamps of 25, 50, 
 and 100 candle-power the resulting illumination curves 
 are shown in Fig. 102, which may be compared with 
 
EXTERIOR ILLUMINATION. 263 
 
 Fig. 99. It is clearly evident therefrom that such lights 
 should hardly be spaced over 120 ft. apart, even when of 
 100 candle-power, while those of 50 and 25 candle- 
 power should be spaced much nearer. If they held 
 their brilliancy well, one might space them further, but, 
 in fact, it is undesirable. 
 
 In using the above curves for mantle burners it 
 should be noted that one such burner corresponds ap- 
 proximately to a 5O-cp incandescent electric lamp, being 
 somewhat better in the early stages of its life, but losing 
 brilliancy rather more rapidly. 
 
 In heavily shaded streets incandescents and mantle 
 burners may best be bracketed out a few feet from the 
 curb, with alternate lamps on opposite sides of the 
 street; loo-cp lamps spaced in this way, with 200 ft. be- 
 tween consecutive lamps on the same side, or even 250 
 ft. under favorable conditions, will produce a tolerably 
 lighted street, and 5O-cp lamps spaced a little closer, say, 
 at 150 to 200 ft. on each side, will do even better, but 
 except for use in shaded streets arcs are generally to be 
 preferred. 
 
 When close economy is an object, 25-cp lamps spaced 
 at 150 ft. between lamps on the same curb, or mantle- 
 burners similarly placed, will give tolerable lighting, but 
 when in suburban districts lights must be economized, it 
 is better to trust to arcs spaced even so widely as to 
 leave regions of comparative darkness between them. 
 
 Whenever possible, lights should be so located at 
 street corners as to shine effectively in all four direc- 
 tions. They should be placed preferably on long mast- 
 arms or on cross-suspensions, unless in a fairly clear 
 street, where pole tops or short brackets are effective. 
 One small but useful point in locating lights is as.fol- 
 
264 THE ART OF ILLUMINATION. 
 
 lows: Never locate a lamp, particularly an arc, at the 
 curb squarely opposite a crossing, for the shadow of one 
 walking on the crossing plunges his way into inky black- 
 ness as soon as he has passed the light. 
 
 Squares and open spaces should be treated somewhat 
 in the same manner as streets, but if the spaces are clear 
 the lights may well be placed higher than in street light- 
 ing, 30 to 40 ft. being advantageous. In well shaded 
 spaces one may use lights of small intensity to con- 
 siderable advantage, but ordinarily arcs do the best 
 work. 
 
 As to intensity, the lighting depends on the character 
 of the space, but it should never be less than in a well- 
 lighted street. Now and then such lighting drifts natu- 
 rally into a species of scenic illumination, the object 
 being to bring some fine square into brilliant relief. Oc- 
 casionally in such work incandescent lamps can be 
 massed with admirable results, although in general it is 
 not advisable to mix arc and incandescent lighting. 
 
 The arrangement of public lights should be more or 
 less influenced by the general illumination of the private 
 premises along the streets. In large cities it often 
 happens that through the evening hours, when the citi- 
 zens are much abroad, the sidewalks are fully lighted by 
 the stray illumination from windows. In such instances 
 the public lighting can be well directed toward special 
 points where it will do the most good, care being always 
 taken to see that the illumination does not fall below 
 minimum requirements when the mass of private lights 
 is out. In small cities there is comparatively little aid 
 of this sort. The most troublesome problems are those 
 connected with the environs of such cities where miles of 
 sparsely settled streets must be dealt with. 
 
EXTERIOR ILLUMINATION. 265 
 
 The practical problem of adequately lighting the 
 streets of a city is one which requires the local data for 
 its solution. The amount and distribution of streets 
 and the needs and distribution of the population are the 
 controlling factors in the matter, and obviously these 
 vary greatly from place to place. 
 
 Then, too, the cost of lights and the funds available 
 for the purpose depend on local conditions. In par- 
 ticular, the price of lights varies so much that it is diffi- 
 cult even to strike an average. Few topics offer less 
 chance for certitude or are more unsatisfactory to in- 
 vestigate. It makes a great difference whether full arcs 
 or half arcs (so-called) are in use, or incandescents, or 
 Welsbachs; also whether the lights are burned all night 
 and every night, every night until midnight, or on 
 "moonlight schedule," that is, on all nights and at all 
 hours of the night when there is not clear moonlight. 
 
 In large cities the tendency is toward powerful arc 
 lights burning every night and all night, supplemented 
 by incandescent lamps and by gas burners. In smaller 
 cities and towns the smaller arcs are apt to be used, 
 burned either on moonlight schedule or until 12 or i 
 o'clock. 
 
 In the latitude of the Northern United States " all 
 night and every night " public lighting usually means 
 from 3900 to 4200 hours of lighting yearly, according 
 to the treatment of twilight and dawn, and the weather. 
 The moonlight schedule is carried out with various 
 slight modifications, and evidently depends considerably 
 upon the weather, but in practice it amounts to not far 
 from 3000 hours per year, while lights run from dusk to 
 midnight, or I a. m., usually will burn 2000 to 2200 
 hours per year. All night and every night lighting is, 
 
266 THE ART OF ILLUMINATION. 
 
 of course, the thing to be desired, but if economy is 
 necessary, places of moderate size can be very satis- 
 factorily lighted on a liberally administered moonlight 
 schedule. 
 
 As between " full " and " half " arcs, the advantage 
 economically lies, on the whole, with the former, pro- 
 vided the same total illumination is to be obtained in 
 each case. The following table gives the spacing re- 
 quired for various radiants on an assumed minimum 
 illumination of 0.02 candle-foot at a point midway be- 
 tween lights: 
 
 DISTANCE LIGHTS 
 
 KIND OF LIGHT. BETWEEN LIGHTS. PER MILE. 
 
 6. 6-ampere enclosed D. C. arc 340 15 
 
 9. 6-ampere open D. C. arc 315 17 
 
 6. 6-ampere enclosed A. C. arc. . 275 19 
 
 6.6-ampere open D. C. arc 260 20 
 
 5O-cp incandescent lamp, electric (or 
 
 mantle burner) 100 53 
 
 The relative expense of lighting by these various 
 radiants depends in great measure upon circumstances not 
 to be predicted. The price charged by lighting companies 
 for public lighting is simply whatever the community 
 will stand. 
 
 The relations between lighting companies and munici- 
 palities are, in most cases, mutually predatory. The 
 former, having acquired and utilized a public franchise 
 can, in a measure, hold the street against competition, 
 and can maintain prices at the highest figure that can be 
 juggled through the city government without public 
 scandal. The latter, through the earnest efforts of its 
 practical politicians, can worry and threaten the former 
 into yielding up a substantial annual rake-off in the form 
 of jobs for heelers, contributions to campaign funds, or 
 plain cash. Occasionally simple and decent business 
 
EXTERIOR ILLUMINATION. 267 
 
 relations are maintained, but seldom for any great 
 length of time. 
 
 The prices actually charged for public arc lights 
 burned all night and every night range very widely, but 
 usually between $75 and $125 per lamp per year. The 
 former price is seldom reached or discounted, save in 
 stations operated by water power or under very strong 
 competition, and even then generally is only for " half " 
 arcs. The latter price is seldom exceeded, save where 
 underground distribution is demanded or in case of very 
 scattered service. 
 
 Incandescent lamps of 50 candle-power, or there- 
 abouts, usually bring $30 to $35 per year, and the 
 former figure is a common one for mantle burners on the 
 gas system. For equal minimum intensity of illumina- 
 tion there is not much to choose between the several 
 illuminants at the prices mentioned, the choice between 
 them being due to suitability. 
 
 At the same total cost, however, the arc lights give a 
 considerably higher average illumination, and experi- 
 ence shows that on the whole the arcs, which have to be 
 inspected at frequent intervals for the purpose of trim- 
 ming, are kept nearer their point of maximum efficiency 
 than either incandescents or gas burners. 
 
 The real cost of public lighting is, of course, im- 
 mensely variable, since amount and price are both varia- 
 ble. In the average New England city most of the 
 lighting is by arcs, and there is an average of one arc for 
 each 175 to 200 inhabitants. The total cost of public 
 lighting is frequently from 50 cents to $i per inhabitant, 
 and sometimes rises to $1.50, and even to $2 per in- 
 habitant. It is, therefore, no inconsiderable item of 
 public expense. 
 
268 THE ART OF ILLUMINATION. 
 
 Whether street lighting should be done by contract 
 with a corporation or directly by the municipality is one 
 of the mooted questions of economics. In theory, it 
 would seem that such a public service should be done by 
 the city or town itself, on the same principle that all 
 towns own their sewerage systems and most own their 
 waterworks. But while experience seems to have 
 shown that public waterworks are in every way desira- 
 ble, the same cannot be said of gas and electric plants. 
 The arguments pro and con are about the same in each 
 case, yet the average results seem to be different. In 
 some municipal lighting plants, mostly small, the eco- 
 nomic results have been excellent, in most they have 
 been unmistakably bad. 
 
 If a municipality could start in de iwvo and erect its 
 complete lighting system according to the best modern 
 practice, and run that system anything like as economic- 
 ally as it would be run by a private company, it could 
 unquestionably do its public lighting at a great saving 
 in the majority of cases. 
 
 But it can rarely start and operate thus freely. It 
 often is compelled by law to buy out the existing plant, 
 generally at a price far larger than would suffice to erect 
 a modern plant, for the art has been changing rapidly. 
 It is lucky if a plant, old or new, can be secured without 
 furnishing pickings and stealings for somebody, and in 
 its operation the finger of the politician is too often in- 
 serted for no honest purpose. 
 
 In this matter of municipal plants statistics are even 
 more utterly mendacious than usual, and conceal rather 
 than disclose the facts in the case. With respect to 
 electrical plants at least, however owned, there is nearly 
 always deliberate concealment or entire neglect of de- 
 
EXTERIOR ILLUMINATION. 269 
 
 preciation, both that due to wear and tear and the larger 
 amount due to improvements in the art, so that from the 
 books or reports it is almost impossible to figure the 
 actual cost of furnishing the electrical energy. 
 
 The matter may, on the whole, be summed up about 
 as follows: If a municipality could both acquire a light- 
 ing plant and operate it at the ordinary current rates for 
 apparatus, labor, and material paid by private buyers 
 and employers, it could effect a large saving in its cost 
 of public lighting; but if the plant is touched by venal 
 politics it will assuredly prove a costly failure. 
 
 Many improvements are possible in street lighting, 
 but for the present the arc lamp must be the main re- 
 liance. At first thought the Nernst lamp would seem 
 to offer advantages, but it does not readily lend itself to 
 the distribution in series which is desirable in street 
 lighting for the sake of economy. Improvements in 
 mantle gas burners may bring them into a position of 
 great usefulness, which they have not yet attained by 
 reason of their rapid deterioration. But for the most 
 part the electric arc is the best available source of light. 
 
 Contracts for arc lighting should never be drawn on 
 the basis of a nominal candle-power. They should 
 clearly specify the kind of arc to be installed^ the amount 
 of energy to be taken in each arc, and the kind of shades 
 to be used. The nature of the fixtures should be spe- 
 cifically designated, whether pole tops, brackets, mast- 
 arms, or cross suspensions. These and the locations of 
 the lamps should be designated by someone familiar 
 with practical street lighting, following the general line 
 of the data which have here been given. The hours of 
 lighting should be distinctly stated, with rebates for 
 failure to provide continuous light within these hours. 
 
2 7 o THE ART OF ILLUMINATION. 
 
 Such rebates should be merely nominal for deficiencies 
 up to, say, i or 2 per cent, of the total hours of lighting, 
 and punitive on an increasing scale for greater de- 
 ficiencies. 
 
 The fixtures used for street lighting are of very vari- 
 ous patterns, but fall into four general classes : pole-tops, 
 brackets, mast-arms, and cross-suspensions. These have 
 been, with the exception of the mast-arm, in use for 
 public lighting for a very long period, going back to the 
 days of oil lamps and candles. The pole-top fixture is 
 essentially a support for a lamp on the top of a post. In 
 arc lighting it is generally combined with a protecting 
 
 Fig. 103. Pole-top. Fig. 104. Bracket. 
 
 weather hood to shield the top of the lamp from the 
 weather, and also sometimes to shield the individual 
 switch for short-circuiting the lamp. Fig. 103 shows a 
 typical pole-top such as is often used for enclosed arc 
 lamps. The thin side rods are placed edge toward the 
 arc to obviate shadows, and the whole affair fits neatly 
 
EXTERIOR ILLUMINATION. 271 
 
 upon the top of the pole, the arc lamp hanging from an 
 insulated hook within the hood. The obvious objec- 
 tion to pole-top fixtures, whether for arc lights or for 
 gas lamps, is that the light must be on the curb, and 
 sometimes does not light the street properly. For open 
 spaces where the pole can be out from the curb the pole- 
 tops work well and may be freely used. 
 
 A very obvious modification is the lateral bracket car- 
 rying the lamp well clear of the curb, yet not so far out 
 as to make it difficult to trim the lamp from the pole 
 without lowering it. Fig. 104 is a specimen of this 
 class, which carries the lamp 2. ft. out from the pole. If 
 longer than this the lamp should be supported by a rope 
 and lowered for trimming. Such brackets in various 
 forms have been in use for a long time i and a neat iron 
 pole and bracket dating back some seventy-five years is 
 shown in Fig. 105. This is fitted with a pulley and cord 
 for lowering the oil lamp for filling. It might be copied 
 to advantage even now. 
 
 A somewhat analogous type of bracket has been intro- 
 duced and very extensively used by the Boston Electric 
 Light Company. It is a hollow casting, fitted to the 
 top of a neat wooden pole, and permits the line wires to 
 be carried within it clear to the lamp without exposing 
 them. For underground service the pole itself may be 
 hollow, thus entirely eliminating exposed wires. The 
 lamp can be trimmed easily from the pole, and a cut-out, 
 A, is fitted in the slight expansion near the base of the 
 upright part of the casting. Fig. 106 shows this very 
 neat and convenient fixture in outline. 
 
 Mast-arms are really modified brackets lengthened so 
 much as to bring the lamps nearly or quite to the center 
 line of the street, and usually arranged to permit the 
 
272 
 
 THE ART OF ILLUMINATION. 
 
 lamp being readily lowered to the street for trimming. 
 Now and then the lamp is carried on a trolley, which can 
 be pulled in to the pole for trimming, but the preference 
 is generally for the former plan. Fig. 107 shows a corn- 
 
 Fig. 105. Antique Iron Pole. 
 
 Fig. 106. Boston Pole 
 Fixture. 
 
 mon form of mast-arm fitted for lowering the lamp. 
 The lamp is usually carried some 14 or 15 ft. out from 
 the pole, hence the truss form becomes necessary to 
 secure the proper degree of strength and stiffness. 
 
EXTERIOR ILLUMINATION. 273 
 
 Mast-arms furnish, on the whole, the best means of 
 carrying the light out over the street. They call for but 
 a single pole at the curb, and put the light exactly where 
 it is wanted, and hold it there steadily. They are far 
 from beautiful, however, and from the aesthetic stand- 
 point the cross suspension is generally to be preferred. 
 This is a very old method of supporting lights, and con- 
 
 Fig. 107. Mast Arm. 
 
 sists merely of a rope stretched across the street and 
 bearing midway a pulley from which the lamp is carried. 
 Fig. 108, from an old French print showing street light- 
 ing in Paris early in the eighteenth century, illustrates 
 the principle as well as a more modern instance. To- 
 day the rope is of wire strands and the lamp is an elec- 
 tric arc, but the rest has changed little. 
 
 In ordinary cases the cross suspension requires a pole 
 set at the curb on each side of the street, which, except 
 at corners where the pole lines cross, is somewhat of an 
 inconvenience. Sometimes a tree is used for one sup- 
 port, but the practice is not to be encouraged, since it is 
 both bad for the tree, and renders the lamp rather un- 
 steady in a wind. When conditions permit, cross sus- 
 
274 
 
 THE ART OF ILLUMINATION. 
 
 pension, however, is a most useful and unobtrusive 
 method of carrying the lamps. 
 
