IRLF 
 
 B 3 E71 T?fl 
 
 
THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
Microscopical Praxis. 
 
PHOTOMICROGRAPH 
 
 OF THE 
 TEST PODURA (LEPIDOCYRTIS CURVICOLLIS^ X 3OOO. 
 
 H. Q. PIFFARD, M.D., NEW YORK. 
 Spencer & Smith, Obj. 1/15. N. A. 1.35. 
 
Microscopical Praxis 
 
 OR 
 
 Simple Methods of Ascertaining the 
 Properties of Various Micro- 
 scopical Accessories * 
 
 BY 
 
 ALFRED C. STOKES, M. D., 
 
 AUTHOR OF "A CONTRIBUTION TOWARD A HISTORY OF THE FRESH-WATER INFUSORIA 
 OF THE UNITED STATES;" "A KEY TO THE GENERA AND SPECIES OF THE FRESH- 
 WATER ALGA: AND DESMIDIE^: OF THE UNITED STATES;" "MICROSCOPY 
 FOR BEGINNERS;" "OBJECTS FOR THE MICROSCOPE," ETC. 
 
 ILLUSTRATED 
 
 PORTLAND, CONN.; 
 
 EDWARD F. BIGELOW 
 
 1894 
 
COPYRIGHT, 1894, 
 
 BY 
 EDWARD F. BIGELOW. 
 

 
 By the Author of " Microscopical Praxis." 
 
 I. 
 
 MICROSCOPY FOR BEGINNERS. 
 Price, postpaid, $1.50. 
 
 II. 
 FRESH-WATER ALGJE AND THE DES- 
 
 Price, postpaid, $1.25. 
 Address E. F. Bigelow, Portland, Conn. 
 
 M351791 
 
MICROSCOPICAL PRAXIS. XI 
 
 Contents. 
 
 PAGE 
 
 Illustrations, - xii 
 
 Preface, - xiii 
 
 The Pocket-Lens, i 
 
 The Microscope Stand and its Parts, 7 
 
 The Micrometer, 58 
 
 How to care for and use the Microscope, 67 
 
 Thin cover-glass, 81 
 
 The Objective, 90 
 
 Refractive Index of Immersion Fluids, 164 
 
 Finders and similar Devices, - 168 
 
 Sub-stage Illuminators, - 169 
 
 The sub-stage Condenser, 169 
 
 Black-ground Illuminators, 181 
 
 The Paraboloid, 182 
 
 The Spot-Lens, 183 
 
 The Woodward Prism, 184 
 
 The Hemispherical Lens, - 186 
 
 Supra-stage Illuminators, 187 
 
 The Lieberkuhn, 187 
 
 The Parabolic Speculum, - *iS8 
 
 The Vertical Illuminator, - 189 
 
 The Amplifter, - 193 
 
 The Erector, 194 
 
 The Polariscope, 196 
 
 Drawing the Object, 200 
 
 Reflectors and Camera Lucidas, "201 
 
 Live Boxes, Growing-Cells and other Accessories, 
 
 ^Index, 
 
Xll MICROSCOPICAL PRAXIS. 
 
 Illustrations. 
 
 PAGE 
 
 Frontispiece. 
 
 Fig. i. Stewart's Safety Stage, 28 
 Fig. 2. Pennock's Method of Centring the Il- 
 lumination, 44 
 Fig. 3. Bullock's Apparatus for measuring the 
 
 Power of Eye-pieces. 54 
 
 Figs. 4-9. The Podura Scale, 97 
 
 Fig. 10. Tube-length used by various Opticians, 102 
 Fig. ii. H. L. Smith's Test-slide for refractive 
 
 Index, - 165 
 Fig. 12. Illumination of opaque Objects by the 
 
 Woodward Prism, - 185 
 
 Fig. 13. The Vertical Illuminator, 192 
 
 Fig. 14. Explanation of Polarised Light, - 197 
 Fig. 15. Mrs. S. H. Gage's Drawing-desk for 
 
 the Abbe Camera Lucida, 207 
 
 Fig. 16. E. A. Apgar's Mechanical Finger, 210 
 Figs. '17, 18, 19. Simple Life-slides, 215, 217, 219 
 
 Fig. 20. Beale's Growing-cell, 221 
 
 Fig. 21. H. L. Smith's Growing-cell, 222 
 
 Fig. 22. Sternberg's Culture-cell, 224 
 
 Fig. 23. Pagan's Life-slide, 226 
 
 The Acme Lamp, - 31 
 
Preface. 
 