 In general, where there is an underground distribu- 
 tion, either of electricity or gas, pole-top and bracket 
 fixtures are most useful for ordinary purposes. Fix- 
 tures like Fig. 105 lend themselves very readily to artis- 
 tic treatment either for electric lights or for mantle or 
 
 Fig. 108. Antique Cross Suspension. 
 
 regenerative gas burners. In streets thickly shaded by 
 trees recourse must generally be taken to mast-arms or 
 to cross suspensions in order to put the lights where 
 shadows will not be troublesome. Sometimes even in- 
 candescent lamps are carried in the latter manner, 
 though being rather closely spaced they, like mantle 
 burners, give fairly good results if placed alternately on 
 each side of the street and bracketed clear of the curb. 
 
 Generally, the lighting of a town will call into useful 
 service all the ordinary types of fixtures, and an attempt 
 to adopt a single standard form will lead to considerable 
 embarrassment in the effective lighting of certain 
 localities. 
 
CHAPTER XII. 
 
 DECORATIVE AND SCENIC ILLUMINATION. 
 
 IN lighting large spaces either indoors or out, effective 
 use may be made of arc lamps as well as incandescents. 
 In some instances fairly good results are obtained by 
 using for this purpose ordinary open or enclosed arc lamps 
 with large metallic reflectors behind them. They produce 
 a powerful and partially diffused illumination that is 
 rather serviceable in many situations, but is neither very 
 uniform nor intensely brilliant. Such places as piers are 
 often thus lighted, the reflectors saving considerable light 
 that would otherwise be thrown in useless directions and 
 wasted. Even a reflector of tin covered with white 
 enamel paint can be made very serviceable for this 
 purpose. 
 
 If for any reason the white or bluish white light of the 
 arc is undesirable, the color can easily be slightly modified 
 by using on the arc lamp a globe of colored glass or coat- 
 ing a clear outer globe with the solution employed for 
 coloring incandescent bulbs. 
 
 These cannot strongly tinge the light without greatly 
 reducing it, since they color only in virtue of absorption, 
 a red screen, for instance, giving a ruddy tinge to the arc 
 by cutting off a large amount of blue and green rays. 
 
 In the absence of electric lamps display and scenic 
 illumination is a rather difficult matter, this part of the 
 art having been developed mainly by the stimulus of 
 electric lighting. A certain amount of display lighting 
 can be done by gas jets with ample reflectors arranged 
 
 275 
 
276 THE ART OF ILLUMINATION. 
 
 much like those already shown, but the results are not 
 generally satisfactory, since on account of the heat evolved 
 the whole apparatus has to be bulky. Mantle burners 
 are nearly useless in this connection, on account of their 
 offensive color. For brilliant scenic work the calcium 
 light is, however, extremely useful, although its use of 
 many years in the theater has now been almost abandoned 
 in favor of electric arcs. 
 
 Theatrical lighting effects really form an art quite by 
 itself. It is quite impossible to give a connected account 
 of it apart from an enormous amount of detail applicable 
 to special problems. Broadly, it may be divided into 
 three branches: The general illumination of the stage, 
 scenic illumination of the stage, and the illumination per- 
 taining to tricks and illusions. 
 
 The first mentioned branch differs radically from 
 ordinary interior illumination in that the lights visible 
 from the auditorium take little or no part in the real work. 
 The footlights, merely incandescents in front of enameled 
 reflectors, are of primary importance, and the remaining 
 illumination has to be furnished from the wings and flies. 
 Contrary to all usual practice elsewhere, such illumination 
 must be nearly or quite shadowless, for it would be most 
 awkward to have a massive stage oak casting the linear 
 shadow appropriate to the board or canvas on which it is 
 painted. Therefore, the general body of the lighting 
 should be thoroughly diffused, as in illumination from a 
 cornice or from concealed lamps above the ceiling, and if 
 for any reason shadows are desired, they should be pro- 
 duced by auxiliary bright lights introduced for that 
 particular purpose and screened from throwing telltale 
 shadows where they are not wanted. 
 
 Not only must the general stage lighting be beautifully 
 
DECORATIVE ILLUMINATION. 277 
 
 diffused, but it must be under perfect control as to amount. 
 To this end theaters usually have an elaborate equipment 
 of rheostats, which can be thrown in series with the lamp 
 circuits, and these latter are in numerous sections, so that 
 the light can be made to fade gradually out without chang- 
 ing its intensity or its direction by perceptible degrees. 
 When alternating currents are used the inductive regu- 
 lators can be made to accomplish this very perfectly, and 
 with an auxiliary storage battery one can do equally well 
 with continuous current. Without these a smooth reduc- 
 tion of illumination is not easy. 
 
 One of the useful devices to this end is to divide the 
 whole body of lights into overlapping groups. For ex- 
 ample, if we imagine 100 incandescents to be massed 
 across the flies, the division would be somewhat as fol- 
 lows: Lamps, i, 6, n, 16, etc., would form one circuit, 
 2, 7, 12, 17, etc., the second circuit, and so on, forming 
 five groups. Then a rheostat of moderate size cut into 
 circuit with each group successively prior to its ex- 
 tinguishment would enable the operator to fade out the 
 light by almost imperceptible steps without altering its 
 distribution. 
 
 Such, or an equivalent arrangement, is quite necessary, 
 and should be capable of producing a uniform shadowless 
 illumination of any required intensity, from the full glare 
 of a spectacle down to a light so faint that a candle in the 
 hand of one of the actors will cast a flickering shadow, 
 for the stage is seldom really dark, however dark it may 
 seem to the audience. 
 
 Whenever the illumination should have a definite direc- 
 tion, it can easily be given by special lights or circuits, 
 but the groundwork of the lighting must be uniform and 
 diffused. 
 
278 THE ART OF ILLUMINATION. 
 
 The mainstay of special lighting effects is the stage 
 projector. In the rough, this is a wide angle searchlight. 
 For the source, there is a first-class focusing arc lamp 
 taking an amount of current which can be regulated by a 
 convenient rheostat. The reflector varies according to 
 conditions. Sometimes it is a polished or enameled me- 
 tallic parabolic mirror, sometimes for other purposes a 
 wide parabolic wedge giving strong lateral distribution. 
 
 Fig. 109 is a good example of the adjustable projector 
 lamp with universal adjustments for height and position 
 of the lamp, and carrying at the base a well ventilated 
 rheostat for the proper regulation of the current at the 
 arc. Such lamps are made to take a considerable amount 
 of current, often up to 10 or 15 amperes, and give a very 
 steady and powerful light. 
 
 For the general purposes of stage illumination a con- 
 densed beam is not required. In this case a rack at the 
 mouth of the reflector is arranged to take colored screens 
 of any shade required. Those most often used are reds 
 and pale blues. By means of such reflector lamps are 
 produced most of the gorgeous spectacular stage effects, 
 although in some cases regular stereopticon lanterns ar- 
 ranged with the dissolving view apparatus and fitted with 
 colored screens are employed with admirable results, 
 particularly in. producing a very concentrated beam. 
 
 No general directions can be given for the amount of 
 illumination required for this theatrical work, for the 
 obvious reason that each stage setting has its own special 
 requirements, which cannot be predicted. Roughly, the 
 stage may require at times fully as much light as the 
 auditorium proper. Considering the fact that the lamps 
 must for the most part be out of sight of the audience and 
 in rather disadvantageous position, it is safe to say that 
 
DECORATIVE ILLUMINATION. 
 
 279 
 
 a maximum illumination of not less than i candle-power 
 per square foot should be provided for, aside from re- 
 flector lamps and the like. Most or all of this should be 
 
 
 Fig. 109. Projector Lamp 
 
 from incandescents, or gas jets, where electric lights are 
 not available, for the more powerful single radiants 
 dominate the illumination too strongly, unless used with 
 great caution. 
 
280 THE ART OF ILLUMINATION. 
 
 The more specialized part of scenic illumination which 
 has to do with local illusions is even less easy to reduce to 
 general principles. It is part of the art of the stage 
 manager and his assistants. Since electric lighting has 
 become general, the range of such work has been enor- 
 mously widened. Stage lightning, which used to be pro- 
 duced by a prodigious flash of lycopodium powder blown 
 across a gas jet, is now beautifully given by the moment- 
 ary flash of a powerful arc. 
 
 Touches like Fafner's gaudy eyes and the forging of 
 the sword in " Siegfried " are due to the skill of the stage 
 electrician, and would have been quite impracticable a 
 quarter century ago. A great deal of temporary work 
 has to be done for any important performance, and much 
 intelligent skill is required on the part of the operators, 
 who sometimes have to follow a rapidly moving object 
 about the stage with the beam from a projector, when a 
 single slip would and sometimes does destroy the illu- 
 sion and provoke unseemly merriment. It is almost need- 
 less to say that in stage illusions much depends on the 
 arrangement of the background. 
 
 Another and very important branch of scenic illumina- 
 tion is the decorative lighting of large buildings and public 
 places. The illumination proper in such cases has been 
 already discussed, but the intelligent use of lights to bring 
 out the full value of their architectural characteristics at 
 night is quite another matter. Even so apparently simple 
 a problem as the adequate illumination of a single monu- 
 ment requires considerable skill and care, and without 
 these almost inevitably fails of producing the proper 
 results. And when a great public building is concerned 
 the task becomes far more difficult. 
 
 As a simple example of scenic illumination of this 
 
DECORATIVE ILLUMINATION. 2*1 
 
 general type let us take an assumed case and see what can 
 be done with it. We will suppose the subject of our 
 study to be a soldier's monument, such as may be found in 
 scores or hundreds of American cities. It will generally 
 be a shaft of marble or granite, surmounted by a figure or 
 group in bronze, and with symbolic panels in bas-relief 
 about the base. Now, the wisest course to pursue is to 
 let the kindly shades of night cover the whole affair, but 
 sometimes the monument is really fine, and so situated that 
 it can be appropriately brought into relief by suitable 
 illumination, or the citizens insist upon lighting it, and the 
 attempt has to be made. 
 
 The broad rule that governs every such case is 
 that it is both impossible and useless to attempt to simulate 
 daylight. Full sunlight brings out details and produces 
 effects that art cannot duplicate, so it is advisable to attack 
 the problem along quite another line. 
 
 The chief difficulty of the task lies in the fact that 
 bronze lights up very badly, particularly after it has 
 acquired the fine patina given by age or skillful chemical 
 treatment. Reflecting little light, it is very hard to bring 
 into proper relief, and the usual result of attempting it is 
 to bring- the funereal shaft into great prominence, and to 
 leave the figures almost imperceptible in the general 
 gloom. 
 
 Lights placed upon the pedestal or shaft almost inevi- 
 tably fail of reaching any useful result, by reason of 
 throwing their light too sharply upwards. The angle 
 of the illumination with the vertical should be at least 45 
 degrees, to obtain even a moderately good effect, and this 
 is very rarely attainable. Arc lights placed on pole tops 
 about the monument are sometimes tried, but since from 
 every direction of view some of them must be visible to 
 
2 S2 THE ART OF ILLUMINATION. 
 
 the observer who is trying to see the object which they 
 are supposed to illuminate, their glare quite defeats their 
 main purpose. 
 
 Lights around the base may be able to illuminate the 
 pediment properly, but they should be enough below the 
 general line of vision to be pretty well out of the field of 
 view. 
 
 About the only way of getting any effective illumina- 
 tion at the top of the monument is to use focusing lamps 
 with projectors, something after the arrangement shown 
 in Fig. 109. Three of four of these mounted symmet- 
 rically about the object to be lighted at any convenient 
 distance will come as near an effective illumination as 
 one may expect to get. Their beams should be inclined 
 upwards enough to keep them effectively out of the field 
 of vision, and the rest of the monument, if of light stone, 
 should be left to itself. A figure wholly light in tone can 
 be very beautifully illuminated by such means, as witness 
 the fine colossal figure of Liberty at the Columbian Ex- 
 position, but if of bronze or similar dark material, e. g., 
 the great Bartholdi Liberty, adequate illumination is both 
 very difficult, and if successful, decidedly expensive. 
 
 The problem of effective illumination is still further 
 complicated when the object is a large building or group 
 of buildings. The arcs with reflectors, which may be so 
 well utilized for illuminating a comparatively small ob- 
 ject, become almost useless on a large scale, owing to the 
 total impracticability of furnishing from suitable direc- 
 tions enough light for the purpose. The lighting of a 
 single monument may be regarded as a special case of the 
 illumination often used on the stage, but architectural 
 illumination is a matter very different from both this and 
 from the illumination of large open spaces purely for 
 
DECORATIVE ILLUMINATION. 283 
 
 utilitarian purposes. It has for its object the display, in 
 the most artistic manner, of the architectural values of 
 great buildings and their surroundings. It is essentially 
 decorative rather than utilitarian, and the methods must 
 be governed by the effect desired to be produced rather 
 than by considerations of rigid economy. 
 
 Such work must be done in connection with great ex- 
 positions, important public places, and sometimes in a 
 temporary manner for great civic functions. The object 
 to be attained is no longer solely the illumination of a 
 plane near the ground, but the bringing into splendid 
 prominence of architectural features which would other- 
 wise be lost in the darkness. 
 
 The first fundamental rule in this class of work is to 
 abandon any attempt to simulate daylight, and after 
 providing for adequate illumination of the ground by 
 means which shall not interfere with higher planes of 
 illumination, to sketch in light the principal effects of the 
 scene before the eye. 
 
 Illumination of a great mass of buildings by reflected 
 light is out of the question. If of dark material, it is a 
 sheer impossibility, and even if the buildings be white, the 
 shadow values of the daylight cannot be successfully 
 imitated by radiants placed near the objects, which will 
 therefore look either white and flat or mottled with petty 
 shadows which melt in the distance into a muddy gray. 
 
 The configuration of the lights to be used in the lumi- 
 nous sketch that seems needful for the best artistic results 
 may be roughly determined by making by daylight, or 
 better, near sunset, a rough, clear, line drawing of the 
 scene to be illuminated from a rather distant viewpoint, 
 the further as the scale of the work increases. Then the 
 distribution of lights following the principal points and 
 
284 
 
 THE ART OF ILLUMINATION. 
 
 outlines of this drawing will give the main effects that one 
 wishes to produce. The sketch may be filled up by adding 
 necessary details not too brightly, and the ground illumi- 
 nation must be such as not to interfere with this general 
 arrangement. Reflected light from the radiants thus dis- 
 
 Fig. no. Illumination of the Eiffel Tower. 
 
 tributed plays a useful part in adding to the general 
 brilliancy of the effect without marring its artistic unity. 
 
 This was the principle applied in lighting the Eiffel 
 Tower at the Paris Exposition of 1900, and the result, 
 shown in Fig. no, is most striking. The treatment must 
 depend somewhat on the distance from which the general 
 view is to be taken. The Eiffel Tower demanded, from 
 its immense height looming against the sky, a simpler and 
 more sketchy treatment than would have been advisable 
 in a smaller structure generally viewed at comparatively 
 
DECORATIVE ILLUMINATION. 
 
 285 
 
 short range. Minute detail is lost at a distance in the 
 general glitter, so that only broad treatment remains 
 practicable. 
 
 The result of this general method has never been so 
 magnificently shown as in the lighting of the Pan- Ameri- 
 can Exposition recently held at Buffalo. This was 
 
 Fig. in. The Electric Tower at Buffalo. 
 
 planned by Mr. Luther Stieringer, a past master in the art 
 of decorative lighting in fact, one of the builders of the 
 art itself. Two buildings at this Exposition show with 
 beautiful distinctness the artistic value of the sketching 
 principle just indicated. One, the great Electric Tower, 
 400 ft. high, shown in Fig. 1 1 1, is a perfect example of the 
 application of the principles just laid down. This tower 
 is the dominating note of the whole scheme of illumina- 
 
286 
 
 THE ART OF ILLUMINATION. 
 
 tion, and it is therefore brought to an intensity greater 
 than would be called for were it considered by itself. 
 Even this characteristic would be indicated by a line sketch 
 of the whole Exposition in grand perspective. 
 
 The treatment of a less important building is admirably 
 shown by the other, the Temple of Music, Fig. 112. It 
 is strikingly beautiful, yet perhaps might have been 
 
 Fig. 112. The Temple of Music at Buffalo. 
 
 improved by indicating some of the vertical lines in the 
 lower part of the dome. A feature worth mentioning in 
 the Electric Tower was the simultaneous turning on of all 
 the lights and their gradual increase to normal brilliancy 
 by the use of a huge water rheostat. 
 
 In this method of illumination powerful radiants are 
 both needless and harmful, since they interfere with the 
 
DECORATIVE ILLUMINATION. 287 
 
 freedom of the sketching and blur the effect by masses of 
 reflected light. If used at all in the architectural work, 
 they should be used very sparingly, and Mr. Stieringer 
 very wisely used the 8-cp incandescent lamp as his unit in 
 the great work at Buffalo. Number of lights, and power 
 of free sketching with them, is what is wanted, and for 
 this an 8-cp lamp is quite as effective and far more 
 economical than one of higher power. 
 
 Arcs must not be allowed to intrude themselves on the 
 effects thus produced, following the principles long ago 
 laid down in this volume. When used, as they may some- 
 times be, for ground or interior illumination, they should 
 be so effectively guarded by opal globes that there shall 
 not be a violent contrast in brilliancy between the various 
 planes of illumination. 
 
 At Buffalo Mr. Stieringer dropped the arc altogether, 
 save in certain features of display lighting, like the illumi- 
 nation of fountains and cascades by reflectors, and pro- 
 duced the ground lighting by clusters of incandescents. 
 The real question, however, is not so much the choice of 
 one or another source of light as the preservation of a 
 uniform or skillfully graded tone of brilliancy in the 
 general illumination. This is most easily secured by the 
 use of incandescents alone, although there certainly are 
 cases in which arcs could be used with admirable results 
 as part of the general scheme. One difficulty with the 
 use of incandescents heavily massed near the ground is 
 the certainty of a number of them being burned out every 
 evening, producing unsightly gaps in the symmetry of 
 the display. Such failures are far less conspicuous at a 
 distance, when the lights melt together. The general 
 effect produced may be greatly modified by varying the 
 number and intensity of the lights used. Small luminous 
 
288 THE ART OF ILLUMINATION. 
 
 units not too thickly crowded give a transparency, an 
 airy, unsubstantial appearance, that is lost when the 
 radiants are so powerful or so numerous as to render 
 much of the structure visible by reflected light. 
 
 The principles of architectural illumination have been 
 well understood and skillfully acted upon, though perhaps 
 not definitely formulated, for many years. Before the 
 introduction of electric light reliance had to be placed on 
 gas jets, lamps, and even candles for such work, and there 
 is no doubt that very beautiful effects were produced, 
 although at great cost of labor, and only temporarily. 
 The nature of the radiants was such as almost to preclude 
 the possibility of overdoing the illumination, and only 
 with the advent of electric lights has there developed a 
 strong temptation to try for daylight effects, always a 
 failure from the artistic standpoint. 
 
 The absolute number of lights required to produce cer- 
 tain effects is more a matter of judgment than of calcula- 
 tion. If a row of radiants is intended to melt into a line 
 of light, of course far more lamps are needed than if one 
 merely desires a row of star-like points. Both arrange- 
 ments may be advantageously used even on the same 
 building. The ordinary 8 or i6-cp lamps melt into a 
 practically continuous line at 500 to 800 times the distance 
 between lamps, so that if, as on high buildings, they are 
 normally to be viewed from a considerable distance, they 
 may be rather widely spaced, while near the ground they 
 may well be more closely spaced. A little tact will enable 
 a certain perspective effect to be attained if desired. 
 
 The use of illumination by reflected light cannot well 
 be combined with any other method, except as the lights 
 used for the illumination may give enough surface reflec- 
 tion to enhance the general brilliancy. Therefore the 
 
DECORATIVE ILLUMINATION. 289 
 
 beams from reflector arcs must be kept away from reflect- 
 ing surfaces which are to be sketched out in lines of light. 
 
 Colored light can be effectively used with reflector arcs, 
 on white surfaces, on cascades, in fountains, and the like, 
 but is seldom successful when tried with incandescent 
 lamps, save on a very small scale. The difficulty lies in 
 the dimness of colored bulbs and the failure of attempts 
 to get delicate tints in this way. Colored glass bulbs are 
 expensive, and coated bulbs accumulate dust and are 
 seldom weather proof. 
 
 Much decorative lighting is for temporary purposes, 
 but with the present facilities for obtaining current and 
 the temporary mountings that can readily be obtained, the 
 work is comparatively easy. 
 
 Special receptacles for signs and decorative designs are 
 now made in convenient form for quickly putting together, 
 and enable temporary work for special occasions to be 
 very easily done. Fig. 113 shows one useful form of 
 mounting device, in which the weather-proof receptacles 
 can be quickly strung together with clamps and held 
 neatly spaced in any way desirable. For decorative work 
 on a considerable scale the retaining clamps would, of 
 course, be much longer than here shown. 
 
 There is a fine chance for art in turning on the lights in 
 architectural and other decorative work. The water 
 rheostat, bringing all the lights simultaneously from a 
 dull red glow to full brilliancy, is by far the most compre- 
 hensive scheme for the purpose. In the absence of this, 
 or in permanent work of which only a part is regularly 
 used, the circuits should be so arranged as to allow a per- 
 fectly symmetrical development of the lighting without 
 throwing on a very large current at any one time. 
 
 In any and all decorative work the illumination must 
 
2 9 o THE ART OF ILLUMINATION. 
 
 be subordinated to the general architectural effect. Sins 
 against art in this respect are all too common. Imagine, 
 for example, a Doric temple with arc lights at the corners 
 of the roof and festoons of red, white, and blue incandes- 
 cents hung between the columns. About a structure of 
 such severe simplicity lights must be used with extreme 
 caution, while more ornate buildings can be treated with 
 far greater freedom of decoration. 
 
 It requires both fine artistic instinct and great technical 
 skill to cope adequately with the problems of decorative 
 illumination. The tricks of the art are manifold, and 
 mostly meretricious. The facility with which electric 
 currents may be manipulated is a continual temptation to 
 indulge in the ingenious and the spectacular without due 
 regard for the unity of the results. Whirligigs, waving 
 banners, rippling water, and the like are better suited to 
 a Coney Island merry-go-round than to serious attempts 
 at decoration. 
 
 Another class of work, hardly a part of ordinary light- 
 ing, but yet of considerable interest, is the use of lights 
 purely for decorative purposes in interiors, in halls and 
 auditoriums for special designs, and as part of the decora- 
 tive scheme of ballrooms and the like. This is really a 
 branch of the art due entirely to electric lighting since 
 only by this means can it be rendered fully serviceable. 
 Most branches of illumination are in a measure indepen- 
 dent of the particular radiants employed. But the ease 
 and safety with which incandescent lamps can be installed 
 renders them peculiarly applicable to such interior work. 
 
 In operating on a comparatively large scale, all sorts of 
 decorative designs can be carried out by means of 8-cp or 
 i6-cp lamps strung together in receptacles, in the 
 manner of Fig. 113, or otherwise temporarily mounted 
 
DECORATIVE ILLUMINATION. 291 
 
 for the purpose. For work on a smaller scale, or in the 
 preparation of very elaborate designs, other means may 
 be employed. 
 
 For purely decorative purposes the miniature lamps 
 serve a very useful purpose. Regular incandescents are 
 made down to 6, or even 4, candle-power, but as has 
 already been explained, the filaments for these powers at 
 ordinary voltages must needs be very slender and fragile, 
 and the lamps are nearly or quite as bulky as those of 
 ordinary candle-power. 
 
 Hence for many uses it is better to make miniature 
 lamps for connection in series, each lamp taking 5 to 25 
 
 Fig. 113. Chain of Receptacles. 
 
 volts to bring it to normal candle-power. Imagine a 
 i6-cp, loo-volt lamp filament cut into four equal parts, 
 and each of these parts mounted in a separate small bulb, 
 and you have a clear idea of the principle involved. Com- 
 monly the miniature lamps for circuits of 100 to 125 volts 
 are of 5 or 6 candle-power, and connected five in series 
 across the ordinary lighting mains. Fig. 114 gives an 
 excellent idea of the size and appearance of the perfectly 
 plain miniature lamp. It is fitted to a tiny socket of the 
 same general construction as the standard sockets for 
 ordinary lamps, but taking up so little room that the lamps 
 can conveniently be assembled in almost any desired form. 
 
292 THE ART OF ILLUMINATION. 
 
 It is not altogether easy to manufacture these lamps 
 so as to attain the uniformity necessary, if the lamps are 
 to be run in series, and this at present constitutes a serious 
 obstacle to their use on a large scale. They are generally 
 not of high efficiency, since great uniformity and good 
 life are the qualities most important. 
 
 They can be fitted with tiny ornamental shades, and 
 
 Fig. 114. Miniature Incandescent Lamp. 
 
 may be obtained of various shapes and colors, so that very 
 elaborate decorative designs can be built up of them. In 
 indoor work colored lamps may be freely used, and are 
 capable of producing some very beautiful effects, but the 
 plain or ordinary frosted lamps are most generally used. 
 
 Owing to the small size of the sockets and fittings, the 
 miniature lamps can be packed so closely as to produce 
 the effect of an almost uniform line of light at com- 
 paratively small distances, so that most ornate schemes 
 of ornamental illumination can be carried out by their aid. 
 They are also very useful in building up small illuminated 
 signs. 
 
DECORATIVE ILLUMINATION. *93 
 
 Lamps of special sizes and shapes, from a tiny 
 bulb, hardly bigger than a large pea, to the candle -shaped 
 lamp of 5 or 6 candle-power, are sometimes used with good 
 effect in interior decoration. Figs. 115, 116, 117, and 
 118 show some of the commoner shapes used for such 
 purposes. When a regular electrical supply is not avail- 
 able, these little lamps can be obtained for very moderate 
 voltages, say, from 5 to 10 volts, and can be run in 
 parallel from storage cells, or even from primary batteries, 
 for temporary use. 
 
 Such small lamps are sometimes used in the table 
 decorations for banquets, and for kindred purposes. By 
 their aid surprising and beautiful effects are attainable, 
 which would be quite impossible with any flame illu- 
 minant. But they must be cautiously used, for their very 
 facility tends to encourage their employment in effects 
 more bizarre than artistic. 
 
 It is well, too, to add a word of caution as regards the 
 possible danger from fire. It is so easy to wire for incan- 
 descents that, particularly when using miniature lamps, 
 there is a natural tendency to rush the work at the expense 
 of safety. Lamps in series on a i zo-volt circuit are quite 
 capable of dangerous results if anything goes wrong, and 
 even the battery lamps are not absolutely safe in the 
 presence of inflammable material. 
 
 It should therefore be an invariable rule not to install 
 a temporary decorative circuit without the same attention 
 to detail that would be exercised in a temporary circuit 
 of the ordinary incandescents. The same precautions are 
 not always necessary, but all the wiring should be care- 
 fully done, joints should be fully protected, and, particu- 
 larly, lamps should be kept out of contact with inflam- 
 mable material. 
 
294 
 
 THE ART OF ILLUMINATION. 
 
 The incandescent lamp is often commended as produc- 
 ing little heat, and, in fact, as compared with other illumi- 
 nants, its heating power is small. But a vessel of water 
 can be boiled by plunging an ordinary i6-cp lamp in it 
 nearly up to the socket, and cloth wrapped about such a 
 lamp will infallibly be ignited within a comparatively 
 
 Fig. 115. Fig. 116. Fig. 117. 
 
 Various Forms of Miniature Lamps. 
 
 Fig. 1 1 8. 
 
 short time. The fact that the cloth does not burst into 
 flame in a few minutes does not indicate safety, for time 
 is an important element in ignition, and even an over- 
 heated steam pipe is capable of setting a fire, low as its 
 temperature is. A good many fires have been started in 
 shop windows by hanging fabrics too near to incandes- 
 
DECORATIVE ILLUMINATION. 295 
 
 cent lamps, and even the miniature lamps are quite 
 capable of similar mischief if in contact with anything 
 easily inflamed. No illuminant has so high an efficiency 
 that it produces a negligible amount of heat from the 
 standpoint of fire risk. 
 
 Special cable is now made to which lights can be at- 
 tached with great facility, and by this means temporary 
 work may be quickly and safely done. 
 
 In ordinary domestic illumination miniature lamps have 
 very little place. Nothing is to be saved by using them 
 so long as they must be used in series at ordinary voltages. 
 Now and then a 4 or 6-cp lamp may be useful as a night 
 lamp, but it is better to use an ordinary lamp of moderate 
 efficiency than to try miniature lamps. Sometimes, how- 
 ever, a circuit of miniature lamps may be installed for a 
 dining room or a ballroom with excellent artistic results. 
 In such cases it is better to use ground than plain lamps, 
 and, as a rule, colored lamps should be eschewed, on 
 account of the impossibility of getting delicate tints to 
 show effectively. 
 
 Temporary decorative circuits may, however, be very 
 useful in domestic illumination for fetes and the like, in 
 which case delicately colored ornamental shades can be 
 applied or the lamps may be used in Japanese lanterns. 
 Any country house fitted for electric lights can be 
 be temporarily wired for such purposes rather easily, and 
 out-of-door temporary wiring can be installed without 
 the rigid precautions necessary indoors. 
 
 In all decorative lighting it is important to recognize the 
 fact that illumination is a means to an artistic end, and not 
 of itself the primary object. One is, in these days of 
 electric lighting, far more likely to err by providing too 
 much light than by failing to supply enough. 
 
296 THE ART OF ILLUMINATION. 
 
 Great brilliancy is far less important than good distribu- 
 tion and freedom from glare. It is highly probable, for 
 instance, that the effect of the illumination of the Electric 
 Tower at the Pan- American Exposition would have been 
 seriously injured by the substitution of 32-cp lamps for 
 the 8-cp actually used, and it is absolutely certain that a 
 dozen arc lights injudiciously placed would have detracted 
 greatly from the harmonious result. 
 
 In interior illumination the same rule holds true. By 
 the reckless use of brilliant radiants one can key the vision 
 up to a point where its power of appreciating values in 
 illumination is almost entirely lost. In decorative light- 
 ing great care must be used not to approach this point, to 
 leave the relief afforded by light and shade, and to realize 
 the perspective in the details of the illumination. 
 
 In the absence of a foreground one's judgment of dis- 
 tances is completely upset, as witness the great difficulty 
 experienced in estimating distances correctly over water 
 on the one hand or in a thin fog on the other. In scenic 
 illumination the distribution of luminous values can be 
 utilized with great facility for producing illusions of 
 distance, giving at will the effect of startling flatness or of 
 interminable vistas. In stage illumination such devices 
 are now and then used to heighten the effects, although 
 other exigencies often interfere with the proper develop- 
 ment of the scheme. 
 
 The commonest cause of failure in proper illumination 
 is thrusting a brilliant light between the spectator and the 
 object to be viewed, with the inevitable result of losing 
 detail and hurting the eyes. Brilliant diffused light is in 
 this particular only less objectionable than direct light, 
 and both should be assiduously avoided. 
 
 It must not be supposed that decorative lighting must 
 
DECORATIVE ILLUMINATION. 297 
 
 necessarily be electric, since very beautiful results were 
 attained before electric light was heard of, but electric 
 lighting is unquestionably the most facile means of secur- 
 ing artistic results on a large scale. 
 