 The writer was ^ recently asked: How is the value of 
 the spaces on the eye-piece micrometer ascertained? 
 What is done with the stage-micrometer when the eye- 
 piece micrometer is in use ? And, How can I learn the 
 magnifying power of a simple microscope ? 
 
 It was these and similar questions which lead to the 
 preparing of this unpretentious little book for the use 
 of those beginning microscopists that are desirous of 
 knowing how to ascertain the matters referred to in 
 these queries, and such other points that are generally 
 unknown by the novice in connection with the various 
 microscopical accessories. 
 
 There seems to be an actual need of a book of simple 
 explanations of such processes as the ascertaining of 
 initial magnifying power of microscope-objectives, of 
 the focus of the concave mirror, of the power of the 
 eye-piece, the measuring of the actual and of the appar- 
 ent field of view, the measuring of the thickness of thin 
 cover-glass, the measuring of the refractive index of 
 immersion-fluids, and other matters of the kind which 
 every amateur microscopist sooner or later is anxious 
 to know, and which the books and the magazines either 
 
XIV MICROSCOPICAL PRAXIS. 
 
 entirely neglect, or publish in such a manner as to be 
 inaccessible to the general reader. 
 
 It is in the explaining of these uncomplicated but im- 
 portant methods in the use of the microscope that the 
 book has its excuse for being, if it have any. Its author 
 has intended that the inquirer should find in its pages, 
 in the form of a few simply-expressed directions, an an- 
 swer to his question, instead of writing to some micro- 
 scopical magazine to ask, for instance, How shall I as- 
 certain the focal length of my pocket-lens ? and then 
 waiting several months for some leisurely microscopist 
 to reply, and perhaps waiting in vain. 
 
 That the book contains all the points that might 
 justly be expected to be found in it, the writer can- 
 scarcely hope; he has not attempted to make a micro- 
 scopical encyclopaedia, but only an elementary praxis 
 of those common things which, at the beginning of his 
 own use of the microscope, he would have liked to 
 know, but could not learn, because the sources of in- 
 formation were then beyond his reach. For the use of 
 all amateur microscopists in a similarly questioning 
 mood, the book is submitted with the author's good in- 
 tentions. 
 
 An attempt has been made to give credit to all those 
 microscopists whose methods have been referred to, 
 but as some of these ways and means are so generally 
 current that they have become common property and 
 their sources have been forgotten, it has not been pos- 
 sible in all cases to discover to whom the credit be- 
 longs. If the reader should happen to find some of his 
 own original and cherished methods described with- 
 out the mention of his name, he may feel sure that the 
 omission is due, not to the writer's intentional lack of 
 courtesy, but solely to his ignorance. 
 
MICROSCOPICAL PRAXIS. XV 
 
 Many, perhaps the majority, of the means here sug- 
 gested to certain ends are not scientifically and mathe- 
 matically accurate, and no such claim is made for them; 
 yet they are intended to be sufficiently correct, and the 
 results sufficiently close, for every practical purpose of 
 the non-professional microscopist, for whom they have 
 been prepared and with whom they are now left. 
 
 TRENTON, N. J., 1894. 
 
Microscopical Praxis. 
 
 The Pocket-Lens. 
 
 The pocket-lens is simply a double-convex lens which 
 is usually mounted in a vulcanite frame. There are 
 several varieties in the-market, but however they may 
 vary in magnifying power and in appearance, they are 
 all essentially simple double-convex lenses. Each may 
 consist of a single lens, or of several so mounted that one 
 or all may be used at a time, the powers varying ac- 
 cording to the combination. With the single lens the 
 magnifying power always remains the same. This is 
 the preferable form. It usually magnifies sufficiently 
 for all practical purposes and it is easily and rapidly 
 used; while with the combination form the power is 
 often inconveniently great and the field of view corres- 
 pondingly limited. The working-distance, too, or that 
 distance between the lens and the object when in focus, 
 is also much shorter than with the single, low-power 
 pocket-lens. 
 