 A special department of lighting, peculiar to the electric 
 branch of the art, is the use of the searchlight for scenic 
 or utilitarian purposes. The searchlight is now a familiar 
 object, consisting of a very powerful arc light, taking 
 
 Fig. 119. Searchlight Lamp. 
 
 from 20 to 50 or more amperes, kept steadily by auto- 
 matic focussing apparatus in the focus of a parabolic 
 mirror, sometimes with an auxiliary lens system. The 
 material of the mirror is most often silvered glass, unless 
 
298 THE ART OF ILLUMINATION. 
 
 the parabolic surface is very deep, when silvered metal is 
 generally employed. 
 
 As the purpose of the searchlight is to give a parallel 
 beam of light, the carbons between which the arc is 
 formed are not in line, but staggered, as shown in Fig. 1 19 
 so that the crater of the arc points obliquely backward, 
 and the carbons are tilted so that this crater faces fairly 
 the apex of the mirror instead of its aperture. An opaque 
 disk between the arc and the mirror aperture cuts off all 
 stray direct light, so that all the light sent out is delivered 
 from the mirror in a nearly parallel beam. The whole 
 affair is mounted in a case having rotation about a hori- 
 zontal and a vertical axis, forming the familiar device 
 shown as a whole in Fig. 120. 
 
 The mirror aperture may vary from a foot or so up to 
 4 or 5 ft., the searchlights most often used having from 
 2 to 3 ft. of aperture. The most perfect results are given 
 by using a rather shallow parabolic mirror of silvered 
 glass, which can be given a better and more permanent 
 figure than a deep metallic mirror, and hence gives a 
 beam more accurately parallel. 
 
 The searchlight, when properly constructed, will throw 
 a dazzling beam many miles on a clear night, but in foggy 
 weather, or even in a comparatively thin haze, its field of 
 usefulness is greatly limited. It is of only casual use in 
 ordinary forms of illumination, and its chief legitimate 
 use is the illumination of special objects, in the manner 
 already described in connection with the simpler reflector 
 arcs. It is often abused by its application to advertising, 
 and to a glaring and offensive simulation of daylight in 
 places that have no need of it. 
 
 It is of considerable military and naval value, serving 
 to detect movements of an enemy's troops or to pick up 
 
DECORATIVE ILLUMINATION. 
 
 299 
 
 hostile vessels, and it is also of no small importance in 
 military signaling over long distances. For this purpose 
 it is turned upwards upon the distant sky, where its glare 
 is visible in clear weather even up to a distance of forty 
 or fifty miles. Then, by deflections of the beam, or by 
 periodically cutting it off by a register shutter over the 
 front, communication may be established by the regular 
 Morse or heliographing code, either openly or in cipher. 
 
 Fig. 120. Search Light. 
 
 Its use in this fashion was especially striking during the 
 recent operations for the relief of Kimberley. 
 
 It is also a valuable adjunct in coast defense, particu- 
 larly of narrow channels and of mine fields, where it can 
 be used both to confuse the hostile pilots and to make a 
 clear target of hostile ships. But its range of effective- 
 ness for such purposes is popularly much over-estimated. 
 
300 THE ART OF ILLUMINATION. 
 
 In clear weather it would quite certainly pick up a large 
 vessel by the time it had come within effective gun range. 
 On torpedo boats and similar small craft, however, 
 painted in neutral tints, as they are for war, the searchlight 
 has a useful scope of little over a mile in distance, and in 
 hazy weather even less. It has therefore for naval, as for 
 general purposes, a somewhat circumscribed field of use- 
 fulness, within which, however, it is undeniably of great 
 value. 
 
CHAPTER XIII. 
 
 THE ILLUMINATION OF THE FUTURE. 
 
 AT the present time the ordinary materials of illumina- 
 tion are pretty well understood, and their proper use is a 
 matter of good judgment and artistic sense. Illumina- 
 tion is not a science with well-defined canons of what one 
 might call illuminative engineering, but an art wherein an 
 indefinable and uncommunicable skill pertains almost as 
 it does in the magic of the painter. 
 
 There are certain general rules that must be followed, 
 certain utilitarian ends to be served, but whether the re- 
 sult is brilliantly successful or hopelessly commonplace 
 depends on the skill that inspires it. There must be in 
 effective illumination a constant adaptation of means to 
 ends, and a fine appreciation of values that quite defies 
 description. One may attack the problem of illumina- 
 ting a great building with all the resources of electrical 
 engineering at his command, and score a garish failure, 
 or he may conceivably be confined to the meager bounds 
 of lamps and candles, and still triumph. 
 
 The general tendency with the modern intense radiants 
 at command is to light too brilliantly, to key the vision 
 to so high a pitch that it fails to appreciate the values of 
 the chiar-oscu/ro on which the artistic result depends. 
 
 The desideratum in illumination, except for a small 
 group of scenic effects, is the possession of cheap and fairly 
 powerful radiants of low intrinsic brilliancy capable of 
 
 301 
 
302 THE ART OF ILLUMINATION. 
 
 modification in delicate color tones. It is doubtful 
 whether these qualities are compatible with very high 
 luminous efficiency in a flame or incandescent radiant. 
 In modern gas and electric lighting the progress toward 
 efficiency is in the direction of very high temperature, 
 which implies high intrinsic brilliancy. 
 
 Vacuum tubes lamps, at present in only a crude experi- 
 mental stage, give hope of better things, but at great risk 
 of color difficulties, particularly if high efficiency is 
 reached. 
 
 Ideally, a gaseous radiant, with nearly its whole lumi- 
 nous energy concentrated in the visible spectrum, would 
 give magnificent efficiency, but it by no means follows that 
 it would give a good light. Sodium vapor meets the re- 
 quirements just noted tolerably well, yet there is no more 
 ghastly light than that given by a salted spirit lamp. 
 
 It mi'ght be possible to work with a mixture of 
 gases such as would give a light approximately white to 
 the eye, and yet be very far from a practicable illuminant, 
 for the phenomena of selective absorption are such, as we 
 have already seen, that the color of a delicately tinted 
 fabric depends on its receiving a certain scale of colors in 
 the light which it reflects. To the eye a much simpler 
 color scheme is necessary to reproduce light substantially 
 white, and such light falling on a colored fabric would by 
 no means necessarily bring out the tints that glow by 
 daylight. 
 
 Even the firefly's secret, could man once penetrate it, 
 might not prove such a valuable acquisition as it would 
 seem at first thought. To the eye the light of most 
 species seems greenish, and, in point of fact, it so com- 
 pletely lacks the full red and the violet rays that its 
 effect as an illuminant on a large scale would be most 
 
ILLUMINATION OF THE FUTURE. 33 
 
 disagreeable, far worse than an early Welsbach at 
 its most evil stage of decrepitude. We must not 
 only steal the firefly's secret, but give him a few useful 
 hints on the theory of color before the net result will be 
 satisfactory from the aesthetic standpoint. Firefly light 
 might do for a factory, but it would find but a poor market 
 as a general illuminant. 
 
 It is a somewhat difficult matter satisfactorily to define 
 the efficiency of an illuminant. Luminosity depends, like 
 sound, upon the physiological relations of a certain form 
 of energy, and cannot be directly reduced to a mechanical 
 equivalent. 
 
 The commonest conception of the efficiency of an 
 illuminant is to regard it as defined by the proportion of 
 the total radiant energy which is of luminous wave 
 lengths. From this point of view the efficiency may 
 approach unity either by the absence of infra-red and 
 ultra-violet rays, in other words, by purely selective radia- 
 tion or by so great an increase of radiation in the visible 
 spectrum as to render the energy of the remainder nearly 
 negligible. 
 
 In the former sense the luminous radiation of the firefly 
 is of perfect efficiency; but, obviously, a purely mono- 
 chromatic light utilizing the same total amount of energy 
 might give a vastly better illumination or a much worse 
 one, according to the wave length of the light in relation 
 to its effect on the eye. 
 
 On the other hand, an arc between tiny pencils of the 
 material used for Nernst glowers is reputed to give, so 
 far as watts per candle-power go, an efficiency nearly as 
 good as can be claimed for the firefly. The experiments 
 in this case are perhaps not beyond cavil, but, even grant- 
 ing their substantial accuracy, it is perfectly certain that 
 
3 o 4 THE ART OF ILLUMINATION. 
 
 such an arc gives radiation by no means confined to the 
 visible spectrum. 
 
 The most that can be said in a definite way is that 
 assuming a continuous spectrum with its maximum 
 luminous intensity in the yellow or yellowish green, there 
 seems to be little chance of doing much better than about 
 0.2 watt per candle-power. 
 
 Until practical illuminants of some kind can be worked 
 at an efficiency within hailing distance of this figure, one 
 need scarcely worry about the possibility of combining 
 nearly monochromatic radiations so as to give true chro- 
 matic values. 
 
 At the present time only the most powerful arcs ap- 
 proach an efficiency of i watt per spherical candle-power 
 when so shaded as to be of much use as illuminants in the 
 ordinary sense. Ordinary arcs properly shaded are good 
 for 2 to 3 watts per candle-power, and even the best 
 incandescents will hardly do better than 4 watts per 
 candle-power. 
 
 For everyday work the thing most needed is an efficient 
 light of moderate candle-power and moderate intrinsic 
 brilliancy combined with low cost and good color. Save 
 under special circumstances very powerful radiants are 
 disadvantageous, particularly if of great intrinsic bril- 
 liancy. 
 
 Casting about the field, it certainly appears at first 
 glance as though most modern radiants had been de- 
 veloped in the wrong direction. In particular, electric 
 lights have been steadily pushed in the direction of 
 enormous working temperature and very great intrinsic 
 brilliancy, gaining greatly in efficiency, of course, but los- 
 ing in convenience. What is most wanted is not a light 
 giving 5000 candle-power at 0.2 watt per candle, but one 
 
ILLUMINATION OF THE FUTURE. 35 
 
 giving 5 or 10 candle-power at even i watt per candle. 
 The vacuum tube lamp seems at present to give the 
 greatest chance for revolutionary improvements, and even 
 this seems to involve very serious difficulties. 
 
 Similarly, in gaslights we have regenerative and 
 mantle burners giving 50 or 100 candle-power at a very 
 good efficiency, but they are too powerful and too bright 
 to be entirely satisfactory, even were they open to no other 
 objections. For most purposes a Welsbach giving 15 
 candle-power on i cubic foot of gas per hour would be 
 vastly more useful than one giving 75 candle-power on 4 
 cubic feet per hour. Of flame radiants none save acety- 
 lene marks any material advance in recent years in point 
 of easy applicability. 
 
 It would seem that modern chemistry might achieve 
 something of value in adding to the materials of illumina- 
 tion. There is a group of occult substances possessing 
 enormous power of giving off radiation akin to that in- 
 volved in the X-ray, whatever that might be. It is perhaps 
 not too much to hope that some material of similar 
 potency with respect to luminous rays may reward the 
 pertinacious investigator. There is no intrinsic reason 
 why an exaggerated type of phosphorescence, capable cf 
 storing sunlight at a high efficiency, may not in due 
 season be evolved. This would settle the artificial light- 
 ing problem unless the color were irremediably bad in 
 a beautifully simple way. Or it might be possible to re- 
 produce by a commercial process the slow oxidation or an- 
 alogous change responsible for the glowing of decaying 
 wood and of certain micro-organisms, and probably also 
 for the light of the firefly and his allies. 
 
 Whatever the method, the aim of improvement should 
 be the production of efficient lights of moderate intensity 
 
306 THE ART OF ILLUMINATION. 
 
 and intrinsic brilliancy, coupled with good color, prefera- 
 bly capable of easy modification. 
 
 The steady tendency as the art of illumination has 
 advanced has been towards more and more complete 
 subdivision of the radiants, and the subordination of 
 great brilliancy to perfect distribution. One of the most 
 important lessons of the Pan-American Exposition was 
 Mr. Stieringer's demonstration of the magnificent useful- 
 ness of 8-cp incandescent lamps, skillfully installed. 
 
 In the art of illumination as much depends on the 
 efficient use of lights as on the efficiency of the lights 
 themselves. A tallow candle, just where it ought to be, 
 is better than a misplaced arc lamp, and, even taking our 
 present illuminants with all their limitations, skill will 
 work wonders of economy. 
 
 It is particularly in the direction of adroit use that the 
 present path of progress lies. One of the fundamental 
 facts in practical lighting which has been many times 
 suggested in these pages, and which lies at the root of 
 improvements, is the need of keeping down intrinsic 
 brilliancy. 
 
 The true criterion of effective and efficient lighting is 
 not simple illumination, which resolves itself into a pure 
 matter of candle feet, but visual usefulness, which takes 
 account of the physiological factors in artificial lighting. 
 
 If one denotes the illumination measured in candle feet 
 or other convenient units by /, then the visual usefulness 
 is measured by the product / 6 7 where a is proportional 
 to the effective area of the iris. This of course is con- 
 stantly shifting as the illumination changes, but, broadly, 
 it is an inverse function of the intrinsic brilliancy of the 
 radiants used. The criterion thus becomes of the form 
 
 * ~~7T#y where B is the intrinsic brilliancy of the 
 
ILLUMINATION OF THE FUTURE. 37 
 
 radiant, and i is the visual usefulness, or the effective bril- 
 liancy of the illumination. 
 
 Now as a matter of practice this is important, for it 
 indicates that a badly placed arc light, for example, may 
 actually work serious injury to the effective illumination, 
 and within reasonable limits one could fairly go> so far as 
 to say that the usefulness of an unmodified radiant varies 
 inversely with its intrinsic brilliancy. Obviously, then, 
 shading the radiant may actually gain useful illumination, 
 although it actually loses light, which in fact experience 
 has shown to be the case. 
 
 As to the permissible intrinsic brilliancy for ordinary 
 cases of illumination, exact figures are from the nature of 
 the case hardly attainable. Yet one may derive a pretty 
 clear idea of the situation from the experiments of Pro- 
 fessor L. Weber given in the following table reduced to 
 candle-power per square inch. 
 
 Horizontal white card reflecting brilliant sunlight, . . 25 
 White cloud reflecting brilliant sunlight, .... 7 
 
 Argand burner, . . . . . . . .6.5 
 
 Horizontal white card under a dull winter sky, . . 0.26 
 
 Now the intensity in the first named case is certainly 
 most painfully great, and even those in the second and 
 third cases are still great enough to be very unpleasant 
 if fairly in the field of view. On the other hand, the last 
 case evidently is one in which the intrinsic brilliancy is 
 unnecessarily low. 
 
 Taking all these things into consideration, it is a safe 
 working rule to keep the intrinsic brilliancy of all 
 radiants within the Held of vision below 5 cp per square 
 inch preferably down to half that value. 
 
 This limit affords a means of determining the approxi- 
 mate size of any diffusing globe or shade, since evidently 
 
3 o8 THE ART OF ILLUMINATION. 
 
 whatever the candle-power of the light, the visible diffus- 
 ing surface must not exceed a brilliancy of 5 cp per square 
 inch. If, therefore, we are dealing with a light of 100 
 candle-power, that amount of light must be scattered over 
 and by at least twenty square inches of diffusing surface. 
 Two conditions enter to modify the situation : On the one 
 hand, a certain amount of the inwardly incident light is 
 actually intercepted by the shade; on the other hand, the 
 diffusion is not uniform, especially if the radiant has great 
 intrinsic brilliancy and the shade is fairly translucent. 
 For heavily ground or fairly dense opal shades the above 
 ratio is not far from right, the modifying factors tending 
 to offset one another. Such shades intercept about one- 
 third of the total light as a necessary feature of keeping 
 the intrinsic brilliancy within bounds, so that it is not 
 unfair to say that for most practical purposes 100 candle- 
 power in a radiant of really low intrinsic brilliancy is as 
 useful as 150 candle-power in a very intense radiant. 
 
 Now practically all our modern sources of light 
 require shading, if within the field of vision. The obvious 
 moral is that one of the great .economies in lighting is 
 centered in keeping the radiant out of this field. 
 
 In electric lighting, incandescent lamps at 3 watts 
 per candle asea , so disposed as to keep clear of the field 
 of vision, are fully as valuable illuminants as lamps at 2 
 watts per candle wrongly installed, so as to either dazzle 
 the eye or to require shading to avoid it. Shaded they 
 must be for hygenic reasons whenever visible. 
 
 In actual practice it is a matter of great difficulty to 
 place lights wholly out of the field of vision, and the more 
 brilliant the lights are the greater necessity for shading 
 them. Hence, it becomes a difficult matter to treat 
 modern illuminants without loss of efficiency. 
 
ILLUMINATION OF THE FUTURE. s9 
 
 Perhaps the most promising line of improvement in 
 artificial lighting, and the one from which most may be 
 expected in the near future, is indirect lighting by dif- 
 fusion. A glance at the tables in Chapter III. shows that 
 with a good diffusing surface scarcely more light is lost 
 than is cut off by proper shading. As the intrinsic bril- 
 liancy of the source rises, the relative importance of 
 diffusion increases, since shading to be effective must be 
 denser. 
 
 Of diffusing shades only the holophanes intercept ma- 
 terially less light than would be lost in a good diffuse 
 reflection, and even in this case the shade must be of con- 
 siderable dimensions to keep the intrinsic brilliancy suf- 
 ficiently low. As compared with a ground glass or opal 
 shade, they should have considerably greater total surface 
 for a light of equal power. 
 
 There is room for splendid developments in diffuse 
 lighting, using arcs, Nernst lamps, incandescents, Wels- 
 bach mantles, and acetylene. In this way such radiants can 
 be used unshaded with the full advantage of their great 
 efficiency, and with good diffusion from white or nearly 
 white surfaces the net efficiency remains high. As has 
 already been noted, lighting by diffusion in ordinary dwell- 
 ings, where the surfaces are not generally good, requires 
 a liberal use of light, but with a careful study of the 
 conditions will come the possibility of very efficient and 
 beautiful lighting in which the radiants shall, save in rare 
 instances, be wholly invisible. 
 
 This method of working, too, has a great artistic advan- 
 tage, in that the light can be successfully modified by 
 tinted diffusing surfaces with far greater success than by 
 any arrangement of colored shades. 
 
 The latter are not available in delicate and easily 
 
3 io THE ART OF ILLUMINATION. 
 
 graduated shades, while pigments can be worked upon 
 diffusing surfaces in almost any desired manner. 
 
 It thus becomes possible to use effectively not only 
 radiants of intrinsic brilliancy too great to be easily 
 managed by shades, but those of naturally objectionable 
 colors. Bad color is of course equivalent to inefficiency 
 in many instances, since a considerable amount of light 
 must be cut off and thrown away to correct the color, but 
 this can be done at as little loss by diffuse reflection as by 
 any other method. 
 
 The weak point of lighting by diffusion is the fact that 
 the radiants are then usually installed in rather inaccessi- 
 ble places, and the globes are likely to suffer from dust, 
 unless special care is taken. A favorite location for such 
 lights is above and partly behind a cornice, a situation in 
 itself very advantageous but difficult to get at. Ordinary 
 ceiling or cornice incandescent lamps can be removed for 
 cleaning by a special handler made for the purpose, but 
 lights behind a cornice must be reached with a step 
 ladder. 
 
 Gaslights, of course, cannot be readily installed in such 
 a situation, and when used by diffusion must be screened 
 like arc lamps. 
 
 The introduction of new illuminants is very likely to 
 effect useful modifications in our methods of lighting. 
 If the vacuum tube line of experimentation leads to any- 
 thing practical, it will probably provide light of rather 
 low intrinsic brilliancy, so that shading will be relatively 
 less important than it now is. There will thus be a 
 practical gain in efficiency even greater than the gross 
 relative efficiencies of the lights would indicate. 
 
 Perhaps the most promising light of the class just 
 mentioned is the mercury lamp, to which some reference 
 
ILLUMINATION OF THE FUTURE. 3 
 
 has already been made. Up to the present its very ob- 
 jectionable color stands in the way of its commercial 
 development, but if this fault can be remedied the mercury 
 lamp certainly has a future, since it is highly efficient, 
 and can be worked successfully on the ordinary con- 
 tinuous current constant potential circuits. Most vac- 
 uum tube schemes, and indeed most of the other devices 
 recently suggested for securing high efficiency, require 
 alternating currents, so that the mercury lamps would be 
 particularly welcome as averting the need of an extensive 
 change of equipment. 
 
 Increase of efficiency in mantle radiants may in some 
 degree be obtained by the use of substances giving more 
 strongly selective emission of light than any now familiar, 
 but, aside from this, efficiency can only be raised by forc- 
 ing the temperature. 
 
 The somewhat promising field of phosphorescence is 
 practically unexplored. A few interesting, but so far 
 futile, experiments have been tried by Edison and others 
 as a result of X-ray investigations. The subject is well 
 worth investigation, both from the electrical and the 
 chemical sides, and will doubtless take its turn sooner or 
 later. 
 
 Meanwhile we must do the best we can with the 
 illuminants which are now at hand, to furnish light of 
 suitable amount and quality. To sum up the suggestions 
 repeatedly made in these pages, the commonest failings 
 in present methods of lighting are a tendency to use too 
 brilliant radiants and to make up in quantity what is lack- 
 ing in quality. More study of the practical conditions 
 of lighting and less blind faith in bright lights would 
 generally both improve practical illumination and tend to 
 economy. 
 
312 THE ART OF ILLUMINATION. 
 
 Imagine, for example, an attempt to light a billiard room 
 where the balls had been stained to match the cloth. Yet 
 this sort of thing, on a less aggravated scale, happens far 
 oftener than would be thought possible. Even in build- 
 ings designed to fulfill hygienic conditions, sins against the 
 fundamental principles of lighting are distressingly com- 
 mon. An observing writer has grimly designated modern 
 schools, " Bad-eye factories," and certainly, even with the 
 full advantage of natural light and buildings in which 
 conditions ought to be favorable, the results are frequently 
 bad. 
 
 With artificial light the task of proper lighting is of 
 increased difficulty, and is further complicated by the 
 sometimes impossible requirements of the latest fashion- 
 able scheme of decoration. The best results can be at- 
 tained only by constant attention to details and a keen 
 perception of the conditions to be met. 
 
 The illumination of the future ought to mean the intel- 
 ligent use of the lights we now have, not less than the 
 application of the lights which we may hope in the full- 
 ness of time to obtain. 
 
CHAPTER XIV. 
 
 STANDARDS OF LIGHT AND PHOTOMETRY. 
 
 OF all important physical constants none are in so un- 
 satisfactory a state as those pertaining to illumination. 
 In spite of the efforts of several scientific congresses, there 
 is no international convention regarding the unit of 
 luminous intensity, nor is there any one practical unit in 
 general use. 
 
 A standard to be worthy the name should be accurately 
 reproducible without extreme difficulty, and ought, as 
 well, to bear a fairly simple relation to other units which 
 are related to it. Now, a standard of light stands quite by 
 itself in kind, and should consist of some determinate light- 
 giving body so constituted that it can be reproduced and 
 used in any part of the world without material error. 
 
 Unhappily, such a light-giving body is not easily, if at 
 all, obtainable, hence the present confusion. 
 
 The only attempt yet made to produce a really logical 
 and scientific unit was that brought to the world's attention 
 by M. Violle at the international electrical congress held 
 in Paris in 1884. Violle proposed as the unit of luminous 
 intensity the light emitted, normal to its surface, by one 
 square centimeter of platinum at its melting point. 
 
 This unit was one based on definite things; was of very 
 convenient magnitude, about 20 candle-power; and of 
 good color, nearly white. But it is a very inconvenient 
 unit to work with, and to reproduce accurately, on account 
 
 313 
 
3i4 THE ART OF ILLUMINATION. 
 
 of the enormous temperature necessary to melt platinum, 
 the uncertainty introduced as to its exact melting point 
 by the presence of trifling impurities, and for other minor 
 but sufficient reasons. So the upshot of the matter has 
 been that this unit has been laid upon the shelf, and while 
 much good came from the agitation of the subject, the 
 world still depends on the curious assortment of units 
 sanctioned by more or less extensive usage. 
 
 The practically and legally adopted unit of light in 
 English-speaking countries is the so-called standard can- 
 dle. This illuminant has its composition, dimensions, 
 weight, and rate of burning specified by law, and can be 
 cheaply and easily obtained. It is a spermaceti candle, 
 weighing 1200 grains avoirdupois (6 to the pound), and 
 burning at the rate of 120 grains per hour. The standard 
 diameter is 0.8 inch at the top and 0.9 inch at the base, 
 and the normal height of the flame is about i% inches 
 over all. 
 
 The rate of burning may vary in practice from about 
 no grains to 130 grains per hour, and in photometric 
 work the luminous intensity is assumed to vary directly 
 with the rate of burning. Selected candles burned under 
 uniform conditions run somewhat closer to the standard 
 rate of burning than the above figures, and burn with a 
 uniformity that, considering their structure, is remarkable, 
 but the presence of the wick, accidental variations in 
 manufacture, and numerous minor causes make the candle 
 at best rather unreliable. With great care in using it 
 may be coddled into a degree of precision of approxi- 
 mately two or three per cent. ; but variations of twice that 
 amount are common. 
 
 In France, and to a considerable extent in Italy, the 
 Carcel lamp is used. This is a standard which was 
 
STANDARDS OF LIGHT. 315 
 
 adopted after the investigations of Dumas and Regnault 
 some forty years since. It is an oil lamp of special con- 
 struction, made according to a very minute specification 
 as to dimensions, including the structure and weight of 
 the wick, and burns refined colza oil, largely used as an 
 illuminant in France. Its normal consumption of this 
 oil is 42 grams per hour, with a permissible variation of 
 4 grams per hour in either direction. It gives a rather 
 yellowish light of nearly 10 candle-power, and has prob- 
 ably about the same possible degree of precision as the 
 English candle, though it should average a little better. 
 
 In Germany a standard paraffine candle, made in pursu- 
 ance of a most elaborate specification, is used to some 
 extent. It carries a longer flame than the English candle, 
 two inches being the standard height, and is about 10 per 
 cent, more powerful, with, in other respects, much the 
 same general properties. 
 
 The standard most used in Germany, however, and 
 often employed in other countries for purposes of refer- 
 ence, is the so-called Hefner unit, being the light given 
 by the amylacetate lamp introduced by Von Hefner- 
 Alteneck. This standard lamp is made from a uniform 
 specification as to dimensions, and has the great advan- 
 tage of burning a perfectly definite chemical compound 
 easily obtained in a state of great purity. It has been 
 very exhaustively investigated at the Reichanstalt, from 
 which certified tested standard lamps can readily be 
 obtained, and its performance under varying conditions 
 of flame height, temperature, barometric pressure, and so 
 forth, has been carefully studied. Its normal flame is 
 40 mm. high and its intensity is then about 10 per cent, 
 less than that of the English candle. 
 
 Being the legal standard in Germany, and widely used 
 
3i6 THE ART OF ILLUMINATION. 
 
 elsewhere on account of its steadiness and the accessibility 
 of certified examples, the Hefner-Alteneck lamp comes 
 nearer to being a real international standard than any 
 other. When used in strict accordance with the minute 
 instructions accompanying each lamp, it is subject to 
 errors less than half as great as those met with in standard 
 candles, and, while not perfectly steady, is far steadier 
 than a candle or a Carcel lamp. Its weakest point is its 
 color, which is distinctly reddish orange. 
 
 This constitutes a rather serious objection to its use 
 as a working standard in measurements made, for in- 
 stance, on mantle burners or incandescent lamps. Even 
 as a primary standard its color and rather small intensity 
 form an obstacle to its convenient use; but all in all it has 
 been rather generally recognized as the best primary 
 standard yet devised. 
 
 Reproducibility is after all one of the most important 
 requirements in a primary standard, and this the Hefner- 
 Alteneck lamp possesses in a very unusual degree. 
 
 In working standards the most necessary qualities are 
 great temporary steadiness and convenience as to color 
 and intensity. These requirements are far more easily 
 met than that of exact reproducibility, and in practical 
 photometry reliable secondary standards are obtained 
 with comparative ease. 
 
 One of the simplest and most useful is obtained from 
 an Argand gas burner, such as has already been described 
 as used for testing purposes. 
 
 Burned at a carefully regulated pressure, with a delicate 
 meter by which to adjust the consumption, and a 
 blackened screen to cut off all the light save that through 
 a narrow aperture of definite dimensions, a gas jet gives 
 a wonderfully steady light, extremely well suited to 
 
STANDARDS OF LIGHT. 317 
 
 photometric work. This arrangement is substantially 
 that of the Methven screen, which is widely used in 
 photometry. If it were practicable to prepare at short 
 notice a gas of definite composition, this apparatus might 
 make a good primary standard, but attempts along this 
 line have not been very successful. Acetylene has been 
 suggested for the purpose, but experience has shown that 
 it is peculiarly subject to variations in luminous intensity, 
 and is worthless as a standard illuminant. 
 
 Aside from the Methven screen, the most generally used 
 secondary standard is the incandescent lamp. If the 
 filament is not worked at too high a temperature, i. e., at 
 too great efficiency, and is aged by several hundred hours 
 of preliminary burning, it constitutes an admirably re- 
 liable standard. 
 
 Burned at a fixed and uniform voltage, its intensity can 
 be accurately determined by comparison with a primary 
 standard, and remains very nearly uniform, having a 
 slight and definitely ascertainable decrement with time. 
 
 In practical photometry such a lamp is merely balanced 
 against an ordinary aged lamp used in the photometer for 
 testing purposes, remaining itself a standard of reference. 
 
 Several attempts have been made at an incandescent 
 lamp as a primary standard, the filament being of definite 
 material and dimensions enclosed in a globe of specified 
 character, and worked with a definite amount of current. 
 
 The result has not so far been encouraging, and in the 
 absence of anything better the Hefner-Alteneck lamp is 
 the main reliance as a reproducible standard. 
 
 At the time the Violle standard was proposed the one- 
 twentieth part of it was tentatively adopted as a working 
 unit and was styled the bougie decimale, but this some- 
 what hypothetical unit has never come into any repute, 
 
THE ART OF ILLUMINATION. 
 
 although its relation to the more common standards has 
 been determined with a fair degree of precision. 
 
 The following table gives the relation between the 
 several primary standards here referred to with as 
 much precision as the nature of the case permits, per- 
 haps rather more, since one must admit that in photometry 
 the third significant figure is of very dubious value : 
 
 
 BOUGIE 
 
 DECI- 
 
 MALE. 
 
 CARCEL. 
 
 HEFNER 
 UNIT. 
 
 GERMAN 
 CANDLE. 
 
 ENGLISH 
 CANDLE. 
 
 Bougie decimale 
 
 
 
 
 
 
 Carcel . 
 
 9 6 
 
 
 
 
 0.6 
 
 
 0.885 
 
 
 
 O.8l5 
 
 0.91 
 
 German candle 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 0.104 
 
 
 
 
 There are here some evident discrepancies which serve 
 to mark the unsatisfactory state of the art, and to measure 
 the uncertainties which exist. 
 
 Given a standard, such as it may be, the process of com- 
 paring a given radiant with it is extremely simple in 
 principle and somewhat troublesome and unsatisfactory 
 in practice. The difficulties come in part from the in- 
 herent difficulties of the process in general, and in part 
 from the complications introduced by variations in the 
 color of the light. 
 
 The Bunsen screen, which in ordinary practice is the 
 backbone of photometry, has already been in some meas- 
 ure described, together with one of its simplest applica- 
 tions. The general principle is that a translucent spot in 
 a nearly opaque screen of light texture disappears when 
 equally illuminated from each side. 
 