 Three simple double-convex lenses usually form the 
 combination pocket-lens, each giving a different mag- 
 nifying power, the highest being in use when the three 
 are employed together, the combination then acting as 
 does the single lens. In such cases there is usually a 
 diaphragm inserted between the highest-power glass 
 and the next powerful, in order to obstruct the passage 
 
2 MICROSCOPICAL PRAXIS. 
 
 of the rays of light that pass through the margin of the 
 lens, and which do not come to the same focus as those 
 that pass through the centre. This spherical aberra- 
 tion, as it is called, being especially noticeable with 
 high-power pocket-lenses. It exists in all forms, ex- 
 cept in the achromatic triplet, in which it is corrected 
 by the use of a combination of lenses cemented in pro- 
 per position by the optician. 
 
 The more the lens magnifies the closer it must be 
 brought to the object in order to focus it. These com- 
 bination lenses are therefore not so pleasant to use as 
 are the single forms. When employing the entire com- 
 bination the highest-power lens should be nearest the 
 object to be examined, while either surface of the 
 single lens may be toward the object. 
 
 A single lens magnifying about five diameters and 
 having about two inches of focal distance is an excel- 
 lent one for ordinary use. If an achromatic triplet 
 should be desired, and it is an admirable thing to 
 possess although rather expensive, one with the half- 
 inch focus should be selected, as it will then magnify 
 about twenty diameters and be amply sufficient for 
 all kinds of field-work. 
 
 To Measure the Focus of the Pocket-Lens. 
 
 To ascertain the focal distance of the lens by day- 
 light, focus the bar of a window on a distant wall, the 
 
MICROSCOPICAL PRAXIS. 3 
 
 distance from the window being as great as possible, at 
 least the width of the room. Move the lens to and fro 
 before the wall opposite the window until the bar, the 
 curtain-fringe or a tassel is sharply in focus on the 
 white surface. Then measure the distance of the lens 
 from the wall, and that distance will be focal length. 
 The image will be small, and it should be perfectly 
 sharp and distinct. 
 
 At night remove the lamp as far as possible from 
 the wall and use the edge of the flame. In this case 
 the focal point will be represented by a minute spot of 
 light of great brilliancy, and the experimenter should 
 be sure that the spot is as small as possible, and that 
 there is not an indistinct haze or halo around its mar- 
 gin. Measure the distance as in the other experiment. 
 In both cases take care to have the lens parallel with 
 the wall. If it is held obliquely, the spot of light will 
 be irregular in form and indistinctly defined. This is es- 
 pecially to be remembered in the evening, when the 
 little spot should be perfectly circular, and intensely 
 bright. 
 
 To Ascertain the Magnifying Power of the 
 Pocket-Lens. 
 
 The opticians have found it necessary to select an 
 arbitrary distance at which vision with the naked eye 
 shall be measured, a ten-inch space having been agreed 
 
4 MICROSCOPICAL PRAXIS. 
 
 upon. The standard bodies of British and of American 
 microscopes are of that length, and the best objectives 
 are corrected for such tubes. The drawing and the 
 measuring of objects under the compound instrument 
 are commonly made at that distance from the eye-piece, 
 and the pocket-lens comes in for similar treatment, and 
 for the same arbitrary reason. 
 
 To measure the magnifying power of this lens place 
 it on a support so that the upper surface of the glass 
 shall be ten inches above the table, and below it spread 
 a sheet of white paper. Have at hand a rule divided to 
 the tenths of an inch. Hold the rule at right angles 
 with the microscopist's body for convenience ot 
 manipulation, that is, with its width parallel with his 
 width. In the right hand lift the ruler into focus under 
 the lens and hold it there steadily, keeping the head 
 immovable in one position, and with one eye, prefer- 
 ably the left eye, closed. When the object is in focus, 
 open both eyes and the lines will appear to be projected 
 on the paper. With a pair of dividers in the left hand 
 ascertain the apparent width of one of these spaces and 
 measure that distance on the rule, when each tenth-inch 
 between the divider's points will represent one diameter 
 in magnifying power. If the space occupies five-tenths, 
 as it probably will with a lens having a two-inch focus, 
 the magnifying power will be five diameters; that is, 
 the object will be enlarged five diameters in length and 
 in width, or twenty-five times. 
 
 A lens of any kind magnifying ten diameters is said 
 to magnify one hundred times, or ten diameters in each 
 direction, "times" representing the square of the "di- 
 ameters," and the diameters the square-root of the 
 times. If this fact is recollected there is no danger 
 of being deceived by the ridiculous claims for power 
 
MICROSCOPICAL PRAXIS. 5 
 
 made for their lenses by some dealers, who are careful 
 to express the amplification by "times," with the inten- 
 tion to lead astray the unsuspecting, extolling a lens, 
 for instance, because it magnifies "a thousand times," 
 which is only one hundred diameters, not a high power 
 with the compound microscope, but utterly impossible 
 with the simple pocket-lens. 
 
 A Test for the Pocket-Lens. 
 
 Mr. Julien Deby, who has suggested the following 
 test for the excellence of a pocket-lens, remarks, that 
 although many objects are accessible for the determi- 
 nation of the merit of the medium-power and of the 
 low-power magnifying glasses of the compound micro- 
 scope, none is on record for that most useful instru- 
 ment to the naturalist, the hand-lens. This needed 
 test he considers to be the elytron, or wing-cover, of 
 the aquatic beetle Gyrinus marinus. The lens should 
 show not only the longitudinal rows of large dots, but 
 also the fine intermediate punctations, the elytra of the 
 male beetles of the genus being more difficult of reso- 
 lution, since the markings are there finer than those on 
 the wing-cases of the females. But unfortunately the 
 species of "whirligig" beetle selected by Mr. Deby is 
 not found in this country. Other forms are here how- 
 ever, and as all the species so closely resemble one an- 
 other that entomologists differ in their opinion as to 
 
6 MICROSCOPICAL PRAXIS. 
 
 what constitutes a distinct species, it is probable that 
 the wing-cases of almost any form will answer the pur- 
 poses of a test. 
 
 To enable the microscopist to identify the forms that 
 he may find, and whose elytra he may experiment with 
 as a test for his pocket-lens, the following key to the 
 species is given. It was originally published in "The 
 Journal of Microscopy," and is reproduced here some- 
 what changed in form. 
 
 Key to the Species of the aquatic Beetle 
 Gyrinus, 
 
 A. Underside entirely rusty-red (a). 
 
 B. Underside wholly or chiefly black, legs reddish (b\ 
 a. Punctures on elytra scarcely feebler toward the 
 
 suture, G. ininutus, Fab. 
 
 a. Punctures on elytra finer toward the suture, G. 
 
 ur ina tor, 111. 
 
 b. Reflexed margin of thorax and elytra reddish (c]. 
 . b. Reflexed margin brassy black (/). 
 
 e. Body ovate or oval (d). 
 
 c. Body elongate-oblong with nearly parallel sides, G. 
 
 bicolor, Payte. 
 
 c. Body oblong-ovate (e). 
 
 J. Punctures on elytra distinctly finer toward the su- 
 ture. G. natator, Sch. 
 
 tl. Punctures scarcely finer, G. stiff riani, Scrip. 
 
MICROSCOPICAL PRAXIS. 7 
 
 c. Interstices on elytra impunctate, G. distinctus, Hub. 
 e. Interstices indistinctly punctured, G. caspius, Men. 
 
 e. Interstices closely and distinctly punctured, G. 
 
 colymbuS) Fr. 
 
 f. Punctures on elytra scarcely finer toward the su- 
 
 ture, G. marinus, Gyll. 
 
 f. Punctures much finer toward the suture, G. opacus, 
 Sahib. 
 
 Our well-known species is common and abundant on 
 the surface of still waters in the spring and the summer, 
 floating together in companies, or swimming rapidly 
 singly and often in circles, their vivacity making them 
 rather difficult to capture without a net. They have' a 
 not unpleasant musk-like odor when held in the closed 
 hand. They are everywhere known as the "whirligig 
 beetles." 
 
 The Stand and its Parts. 
 