 But for this to happen requires that the screen be en- 
 tirely symmetrical. Light falling upon it must be trans- 
 mitted through and reflected from the surface of the 
 
STANDARDS OF LIGHT. 319 
 
 grease-spot in precisely equal measure irrespective of the 
 side of the spot on which the light falls. Jf not, when 
 viewed obliquely from one side the spot will seem to 
 disappear at a particular point, but when viewed from the 
 other side this point will be shifted. Moreover, unless 
 the screen be viewed from the same angle on each side, 
 it will not balance at the same point, even if the spot be 
 
 Fig. 121. Bunsen Photometer Screen. 
 
 absolutely symmetrical as regards its two faces. If this 
 condition is fulfilled, one side of the spot will generally 
 accumulate dust a trifle more freely than the other, and 
 throw things out of balance again. 
 
 To eliminate as far as possible such difficulties, it is 
 usual to arrange the Bunsen screen so that both sides can 
 be observed simultaneously, and from the same angle. 
 To this end the apparatus is arranged as in Fig. 121. 
 The screen marked sc in the cut is placed in a blackened 
 box having openings in the ends along the line xy between 
 the lights to be compared, and a lateral opening o, in 
 which the edge of the screen is central. Two ordinary 
 pieces of mirror, cut side by side from the same glass, are 
 set vertically in the screen box in the positions mm', as 
 
320 THE ART OF ILLUMINATION. 
 
 shown. To the observer looking fairly into o the re- 
 flected images of the two sides of the screen then appear 
 side by side, and the slightest change in the appearance of 
 either may be at once noted. Sometimes the mirrors are 
 fitted to slide out so that they may be interchanged and 
 another reading taken, and sometimes the sight box itself 
 is arranged to revolve 180 degrees about a horizontal axis 
 in the plane of the screen. The interior of the box must 
 be blackened with extreme care to avoid diffused light. 
 
 In observing with this sight box one soon falls into a 
 very uniform habit of setting the screen by reference to 
 
 Fig. 122. Bunsen Photometer. 
 
 both its sides, and can take wonderfully concordant read- 
 ings. But vision differs in different persons, and the 
 " personal equation " in photometric work is of consider- 
 able importance. 
 
 Aside from the sight box, the essential parts of a 
 photometer are a long, graduated bar along which the 
 sight box can be slid, suitable supports for the lights to be 
 compared, so that they may always be in their proper rela- 
 tion to the graduated bar, and the screens already referred 
 to for cutting off stray light. The elementary arrange- 
 ment of a Bunsen photometer, except for the screens, is 
 very well shown by Fig. 122. The two lights are sup- 
 
STANDARDS OF LIGHT. 321 
 
 ported at known equal distances from the ends of the 
 graduation, and the sight box is then slid along the bench 
 until the grease spot shows a balance between the illumina- 
 tion from the two sides. Then the intensities of the two 
 lights are inversely as the squares of their respective dis- 
 tances from the grease spot. 
 
 This relation assumes that the lights illuminate their 
 respective sides of the Bunsen screen strictly according to 
 the law of inverse squares, uncomplicated by any sensible 
 regular or diffused reflection. Right here is where the 
 trouble begins. No one who has not tried it realizes the 
 difficulty of eliminating reflection. There must be no 
 reflecting surfaces about the photometer, and it must be 
 in a darkened room with non-reflecting walls, as far as it 
 is possible to obtain them. Several coats of dead black 
 paint prepared from lampblack with just enough thin 
 shellac to serve as a medium answers the purpose fairly 
 well. The photometer bench should allow not less than 
 six feet between the lights, and better eight or ten, A 
 room about twelve feet by six feet is a convenient size for 
 photometric work, and the higher the better, as a low 
 room is apt to become unpleasantly hot after working in 
 it awhile. The bench should run along one side, and all 
 the apparatus should be stowed on a shelf under it within 
 easy reach of the hand, for the room should be kept as 
 dark as possible to avoid loss of sensitiveness in the eye. 
 
 A couple of small heavily shaded incandescent lamps, 
 with switches in easy reach, form a convenient means for 
 securing what little light is needed, and it is convenient 
 also to have a tiny miniature lamp, with a ground bulb 
 and an opaque screen to keep the light from the observer, 
 carried on the sight box just above the pointer. This 
 lamp should be furnished with a mere contact key on the 
 
3 22, THE ART OF ILLUMINATION. 
 
 carriage of the sight box, so that it can be momentarily 
 lighted to read the graduated scale. 
 
 For very precise work the Lummer-Brodhun photo- 
 metric screen is sometimes used. This need not be de- 
 scribed here, further than to say that it is a somewhat 
 complicated but beautifully effective device, rather costly, 
 and not as widely used in this country as the simpler 
 Bunsen screen. Opinions differ widely as to the real 
 relative merits of these two devices. In the writer's judg- 
 ment the Lummer-Brodhun screen, when carefully used 
 for the comparison of lights not differing greatly in in- 
 tensity or color, permits a somewhat closer balance than 
 the Bunsen screen, but under ordinary conditions the 
 latter is nearly or quite as effective, and much easier to use. 
 
 The general structure of the photometric apparatus 
 should be rigid and substantial. All the working parts 
 should move easily and smoothly, and all the accessories 
 should be as conveniently placed as possible, so as not to 
 distract the attention of the observer from the work in 
 hand. 
 
 Attempts are sometimes made to reduce the photometer 
 to a compact, portable form, that can be easily set up for 
 testing in any convenient location. As a rule, such porta- 
 ble photometers are rather unreliable. It is hard enough 
 to do precise photometric work under the most favorable 
 conditions, and in portable apparatus the tendency is to 
 sacrifice too much to compactness. For certain classes 
 of work in which high precision is not necessary, the 
 portable photometers are convenient, but they are not to 
 be advised for general purposes. 
 
 Many commercial photometers, both permanent and 
 portable, are provided with scales so graduated as to read 
 candle-power directly, assuming a certain fixed distance 
 
STANDARDS OF LIGHT. 323 
 
 between the lights under comparison and a fixed intensity 
 of the standard. It is, of course, much easier to make 
 photometric tests rapidly with such a scale, but it should 
 be used with extreme caution, and as an auxiliary. When 
 once correctly adjusted it is most convenient, but it should 
 be assumed to be mal-ad justed at the start, and its correct- 
 ness carefully verified before it is regularly used, and it 
 should be subsequently checked at frequent intervals. 
 The same precaution should be taken with any other 
 apparatus graduated for convenience in arbitrary units. 
 
 The holders for the lights to be compared should be 
 easily adjustable, so as to enable the operator to bring 
 the luminous areas into exactly the right position with 
 respect to the graduated scale. When incandescent lamps 
 are under test it is convenient to mount the lamp to be 
 tested upon a rotating spindle, so that by revolving it at 
 the rate of three or four turns a second the mean hori- 
 zontal candle-power may be obtained at a single reading. 
 Other sources of light are generally also measured hori- 
 zontally, but in a single conventional azimuth, and it is a 
 question whether in the long run it is not better to 
 measure incandescent lamps in a similar fashion. If any 
 mean value of the luminous energy is to be considered 
 important, it is the mean spherical rather than the mean 
 horizontal, and it has already been explained how by 
 changing the shape of the lamp filament the distribution 
 may be widely altered without being changed in amount, 
 so that spherical candle-power is really the significant 
 thing. 
 
 Rotators for incandescent lamps are, however, con- 
 venient, and particularly so if arranged so as to allow the 
 axis of rotation to be tilted at any required angle. But 
 they require watching, if accurate work is desired, since 
 
3 2 4 THE ART OF ILLUMINATION. 
 
 it is very difficult to avoid small and variable losses in 
 voltage at the lamp due to varying resistance at the 
 brushes which convey the current from the fixed to the 
 rotating part of the device. Mercury-cup contacts are 
 somewhat more reliable, but do not lend themselves readily 
 to tilting the axis of rotation. 
 
 Fig. 123 shows an excellent typical form of photometer 
 intended primarily for testing incandescent lamps, but 
 readily adaptable to more general purposes. It consists 
 
 Fig. 123. Photometer Bar Complete. 
 
 of a pair of little standards supported by cast iron columns, 
 and supporting in turn the lights and their accessories and 
 the pair of steel shafts extending between them and bear- 
 ing the photometer carriage. The forward bar carries the 
 graduation. On the left is the carriage for the standard 
 lamp, screened in front and curtained behind, and on the 
 right is the rotator, similarly screened, for the lamp to be 
 tested. A pair of sliding screens help to cut off extrane- 
 ous light from the sight box, and each lamp is provided 
 with a rheostat for the exact adjustment of its voltage, 
 and with the necessary electrical connections. 
 
 In setting up such a photometer, even in a room painted 
 dead black, the screens supplied should be supplemented 
 by other and larger ones placed nearer the sight box to cut 
 off indirect illumination. It would also be advisable to 
 
STANDARDS OF LIGHT. 3 2 5 
 
 place a long shelf from standard to standard under the 
 photometer bar. This should be painted dead black, and 
 used to carry instruments and accessories ready to the 
 observer's hand. In this instance the distance between 
 lights is made either two or three meters, the longer bar 
 being preferable for measurements of precision. 
 
 The sight box is mounted on trunnions, so as to be 
 reversible as a whole with respect to the ends of the bar. 
 In thus reversing, the errors due to difference in the re- 
 flecting mirrors or in the sides of the Bunsen screen 
 proper, as well as the personal errors between the ob- 
 server's two eyes, are eliminated from the result. There 
 is, however, a personal error as between different observers 
 that it not easy to be rid of. The idiosyncrasies of the eye 
 in photometric work almost pass understanding. Two ob- 
 servers setting the Bunsen screen alternately on the same 
 lights in quick succession will not infrequently obtain 
 results differing by nearly 10 per cent., each man's read- 
 ings, however, being closely consistent. The same ob- 
 server will, as a rule, get consistent results from day to 
 day, but has his own habit of seeing the spot on the screen 
 disappear. Such individual differences are particularly 
 marked when comparing lights differing in color. 
 
 In comparing, however, lights of approximately the 
 same intensity and color, as in testing incandescent 
 lamps, there is a convenient way of avoiding most of the 
 errors in photometry, which can hardly be too strongly 
 commended. It is one of the general processes of physi- 
 cal investigation, known as the " method of substitution." 
 It consists of comparing the standard, which we will call 
 A, with an intermediary standard B, and then, leaving 
 everything else unchanged, replacing A by the object to 
 be tested, C. 
 
326 THE ART OF ILLUMINATION. 
 
 In applying this method to photometry proceed as 
 follows : Place the standard lamp on the rotator of 
 Fig. 123, and the intermediary standard in the socket 
 at the other end of the photometer bar, setting the sight 
 box at the midway point. Then vary the intermediary, 
 either by turning it slightly or by shifting the rheostat 
 belonging to it, until an accurate balance is obtained. 
 Then any lamp of equal intensity with the standard on the 
 rotator may replace it without changing the balance. 
 This eliminates all the errors of comparison save two : first, 
 that due to possible variation of resistance in the rotator, 
 and, second, that due to a possible variation in the ob- 
 server's habit of seeing during the progress of subsequent 
 observations. 
 
 Most standard lamps are intended to be used in a fixed 
 azimuth, and not in rotation, so that the former error may 
 enter unless the rotator is in first-class order. The ex- 
 istence of this error is a strong argument in favor of 
 measuring lamps in one or more fixed azimuths. 
 
 As to the second error, it is seldom of much moment in 
 the case of a practiced observer, but may be detected and 
 approximately evaluated by repeating at the close of the 
 observations the original observation with the funda- 
 mental standard, setting in this case the sight box without 
 varying either lamp. If the observer has been uniform in 
 his habit of setting, and the resistance in the rotator has 
 not varied materially, the sight box will give balance at 
 the same point as before. 
 
 The possible residual error is that due to the varying 
 resistance of the rotator when at rest from its resistance 
 when in motion. This error may be detected, if it exists, 
 by measuring the mean horizontal candle-power of a lamp 
 having quite uniform horizontal distribution, first, by 
 
STANDARDS OF LIGHT. 327 
 
 rotating, and, second, by averaging the readings taken, 
 say, in six azimuths 60 degrees apart. If the rotator has 
 introduced no error, the two values thus obtained should 
 check each other within the ordinary error of observation. 
 In incandescent lamp testing there are two general ways 
 of arranging the connections. In the first, called the two- 
 circuit method, the working standard is placed on an 
 independent source of energy, generally a storage battery, 
 brought to its proper voltage by means of a rheostat in 
 circuit with it and a voltmeter, and kept constant during 
 the observations by occasional adjustment of the rheostat, 
 if necessary. The lamps being tested are similarly 
 treated. When a storage battery is available, the method 
 
 Fig. 124. Photometer Circuits. 
 
 is a very satisfactory one, the only errors involved being 
 those in the voltmeters, which need to be frequently com- 
 pared, and very carefully read. 
 
 The second or single-circuit method is shown in dia- 
 gram in Fig. 124. Here the two lamps to be compared 
 are put in multiple off the same set of mains worked at the 
 usual voltage. The standard, B, is brought to its proper 
 voltage by means of the rheostat and a voltmeter, and 
 afterwards the voltages at the two lamps vary together, 
 if at all. This method of connection is very much more 
 convenient than the two-circuit method, especially in 
 alternating-current stations. It is, moreover, sufficiently 
 precise if carefully applied. A second rheostat is em- 
 ployed for lamp A, if the voltage of the supply circuit 
 varies from the rated voltage of A. 
 
328 
 
 THE ART OF ILLUMINATION. 
 
 The essential difference between the two methods is that 
 in the two-circuit scheme each lamp is tested rigorously 
 at its rated voltage, while in the single-circuit method the 
 two lamps are tested either at their rated voltages or at 
 voltages equally at variance from these ratings. In the 
 latter case there is a chance for error, unless equal incre- 
 ments of voltage correspond to equal increments of in- 
 
 
 : 
 
 -J-|- 
 
 -H f 
 
 
 
 
 
 
 19 
 18 
 
 Cur 
 
 ve Af 
 B 
 C 
 
 or 
 
 26Wa 
 t.O 4 
 3.1 ' 
 
 ttLamp 
 
 L 
 
 4 
 
 ' - H 
 
 
 - 
 
 H 
 
 j 
 
 ~7 
 
 
 
 
 Z 3 
 
 > 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 77 
 
 
 
 
 
 
 
 
 s 
 
 
 ^A 
 
 
 16 
 15 
 14 
 
 L 
 
 
 
 
 
 
 > 
 
 
 
 . 2 
 
 
 j 
 
 
 
 
 
 
 ^ X 
 
 
 i- /- 
 
 
 
 f 
 
 
 
 
 
 
 
 
 2^~" 
 
 
 
 5- 
 
 
 
 
 ^ 
 
 7 
 
 . 2 
 
 
 
 
 
 
 ' ,^ 
 
 
 
 
 
 j 
 
 
 
 
 f 
 
 
 " 7 ^ 
 
 
 
 
 
 JO 
 
 
 
 
 
 ^ 
 
 F 
 
 ^ 
 
 
 
 
 
 
 
 
 
 ^/ 
 
 
 ^ 
 
 
 
 
 
 
 
 12 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Variation of 
 Candle Power witl 
 Applied E. M. F. 
 
 i 
 
 
 
 
 
 
 >* 
 
 
 
 
 
 
 
 _ ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 -Vc 
 
 1 
 
 
 
 
 
 
 Nil 
 
 r IT 
 
 
 
 
 
 
 
 
 
 
 91 95 96 97 98 99 100 101 102. 103 104 105 106 107 108 109 
 
 Fig. 125. Variation of Light with Voltage. 
 
 tensity. In other words, if A and B are once balanced, 
 will variation in the voltage of supply destroy that balance ? 
 In general terms, two incandescent lamps of the same 
 candle-power at some particular voltage will not remain 
 equal if the voltage be changed. On the other hand, for 
 small variations of voltage the difference will generally be 
 so small as to be within the ordinary errors of observation, 
 and, in fact, practically negligible. Fig. 125 shows the 
 curves of variation of candle-power with voltage, in three 
 typical lamps of differing efficiencies. All three show 
 an approximation to the common rough-and-ready rule 
 of a variation of one candle-power per volt. Of course, 
 
STANDARDS OF LIGHT. 329 
 
 the slope of the curves is the important consideration, and 
 this generally decreases slightly with the efficiency of the 
 lamp. 
 