 The compound microscope includes the eye-piece 
 and the objective, with the brass parts which support 
 the mirror and the optical portions; that is, it includes 
 the entire instrument as prepared for the examination 
 of objects, while the stand is only the brass part with 
 the mirror and usually the eye-piece. To convert the 
 
8 MICROSCOPICAL PRAXIS. 
 
 stand into a microscope the addition of an objective is 
 essential. It is well to remember the distinction be- 
 tween these terms, especially in talking with the deal- 
 ers. If you ask a dealer for a microscope, he will antic- 
 ipate the sale of not only the stand but of a series of 
 objectives as well; but it is often to the purchaser's ad- 
 vantage to buy a stand of one manufacturer and the 
 objectives from another. The following is a list of the 
 parts of the stand, with the definition of each. 
 
 FOOT OR BASE: The part on which the entire instru- 
 ment rests, usually in the form of a low and flattened 
 tripod. 
 
 PILLARS: The upright, cylindrical rods attached to 
 the centre of the foot and supporting the portions 
 above. Many stands have only a single pillar. 
 
 ARM: The part, usually curved, attached to the pil- 
 lars by means of a joint, so that the working-portions 
 may be inclined at the convenience of the observer. 
 The upper frontal end of the arm carries the body. 
 
 BODY: The movable tube to which are attached the 
 magnifying or optical parts. 
 
 DRAW-TUBE: An additional, movable tube within 
 the body, whence it may be withdrawn at the will of the 
 microscopist to increase the magnifying power, or for 
 other purposes. 
 
 EYE-PIECE OR OCULAR: The short tube containing 
 an upper and a lower lens, and fitting loosely into the 
 upper end of the body, or of the draw-tube. It is so 
 called because it is near the observer's eye when the 
 microscope is in use. 
 
 OBJECTIVES: The compound magnifying lens applied 
 to the lower end of the body. It is so named because 
 it is near the object to be examined when the microscop- 
 ist is working with the instrument. 
 
MICROSCOPICAL PRAXIS. 9 
 
 Focus: The position of the lenses in which the ob- 
 ject is seen most distinctly. Seeking this point by 
 moving the lens closer to the object or further away, is 
 called focussing. 
 
 FIELD OF VIEW: The circular, lighted space seen by 
 looking through the microscope. The object to be ex- 
 amined, when it is in this space, is said to be in the 
 field. 
 
 COARSE ADJUSTMENT: The means by which the 
 body is moved up and down rapidly, and the objective 
 brought into approximate focus. In the cheaper stands 
 this is accomplished by sliding the body through a col- 
 lar which is immovably attached to the arm. In other 
 and better grades it is by rack and pinion. 
 
 RACK AND PINION: The rack is the straight, narrow 
 piece of metal at the back of the body, with cogs or 
 teeth on its edge. The PINION is the toothed wheel on 
 the arm; it lifts the body by its rotation and by the ac- 
 tion of its "leaves" on the rack. It is usually not visi- 
 ble unless the body is taken off the arm. 
 
 FINE ADJUSTMENT: The slow movement which 
 brings the objective exactly into focus. It is accom- 
 plished by the action of a fine-threaded screw, at the 
 back of the arm in the better class of modern instru- 
 ments, or at the lower end of the body in the older and 
 less praiseworthy stands. It is also sometimes under 
 the front of the arm, or even attached to the stage and 
 moving it. 
 
 STAGE: The thin, circular or oblong plate of metal 
 or of glass attached to the lower, frontal end of the 
 arm, and used to support the object to be studied. It 
 is pierced by a central opening for the passage of the 
 light that illuminates the object. 
 
IO MICROSCOPICAL PRAXIS. 
 
 SPRING CLIPS: The two, narrow, curved strips of 
 metal attached to the back part of the upper surface of 
 the stage, and which hold in place the object-carrier or 
 the glass slip. 
 
 SLIP: The strip of glass, usually three inches long 
 by one inch wide, on which the object to be examined 
 is "mounted." It is placed under the spring clips, 
 when they are present, as they are not on all stands. 
 It is held loosely in position and is easily moved to and 
 fro under the objective by the fingers, or by movement 
 of the whole stage when the spring clips are absent. 
 
 SLIDE: The slip with the object prepared for exami- 
 nation. The addition of the object changes the slip 
 into the slide. The object is usually covered with a 
 square or a circular piece of thin glass made for the 
 purpose. 
 
 DIAPHRAGM: The circular, rotating disk of metal 
 pierced by openings of various sizes and placed beneath 
 the stage. It is used to modify the light, its different 
 apertures Joeing turned below the stage-opening for 
 that purpose. In microscopical literature and conver- 
 sation any disk with one or more apertures is a dia- 
 phragm. Such disks are to be found in the body-tube, 
 in the eye-piece and in some other parts of the optical 
 accessories. 
 
 STOP: A metal disk with a central, circular piece to 
 obstruct the passage of light, and supported by narrow, 
 radiating arms; or a metal disk with a semi-circular or 
 rectangular portion cut from one side, and used to ob- 
 struct all light except that which passes obliquely 
 through the opening. 
 
 MIRROR: The silvered glass that reflects the light 
 through the object or upon it, and through the object- 
 ive, the body and the eye-piece. It is attached movably 
 
MICROSCOPICAL PRAXIS. II 
 
 to an arm at the back of the stage by means of the mir- 
 ror bar, and has independent movements of its own. It 
 should have both plane and concave surfaces in the 
 same mounting. 
 
 SUB-STAGE: Those parts, except the mirror, which 
 are below the stage and intended to carry various ac- 
 cessories to affect the illumination. In the best stands 
 the sub-stage is attached to the arm by a movable sub- 
 stage bar; in others it is borne on the mirror-bar, 
 while in the cheaper forms it is attached to the under 
 surface of the stage. From the lowest priced instru- 
 ments it is usually absent, yet it should always be pres- 
 ent in some form, as it is of great importance in many 
 kinds of work, since certain essential accessories can- 
 not be used without it. 
 
 MILLED HEADS: The disks, roughened or milled on 
 the edges to give the fingers a firm hold, and attached 
 to the fine-adjustment screw, to the pinion of the 
 coarse adjustment, and to the various sub-stage appa- 
 ratus of first-class stands. 
 
 The Body-Tube. 
 
 Many stands with the body-tube about six inches 
 long have a draw-tube by which to increase it to the 
 standard length of ten inches. Most instruments of 
 this kind have the short body lined with cloth, an ar- 
 
12 MICROSCOPICAL PRAXIS. 
 
 rangement that for a time ensures a smooth and easy 
 movement of the draw-tube. Soon however, espec- 
 ially if much used, the latter begins to move less 
 smoothly, until finally it may demand considerable 
 muscular effort and both hands. The difficulty is 
 caused by the roughening of the cloth lining, which 
 must be remedied before the tube can again be moved 
 easily. To do this, take out the draw-tube and heat it 
 until it is so hot that it cannot be held without some 
 discomfort, and gently force it into the body where it 
 should remain until cold. This in effect irons the lin- 
 ing, which the experimenter must be careful not to burn. 
 If one ironing is not sufficient the heating should be re- 
 peated. 
 
 It not rarely happens, too, that after the cloth-lined 
 tube has been used for some time, the draw-tube will 
 slowly slip downward, urged forward by its own weight 
 alone. The object will gradually become dimmer un- 
 til it will slowly fade away, unless it is followed up by 
 a change of focus, or unless the draw-tube is extended. 
 This is even a greater annoyance than to have the 
 cloth lining of the body too thick to admit the draw- 
 tube, as it continually interferes with the appearance of 
 the image. To correct it, loosen the texture of the 
 cloth by the fingers alone, or by teasing it out gently 
 with the forceps. 
 
 The inside diameter of the opening in the lower end 
 of the body and the outside diameter of the screw-end 
 of the objective are about three-fourths of an inch. 
 This is the society screw, so called because first sug- 
 gested by the Royal Microscopical Society of London. 
 For objectives as commonly made, it is amply suffici- 
 ent, but for a few exceptional ones it has been found 
 too small. A few opticians make a few objectives 
 
MICROSCOPICAL PRAXIS. 13 
 
 whose angle of aperture is so uselessly great, and the 
 diameter of their component lenses so uselessly large, 
 that a special screw is demanded on the end of the 
 body. This, at the suggestion of Dr. W. W. Butter- 
 field, of Indianapolis, is made about one and one- 
 fourth inches in diameter. It is called after the in- 
 ventor's name the Butterfield screw, and is to be found 
 on first-class American stands, which have almost every 
 microscopical convenience, being placed above the 
 society screw so that, to use it, a part of the lower end 
 of the body must be removed. It is not on the cheaper 
 stands, and is not needed. Indeed it is a worthless 
 thing anywhere. The society screw is an indespensa- 
 ble adjunct of every body-tube, and is found on all 
 American and English stands, however small and 
 cheap they may be. And when the reader goes to the 
 optician to select a stand, he will find it to his advan- 
 tage to recollect the size of the society screw and its 
 position. 
 