 A brief examination discloses the relations between the 
 curves. Suppose the working standard to be a lamp of 
 moderate efficiency, say, 4 watts per candle, as shown in 
 Curve B. Assuming the working voltage as 100, a rise 
 of one volt or a fall of one volt increases or diminshes the 
 light by, as nearly as possible, .85 candle-power. If the 
 lamp under comparison corresponds to Curve A, the 
 increment or decrement is not far from .75 candle-power 
 per volt, while with the lamp of Curve C the change is a 
 little over .9 candle-power per volt. These three lamps 
 represent about as large differences as will generally be 
 found, and it is therefore safe to say that for variations of 
 less than one volt on either side of the normal the differ- 
 ences in candle-power as between the lamps tested will be 
 less than o.i candle-power, and for practical commercial 
 testing may generally be neglected. But a difference of 
 four or five volts would obviously lead to variations of 
 a considerable fraction of a candle-power, which would 
 evidently be quite inadmissible. 
 
 The single-circuit method then must be used with cau- 
 tion, but when so used is generally quite as good as the 
 two-circuit method, unless the latter be applied with ex- 
 traordinary care. It should be remembered that unless 
 the voltmeters employed have very open scales, the ordi- 
 nary errors in reading them involve errors in candle- 
 power quite as great as those between two lamps on the 
 same circuit under a slightly shifting voltage. 
 
 Of the two methods the writer, on the whole, prefers 
 the single circuit one for ordinary use. It is usually easy 
 to find a time for testing when the variations in voltage 
 
330 THE ART OF ILLUMINATION. 
 
 are small and slow enough to be easily reduced by a little 
 attention to the rheostat. 
 
 It is not advisable in commercial testing to attempt the 
 comparison of incandescent lamps with standards of an- 
 other character. Such comparisons depend for their cor- 
 rectness on a knowledge of the absolute value of the 
 voltage a knowledge seldom very precise. They also 
 introduce the factor of color difference, which is enor- 
 mously troublesome, even with trained observers and the 
 full resources of a well-equipped laboratory. 
 
 When the lights compared by means of a Bunsen or 
 Lummer-Brodhun screen differ considerably in color, 
 absolute balance is attainable at no one point on the scale. 
 The same observer will obtain very regular apparent 
 values for the comparison, but another observer is likely 
 to obtain a somewhat different set of values. Such per- 
 sonal differences may easily amount to 5 per cent, or more, 
 in comparing, for instance, a Welsbach with a Hefner 
 lamp, or an incandescent lamp with an enclosed arc. 
 
 There will also be considerable differences in the results 
 with a single observer if the absolute brightness of the 
 colored radiants changes, even when the relative bright- 
 ness remains the same. That is, if one were comparing 
 a Welsbach and a Hefner lamp, and obtained what ap- 
 peared to be a satisfactory balance, that balance would be 
 destroyed by doubling the distance of each light from the 
 screen. 
 
 These color difficulties are physiological and subjective. 
 They depend upon a property of vision sometimes known 
 as Purkinje's law, stated by Von Helmholtz as follows: 
 " Intensity of sensation is a function of the luminous in- 
 tensity which differs with the kind of light." 
 
 This difficulty in color photometry is precisely akin to 
 
STANDARDS OF LIGHT. 33' 
 
 that involved in comparing the loudness of two noises of 
 differing quality, although fortunately somewhat less 
 serious. For example, one would have extreme difficulty 
 in forming any notion of the relative loudness of a bugle 
 note and a pistol shot, or a shout and a steam whistle. 
 One's first instinctive effort at comparison would probably 
 be made by investigating the distance at which each sound 
 became inaudible, or barely audible. 
 
 A similar procedure based on visual acuteness has often 
 been tried in rough color photometry. In its crudest 
 form it consists of noting the distance from each radiant 
 at which a printed page held at arm's-length just becomes 
 legible. A very little experience will convince the experi- 
 menter that the results depend upon the general state of 
 the eye, the personal equation of the observer, practice, 
 preconceived notions of the relative intensities, and other 
 factors so variable that the result is little better than 
 guesswork. Yet this wildly inaccurate method has not 
 infrequently been used in estimates of street lighting. 
 
 With proper apparatus and a careful and unprejudiced 
 observer, however, the principle involved is capable of 
 giving useful approximate determinations of illumination. 
 An instrument for this purpose which has become fairly 
 well known in this country is Houston and Kennelly's 
 illuminometer, shown in section in Fig. 126. In the cut, 
 X, X is a small box thoroughly blackened on the inside and 
 provided with an eye tube T, T, pointing directly at a re- 
 movable inclined block B, on the face of which is placed 
 a group of printed test characters. A focusing eye-piece 
 E enables any observer to see the test object distinctly. 
 In the top of the box is a window W, closed by a translu- 
 cent diaphragm of porcelain, opal glass, or the like, which 
 serves to illuminate the test object. This window can be 
 
332 
 
 THE ART OF ILLUMINATION. 
 
 closed by an opaque shutter S, moved by a rack and 
 pinion, the latter turned by a milled head outside the box. 
 
 The instrument is used by facing the window toward 
 the source of illumination, and opening or closing the 
 shutter until the test characters are just legible. A scale 
 attached to the shutter then gives the illumination directly 
 in bougie-metres. 
 
 The scale is calibrated empirically by testing with a 
 source of light of known intensity at definite distances. 
 
 Fig. 127. Illuminometer. 
 
 The instrument is small enough for the pocket, and is 
 very convenient for relative measurements. So far as 
 absolute values of the illumination are concerned, it can 
 hardly be considered seriously, unless in experienced 
 hands, and calibrated by the user; but in comparative 
 measurements the average error of a single careful read- 
 ing is less than 10 per cent., which is a great improvement 
 on guesswork. A skillful observer by frequently check- 
 ing the calibration of his instrument could bring the 
 absolute errors somewhere nearly down to this figure. 
 The question of color is partially eliminated by the great 
 reduction in intensity of the light. As has been noted 
 in a previous chapter , color differences are inconspicuous 
 in very faint light. 
 
STANDARDS OF LIGHT. 333 
 
 To return to photometry proper, this same expedient 
 of reducing the intensity of the light that reaches the eye 
 from the photometer screen is of material assistance in 
 comparing colored lights. Observing the screen with 
 nearly closed eyes makes comparisons very much easier, 
 and leads to fairly consistent results. But color percep- 
 tion changes so notably in dim illumination that results 
 so obtained do not represent working conditions nearly 
 enough to justify making any pretense of precision. 
 
 Numerous expedients to avoid these difficulties have 
 been devised, all amounting in the last resort to the selec- 
 tion of conventional conditions, representing the practical 
 requirements of illumination. None of them are perfectly 
 satisfactory under all conditions, but probably the best 
 available method is Crova's. This is based on the experi- 
 mental fact that in comparing two lights, even of very 
 different color, their total intensities are sensibly propor- 
 tional to their relative intensities in the region of the 
 spectrum of wave length, about 0.582 M> that is, in the 
 clear yellow of the spectrum. 
 
 The troublesome part of such a comparison is to segre- 
 gate the rays of about this wave length without resorting 
 to spectro-photometry, which necessitates the formation 
 of two spectra from the two sources side by side. Crova 
 found that a solution of 22.3 grams anhydrous perchlo- 
 ride of iron and 27.2 grams chloride of nickel in 100 cubic 
 centimeters of distilled water forms an absorbing screen 
 that serves the purpose. The former constituent cuts out 
 the green and blue, the latter the red. A layer of this 
 standard solution 7 millimeters thick, used as a screen 
 through which to observe the photometer screen, serves 
 the purpose, although a thicker layer limits the desired 
 region more closely. 
 
334 THE ART OF ILLUMINATION. 
 
 The objection to the method is principally the large 
 amount of light cut off by the screen, so that it works best 
 in comparing rather powerful lights. 
 
 As a matter of general practice such refinements are 
 seldom used. Excepting arc lamps, the ordinary sources 
 of light can be compared without serious difficulty from 
 differences of color. With flame radiants a well stand- 
 ardized Methven screen forms by far the best working 
 standard, while in comparing incandescent lamps the 
 working standard should be an incandescent of moderate 
 efficiency. 
 
 In comparing arc lamps serious trouble is encountered. 
 In the first place, the difference between the intensity of 
 an arc and any feasible standard is inconveniently great, 
 and in the second place the colors are widely different, 
 especially in dealing with enclosed arcs. The first diffi- 
 culty may be averted by using the arc at a sufficient dis- 
 tance from the screen to give a proper working distance, 
 say, three or four feet, between the screen and the stand- 
 ard. In the tests by the committee of the National 
 Electric Light Association already quoted, the color 
 trouble was dealt with by observing the screen through 
 a rapidly rotating disk having narrow radial slits. This 
 in effect cut down the brilliancy of the screen to a point 
 where color perception was considerably weakened. It 
 is rather doubtful whether this procedure affected the 
 standard and the arc in equal ratios. 
 
 In arc photometry still another troublesome factor is 
 met, in the tendency of the arc to wander from side to 
 side of the carbon, or to slowly rotate, so that the real 
 luminous intensity is very difficult to catch. In the re- 
 search just mentioned this was escaped by using a pair of 
 mirrors simultaneously reflecting light from two sides of 
 
STANDARDS OF LIGHT. 335 
 
 the arc lamp, diametrically opposite, upon the photometer 
 screen, the direct radiation being screened off. The line 
 joining these mirrors was, of course, perpendicular to the 
 line of the photometer bar, and the absorption of the 
 mirror surfaces could readily be allowed for. 
 
 There is at present no conventional method of compar- 
 ing the brilliancy of different sources of light. Flames 
 are universally rated by their intensity as measured in a 
 horizontal plane, in a direction generally 45 degrees from 
 the plane of the flame, if the flame is flat, or irrespective 
 of direction in Argand and other symmetrical round 
 burners, including mantle burners. 
 
 In the early days of electric lighting the photometric 
 question assumed some importance, and all sorts of wild 
 statements were afloat as to the power of the new illumi- 
 nant. Arc lamps were apparently rated at their momen- 
 tary maximum intensity on the most favorable direction. 
 The rivalry between makers of arc lamps did not tend to 
 depreciation of their intensity, and so it came about that 
 an open arc taking about 450 watts was rated at 2000 
 candle-power, while a similar arc of about 325 watts was 
 rated at 1200 candle-power. 
 
 While it is possible that some experimenter at an 
 especially favorable moment may have obtained these 
 intensities in a single direction, it is certain that the 
 ratings were very soon regarded as merely conventional. 
 They have long since been relegated to the category of 
 merely commercial designations, the rating bearing no 
 more precise relation to the thing than does the term 
 " best," as applied to flour or other commodities. 
 
 When an individual or a municipality contracts for a 
 2OOO-cp arc light, the thing bought, received, and paid 
 for is an arc light taking about 450 watts of electrical 
 
336 THE ART OF ILLUMINATION. 
 
 energy, and such is the general understanding of the 
 term as interpreted at various times by the courts. There 
 is not, nor has there ever been, in commercial use in this 
 country or elsewhere an arc lighting system using lamps 
 actually giving anywhere near 2000 candle-power, either 
 as maximum zonal intensity or as mean spherical intensity. 
 The former requirement would demand about 750 watts 
 at the arc, the latter nearly 1200. Lamps of such power 
 have only been used for searchlights and similar purposes, 
 and are far too powerful to be advantageously used for 
 ordinary illumination. 
 
 In incandescent lighting the ratings are intended to 
 express the real candle-power of the lamps. Sixteen 
 candle-power is a figure borrowed from the legal require- 
 ments for gas, and corresponded originally to a measure- 
 ment corresponding to that applied to gas flames, i. e., 
 in a horizontal plane 45 degrees from the plane of the 
 curve formed by the filament. 
 
 With the introduction of looped and spiraled filaments 
 giving a better distribution of light than the simple U- 
 shaped filament, demand arose for a method of measure- 
 ment which would credit these lamps with their just due.. 
 Hence arose the measurement of mean horizontal candle- 
 power by rotating the lamp. This credits the lamp with 
 its just horizontal candle-power as against a lamp giving 
 1 6 candle-power only in certain horizontal direction, but 
 it fails to give credit for gains in spherical distribution, 
 and puts a premium on lamps with long Li-filaments 
 adapted to throw out a large proportion of horizontal 
 illumination. 
 
 Mean spherical candle-power, i. e., total luminous flux, 
 is unquestionably the fairest basis of comparison between 
 various sources of light, but it is somewhat troublesome 
 
STANDARDS OF LIGB 337 
 
 to measure, and runs counter to long- established custom 
 and legal requirements as to gas lighting. It is certainly 
 desirable that a uniform method should be established for 
 all radiants, and this is no easy matter. There is a strong 
 tendency to apply the mean spherical measurements to arc 
 lamps, although the lower hemispherical candle-power 
 is sometimes used instead, on the ground that downward 
 light is the proper criterion of useful illumination. This 
 rating is approximately true of lamps having reflectors 
 over them, but it is certainly not true in general, for it 
 neglects the very great effectiveness of diffuse reflection 
 from walls and ceiling. 
 
 The fact is that no simple rating can be applied with 
 equal fairness to all commercial sources of light, by reason 
 of their very great diversity in the nature of the light- 
 distribution. 
 
 The mean spherical measurement comes nearer to 
 general fairness than any other, and could it be uni- 
 versally adopted it would afford a very satisfactory basis 
 of comparison. As a practical standard at the present 
 time, it leaves considerable to be desired. 
 
 Mean horizontal candle-power is by far the easiest thing 
 to measure, and it is to be recommended, save in the com- 
 parison of radiants deliberately planned, as in case of 
 intensive gas burners, the American type of Nernst lamp, 
 and certain arcs and incandescents, to give particularly 
 strong illumination in some other direction. 
 
 The thing most to be desired in practical photometric 
 work is a general international convention defining em- 
 pirically, if need be, certain bases of work. A working 
 reproducible standard of greater intensity and better color 
 than the Carcel or Hefner lamp is badly needed. As 
 actual standards for use on the photometer bar, nothing 
 
338 THE ART OF ILLUMINATION. 
 
 can be better than incandescent lamps, but as has already 
 been noted, they are not reproducible. The nearest ap- 
 proach to a reproducible standard of good size and color 
 at present available seems to be the Vernon-Harcourt 
 lo-cp pentane lamp, which is the present official standard 
 in London. It has not been subjected to as searching 
 and protracted an investigation as the Hefner lamp, but 
 the reports so far obtained from it are highly encourag- 
 ing, while its intensity and color are great advantages in 
 passing from it to the more powerful modern radiants. 
 
 Granted a proper standard, there is also needed a 
 definite conventional method of dealing with the color 
 difficulty. This involves a tougher problem even than 
 the standard itself. Possibly Crova's method, or some 
 modification of it, might be made to serve a useful pur- 
 pose. Finally, aside from the difficulty of comparing 
 lights differing widely in color, there remains the question 
 of the different illuminative values of such lights when put 
 into practical service.. This again suggests the question 
 of measuring illumination, instead of the intensity of the 
 radiants, but as has already been indicated there are no 
 methods of measuring illumination comparable in pre- 
 cision with ordinary photometry, which is saying little 
 enough. 
 
 It is to be hoped that the recently organized Bureau of 
 Standards may facilitate the study of these puzzling mat- 
 ters, and promote an international photometric congress 
 that can give general sanction to a definite programme 
 in commercial photometry. 
 