 In this connection there* is one rule to be remem- 
 bered, and it is without exception. It is that any 
 body-tube with a screw on its lower end of a diameter 
 less than three-fourths of an inch should be rejected 
 without a moment's hesitation. And any objective 
 with a screw on its upper end of a diameter less than 
 three-fourths of an inch should be rejected even more 
 speedily. Any stand or any objective without the 
 society screw may safely be set down as a good thing 
 to be avoided, and the reader may also justly view with 
 suspicion any objective which must have an adapter to 
 fit it to the society screw of the stand. 
 
 In some first-class American stands a useful contri- 
 vance is applied to the lower end of the body, and 
 named 'the safety nose-piece.' It consists of a short 
 
14 MICROSCOPICAL PRAXIS. 
 
 tube sliding easily within the body, and pressed upon 
 by a spiral spring so that when forced upward, the 
 pressure of the spring tends to return it to its former 
 position. Its use is to protect high-power objectives 
 and also the object. High-power lenses usually have a 
 short working-distance, so that there is danger, when 
 focussing, of touching the front of the lens against the 
 glass slip, a thing that every careful microscopist is anx- 
 ious not to do, since the thin-glass cover over the ob- 
 ject may be cracked, or the lens scratched or broken, 
 a much more serious matter than the breaking of a 
 cover-glass. But should such an accidental contact 
 occur, before injury can be done the safety nose-piece 
 will slide upward, at once relieving the pressure on the 
 objective and calling the microscopist's attention to the 
 danger. But any microscopist that will focus an ob- 
 jective while looking through it deserves to injure it. 
 
 Every objective, it makes no difference what its 
 focal length may be, should always be focussed while 
 the microscopist is looking under it, not while he is 
 looking through it. In the last-mentioned position the 
 finishing touches may be given by the fine -adjustment, 
 if necessary, but in all other events the objective should 
 be gently racked down toward the object while the mi- 
 croscopist is looking across the slide and under the de- 
 scending lens. In this way he can see how near the 
 objective is to the slide, and can guard it from danger. 
 Then while looking through it he should rack it upward 
 until it is in accurate focus, if it is a low or a medium 
 power, or if a high-power the focussing should be ac- 
 complished by a few touches to the fine-adjustment 
 screw. In no instance should any objective be fo- 
 cussed downward, not even the four inch, while the ob- 
 server is looking through the instrument. 
 
MICROSCOPICAL PRAXIS. 15 
 
 Another interesting and often useful device on first- 
 class American stands and on a few foreign ones, is the 
 scale and vernier on the side of the body and on the 
 arm of the instrument, for the measurement of the 
 working-distance of objectives. It is never found on 
 smaller and cheaper stands, where it might well be ap- 
 plied without great additional cost, and be used to as- 
 sist the beginner in focussing his objectives, or in 
 measuring the working-distance of his lenses. 
 
 The Draw-tube. 
 
 On the majority of stands a draw-tube will be found 
 within the body if the latter is of the standard length 
 of ten inches. On those whose body is shorter than 
 ten inches, the draw-tube is used only to make the ex- 
 tension to the standard length, and no additional tube 
 will be on the instrument. The length of the draw- 
 tube is usually almost that of the body, so that it will 
 lengthen the latter enormously when fully extended, 
 and correspondingly increase the magnifying power. Its 
 upper end commonly projects somewhat beyond that 
 part of the body, affording a means of manipulation, 
 and for carrying the eye-piece. 
 