 A great deal of time and effort has been wasted in this 
 world in the promulgation of so-called " absolute " stand- 
 ards, referred in a perfectly definite way to immutable 
 constants of nature. Desirable as they are, it is of far 
 
STANDARDS OF LIGHT. 339 
 
 greater importance to have a convenient, reproducible, and 
 international set of units in universal use. The metric 
 system started on its career as an absolute system, but its 
 value lies not in the supposed relation of its units to 
 natural constants, but in their relation to each other, and 
 in its well-nigh universal acceptance as the basis of 
 scientific measurements of length and mass. 
 
 Standards as concrete things may be constantly suscep- 
 tible of improvement without limit. They are important 
 practically only in proportion to their general recognition 
 at a certain conventional determinable value. 
 
INDEX. 
 
 Absorption, selective, 28 
 
 Acetylene, 75 
 
 , burners for, 79 
 
 ,' generators for, So 
 
 , dangers of, 77 
 
 , preparation of, 76 
 
 , value of, 8 1 
 
 After-images, 10 
 
 Air-gas 66 
 
 , cost of, 67 
 
 machines, 66 
 
 Architectural illumination, 283 
 
 illumination, funda- 
 mental principles of, 284 
 
 Arc, best length of, 143 
 
 , relation between length 
 
 of, and voltage, 143 
 
 , relation between cur- 
 rent density and light in, 142 
 
 . .relation between cur- 
 
 rent and efficiency in, 160 
 Arcs, actual intensities of, 248 
 
 , alternating and direct 
 
 current comparison of, 158 
 , alternating, best fre- 
 quency for, 157 
 . alternating, distribu- 
 
 tion of light from, 156 
 , alternating, advantages 
 
 of, 155 
 
 -, alternating, annual sav- 
 
 ing from, 260 
 
 , alternating current, 154 
 
 , alternating current 
 
 series, 259 
 
 , enclosed, 144 
 
 , low voltage, 144 
 
 , classification of, 248 
 
 computing illumina- 
 
 tion from, 251 
 
 , distribution curves 
 
 from various, 249 
 
 , distribution of light 
 
 from, 148 
 , enclosed, amperage of, 
 
 150 
 
 Arcs, enclosed, distribution 
 of light from, 150 
 
 , enclosed, character- 
 istics of, 147 
 
 , enclosed, consumption 
 
 of carbon in, 145 
 
 enclosed, voltage in, 
 
 146 
 
 , illumination 
 
 from, 255 
 , for lighting 
 
 large 
 
 rooms, 217 
 
 alternating, objections 
 
 to, 155 
 
 , rating of, 335 
 
 result of distribution 
 
 from, 248 
 
 Bougie d ecu n ale, 317 
 Bougie-meter, 5 
 Bracket fixtures, 271 
 Bulbs, exhaustion of, 100 
 
 , Malignani process for 
 
 exhausting, 100 
 Bunsen screen, construction 
 
 of, 318 
 Burner, Argand, 71 
 
 bat's-wing, 71 
 
 fishtail, 71 
 
 oxy-hydrogen, 83 
 
 Siemens, 74 
 
 Welsbach, 85 
 
 Welsbach, form of, 86 
 
 Wenham, 73 
 
 Burners, regenerative, 73 
 Burning fluids, 59 
 
 Calcic carbide, 76 
 
 carbide, cost of, 81 
 
 Candle-foot, 5 
 
 Candle power, mean spherical, 
 105 
 
 , standard, 314 
 
 Candles, 62 
 
 , efficiency of, 62 
 
 Carcel lamp, 314 
 
 341 
 
342 
 
 INDEX. 
 
 Ceiling lighting, advantage of, 
 
 232 ' 
 lights in halls, 216 
 
 lights, practical effect 
 
 of, 196 
 
 Churches, amount of light for, 
 225 
 
 , choice of lights for, 224 
 
 , illumination of, 224 
 
 Coal gas, 68 
 
 gas, composition of, 68 
 
 gas , impurities in, 69 
 
 Color, effects of dilution on, 
 
 44 
 
 , fundamental law of, 23 
 
 , in illumination, 23 
 
 , of walls in illumination, 
 
 55 
 
 Color-blindness, effect of, 30 
 Color-photometry, 330 
 
 Crova's method of, 333 
 
 Colors, changeable, 26 
 
 , from pigments, 25 
 
 in very dim light, 29 
 
 , luminosity of, 32 
 
 , matching, 29 
 
 Colored illumination, limita- 
 tions of, 289 
 
 light on fabrics, 34 
 
 light, effect of, 33 
 
 Common illuminants, cost of, 
 
 94, 
 
 illuminants, properties 
 
 of, 93 
 
 Crater, temperature of, 142 
 Cross-suspensions, 273 
 
 Danger from light oils, 92 
 Daylight, intensity of, 21 
 Decorative lighting of large 
 
 buildings, 280 
 lighting, temporary, 289 
 
 lighting, miniature 
 
 lamps for, 291 
 
 Diffuse lighting, development 
 of, 309 
 
 lighting, objections to, 
 
 310 
 
 Diffusion, difficulty of check- 
 ing, 54 
 
 , help received from, 188 
 
 in large rooms, 211 
 
 , relation of, to quantity 
 
 of light, 189 
 
 Display and scenic illumina- 
 tion, 275 
 
 Domestic lighting, illuminants 
 for, 183 
 
 lighting, importance of 
 
 low intrinsic brilliancy in, 
 184 
 
 illumination, 8 c. p. 
 
 lamps in, 210 
 
 illumination, mantle 
 
 burners in, 210 
 lighting, quantity of 
 
 light for, 190 
 Draughting rooms, inverted 
 
 arcs in, 236 
 
 rooms, light required 
 
 in, 235 
 
 Electric arc, 140 
 
 arc, crater of, 141 
 
 Enclosed arcs, annual saving 
 from, 259 
 
 arcs, illumination from, 
 
 258 
 
 Exposition buildings, illumina- 
 tion of, 242 
 
 lighting, principles of, 
 
 285 
 
 Eye, properties of, 2 
 
 Factories, illumination re- 
 quired for, 240 
 
 Fats, 58 
 
 Fechner's law, 4 
 
 Filament of Auer von Wels- 
 bach, 124 
 
 Filaments of refractory mate- 
 rial, 123 
 
 , disintegration of, no 
 
 , flashing, 98 
 
 . effect of flashing on, 99 
 
 , manufacture of, 96 
 
 , material of, 96 
 
 , practical dimensions of, 
 
 114 
 
 , section of, 101 
 
 , shapes of, 101 
 
 from soluble cellulose, 
 
 96 
 Firefly, efficiency of light of, 
 
 138 
 , emission spectrum 
 
 from, 137, 138 
 , light of, 136 
 
INDEX. 
 
 343 
 
 Fire risks of illumination, 295 
 Flames, luminous, 56 
 Fraunhofer lines, 24 
 
 Gas-burners, 71 
 
 Hefner unit, 315 
 Holophane globes, 175 
 
 globes, classes of, 177 
 
 globes, distribution of 
 
 light from, 180 
 
 - globes, structure of, 175 
 
 globes, weak points of, 
 
 179 
 
 Illuminants, choice of, 193 
 
 , colors of, 36 
 
 , color properties of, 27 
 
 , conception of efficiency 
 
 in, 303 
 
 , incandescent, 56 
 
 , ultimate efficiency of, 
 
 304 
 Illumination, apparent, 12 
 
 , common fault in, 296 
 
 , computation of, for a 
 
 room, 191 
 
 , best direction of, 17 
 
 , effect of direction in, 3 
 
 , effective, 186 
 
 , effect of height on, 223 
 
 , formulae for computing, 
 
 194 
 
 , general, 2 
 
 of a hall, computing, 213 
 
 of very high rooms, 220 
 
 of a modern house, 202 
 
 , predominant direction 
 
 of, 217 
 
 -, needed improvements 
 
 m. 304 
 
 over-brilliant, 301 
 relation of, to yellow 
 component, 33 
 scenic, i 
 standards of, 5 
 necessary strength of, 
 18 
 
 Illuminometer, 331 
 Interior illumination, limita- 
 tions imposed upon, 195 
 Incandescent lamps, rotators 
 
 for, 323 
 lamps, value of, 103 
 
 Incandescent electric light. 95 
 Incandescents, color of light 
 
 in, 113 
 
 , actual cost of, 117 
 
 , effect of temperature 
 
 on life of, 116 
 
 , high efficiency, no 
 
 importance of sorting, 
 
 intrinsic brilliancy of, 
 
 120 
 
 in 
 
 , illumination curves 
 
 from, 262 
 
 , light-curves from, 104 
 
 , life of, 115, 116, 117 
 
 life of, in candle-hours, 
 
 118 
 
 , low efficiency, 113 
 
 , nominal candle-power 
 
 of, 104 
 
 , position of axis in, 108 
 
 , rated efficiency of, 109 
 
 , real efficiency of, 122 
 
 , rating of, 336 
 
 , rating, 107 
 
 relation between in- 
 
 tensity and energy in, 112 
 
 relation of light and 
 
 voltage in, in 
 
 , relation of temperature 
 
 and efficiency 'in, 109, in, 
 
 112 
 
 -, need of good regulation 
 
 for, 119 
 
 , standard sizes of, 113 
 
 , total light from, 104 
 
 , value of, 118, 119, 120 
 
 variation of light, with 
 
 permis- 
 
 voltage in, 328 
 
 Inertia, visual, 13 
 
 Intrinsic brilliancy 
 sible, 307 
 
 brightness, 8 
 
 brightness, table of, 9 
 
 Inverse squares, law of, 6 
 
 Inverted arcs, unilateral light- 
 ing by, 239 
 
 Iris, action of, n 
 
 Kerosene, 61 
 
 Lamp, arc, fluctuations of, 14 
 
 , incandescent electric, 
 
 13 
 
344 
 
 INDEX. 
 
 Lamp, incandescent flickering 
 of, 13 
 
 , Nernst, 124 
 
 , Nernst, advantage of, 
 
 on high voltage, 133 
 
 , Nernst, American form 
 
 of, 129 
 
 , Nernst, arrangement 
 
 of, 126 
 
 , Nernst, ballast resist- 
 ance in, 127 
 
 , Nernst, on continuous 
 
 current, 132 
 
 , Nernst, intrinsic bril- 
 liancy of, 131 
 
 , Nernst, life of, 129, 131 
 
 , Nernst, light-curve 
 
 from, 133 
 
 , Nernst, tests of, 130 
 
 -, Nernst, variation of re- 
 
 sistance in, 124 
 
 , " Rochester," 63 
 
 , vacuum tube, 134 
 
 -, vacuum tube, color of 
 
 light from, 135 
 
 vacuum tube, difficul- 
 
 ties with, 134 
 Lamps and candles, function 
 
 of, 194 
 , incandescent, of large 
 
 power, 233 
 
 , kerosene, 63 
 
 , oil, 63 
 
 , Roman, 57 
 
 -, silvered bulb, 171 
 
 Light, artificial, sources of, 
 
 57 
 
 , diffused, value of, 187 
 
 Lights, importance of steadi- 
 ness in, 241 
 
 Lighting, criterion of effective, 
 306 
 
 " Lucigen " torch, 64 
 
 Lummer-Brodhun screen, 322 
 
 Lux, 5 
 
 Mantle burners, 86 
 
 burners for air-gas, 91 
 
 burners, color of light 
 
 of, 89 
 
 burners, efficiency of, 88 
 
 burners, life of, 89 
 
 Mantles, composition of, 85 
 Mast-arms, 271 
 
 Mean spherical candle power 
 as a basis of rating, 336 
 
 Methveivscreen, 317 
 
 Miniature lamps, objections to, 
 292 
 
 Monuments, illuminating, 281 
 
 Moonlight schedule, 265 
 
 Municipal lighting, 268 
 
 Nernst filament, efficiency of, 
 129 
 
 Oils, combustion of, 63 
 
 Paraffin, 61 
 
 Petroleum, 60 
 
 , composition of, 60 
 
 products, 61 
 
 Phosphorescence, possible 
 value of, 305 
 
 Photometer bench, 321 
 
 , Bunsen, 20, 320 
 
 circuits, 327 
 
 , daylight, 19 
 
 , practical arrangement 
 
 of, 324 
 
 Photometers, portable, 322 
 
 Photometry of arc lamps, 334 
 
 , " method of substitu- 
 tion " in, 325 
 
 Plane of illumination, 190 
 
 Pole-top fixtures, 270 
 
 Projectors, stage, 278 
 
 Public buildings, lighting, 227 
 
 lights, relation of gen- 
 eral illumination to, 264 
 
 squares, lighting of, 264 
 
 Quantity of light required in 
 large buildings, 212 
 
 Railway stations, lighting, 233 
 stations, spacing of arcs 
 
 in, 234 
 
 Rare earths, properties of, 86 
 Reflection, 38 
 
 , asymmetric, 42, 49 
 
 , asymmetric, in fabrics, 
 
 50 
 
 , coefficient of, 47 
 
 , diffuse, 39 
 
 , coefficients of diffuse, 53 
 
 , coefficients of regular, 
 
 47 
 
INDEX. 
 
 345 
 
 Reflection, selective, effect of, 
 
 total intensity of, 41 
 
 losses in, 48 
 
 multiple, 45 
 
 diffuse, nature of, 40 
 
 regular, 38 
 
 selective, 43 
 
 Reflector lamps, 171 
 
 lamps, objections to, 173 
 
 Reflectors, 163 
 
 for inverted arcs, 236 
 
 , economy o, 198 
 
 Rooms, light required to illu- 
 minate, 199 
 
 Search light, 7 
 
 lights, 297 
 
 lights, use of, 298 
 
 Selective coloration, effect of 
 
 material on, 50 
 Shades, 163 
 
 light intercepted by, 166 
 
 paper and fabric, 165 
 
 reflecting, 167 
 
 reflecting, tests of, 169 
 
 requirements for, 165 
 
 Shadows, function of, 16 
 " Shotgun diagram," 120 
 diagram, interpreta- 
 tion of, 121 
 
 Single-circuit method, 327 
 Snow-blindness, 3 
 Spectra of colors, 25 
 Spectrum, 24 
 Standards, requirements for, 
 
 3i3 
 
 , relation between pri- 
 mary, 318 
 
 , secondary, 316 
 
 Stearin, %Q 
 
 Street lighting, 244 
 
 lighting, annual hours 
 
 of, 265, 
 
 lighting, contracts for, 
 
 269 
 r lighting, cost of, 267 
 
 Street lighting, fixtures for, 270 
 
 lighting, incandescents 
 
 in, 261 
 
 lights, location of, 263 
 
 light, spacing of vari- 
 ous, 266 
 
 Streets, amount of light re- 
 quired for, 254 
 
 , effective illumination 
 
 in, 257 
 
 -, computing illumination 
 
 for, 244 
 , minimum illlumination 
 
 in, 247 
 , principles of lighting, 
 
 246 
 
 Temporary lighting effects, 233 
 lighting, installation of, 
 
 293 
 Theaters, ceiling lighting for, 
 
 231 
 , actual floor space of, 
 
 229 
 
 , illumination of, 228 
 
 , light required in, 230 
 
 , location of lights in, 230 
 
 , stage lighting in, 276 
 
 Two-circuit method, 327 
 
 Vacuum tube, efficiency of, 
 
 135 
 
 Velvet, action of dyes in, 51 
 Vernon Harcourt pentane 
 
 standard, 337 
 Violle's unit, 313 
 Visual usefulness, 306 
 
 Walls, diffusion from, 197 
 Water-gas, 69 
 
 , composition of, 70 
 
 , danger from, 70 
 
 Waxes, 58 
 
 White light, composition of, 24 
 
 Workshops, arc lights in, 218 
 
 , light required in, 241 
 
 , mantle burners in, 219 
 

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