 The lower end bears a diaphragm whose aperture 
 sometimes contains the society screw, so that very low- 
 
l6 MICROSCOPICAL PRAXIS. 
 
 power objectives may be placed there and used while 
 entirely within the body. This is often a great conven- 
 ience in the employment of such objectives as the four, 
 or three inch, which have such an exceedingly long 
 working distance that occasionally the body must be 
 raised so high above the stage that it will run off the 
 rack before the focus is obtained, but if the objective is 
 on the diaphragm of the draw-tube it may be approxi- 
 mately focussed by pulling out that tube, the focussing 
 being completed by the rack and pinion on the body. 
 These low-power objectives are at times very useful in 
 the study of large objects, where not much amplifica- 
 tion is desired. 
 
 What are styled 'Student's stands' often have the 
 draw-tube diaphragm supplied with the society screw. 
 It is not restricted to first-class instruments, as it is 
 almost a necessity on any stand, for it not only may 
 carry the low powers, but also the amplifier, and the 
 analyser of the polariscope. As a diaphragm of some 
 kind must always be in the tube, and as the society 
 screw can add but the veriest trifle to the cost of the 
 stand, the purchaser might do well to seek an instru- 
 ment with this convenience. 
 
 The draw-tube is also often externally graduated to 
 parts of an inch or of centimeters or both, so that the' 
 distance to which it is extended may be recorded and a 
 desirable result be reproduced at any future time. The 
 only graduation on the cheaper stands is a single circle 
 engraved at the point to which the tube must be ex- 
 tended to make the body of the standard length. 
 
 When the part is to be drawn out, do so with a strong 
 and steady pull, holding the body firmly in position at 
 the same time, otherwise it may be run off the rack^and 
 entirely off the arm. Do not turn the draw-tube from 
 
MICROSCOPICAL PRAXIS. 1 7 
 
 side to side while extending it. The latter method puts 
 a severe strain on the rack and pinion, and on the fine 
 adjustment if it be at the back of the arm. When it is 
 to be pushed in, grasp the milled head of the pinion, or 
 hold the body so that the latter can not move, while the 
 tube is slowly and steadily pressed down. If these 
 precautions are not taken, and the objective is on the 
 body, it may be forced against the slide, or the air 
 compressed within the body may throw out the eye- 
 piece. 
 
 Increase of Magnifying Power by the Use of the 
 Draw-tube. 
 
 Every increase in the length of the body-tube in- 
 creases the magnifying power of the objective by 
 increasing the distance between the lens and the 
 eye-piece. The following tabulated statement of 
 the increase in power to be added to the original ampli- 
 fication for each inch to which the draw-tube is ex- 
 tended is only approximately correct, but it is near 
 enough for all practical purposes. It was originally 
 calculated and published by Messrs. R. and J. Beck to 
 accompany their list of British microscopes. The re- 
 sults will not hold good with the objectives or with the 
 oculars of all makers, since all A, B, C, eye-pieces have 
 not the same power. 
 
i8 
 
 MICROSCOPICAL PRAXIS. 
 
 Objective. 
 Focal length. 
 
 Add for each inch of draw-tube with 
 eye-piece, 
 
 A (2 in.) 
 
 B(iiin.) 
 
 C (i in.) 
 
 4 inches. 
 
 i^ diam. 
 
 3 diam. 
 
 5 diam. 
 
 3 
 
 2 
 
 4 " 
 
 6 " 
 
 2 
 
 4 " 
 
 6 " 
 
 8 " 
 
 T i " 
 
 5 "' 
 
 7 " 
 
 12 " 
 
 i 
 
 5 " 
 
 J 5 " 
 
 20 , u 
 
 * " 
 
 8 " 
 
 14 u 
 
 25 " 
 
 4_ " 
 
 14 " 
 
 24 " 
 
 34 " 
 
 i " 
 
 24 
 
 42 " 
 
 63 
 
 i " 
 
 18 
 
 35 " 
 
 60 " 
 
 i " 
 
 50 " 
 
 85 - 
 
 140 " 
 
 1 U 
 
 60 
 
 100 " 
 
 1 80 " 
 
 A " 
 
 80 
 
 150 " 
 
 300 " 
 
 The Coarse Adjustment. 
 
 This part consists of the rack on the body-tube, the 
 pinion with its cogged wheel acting in the depressions 
 or teeth of the rack, and the milled heads on each side 
 by which it is manipulated. Its action is to raise or lower 
 the body rapidly so that the objective may be approx- 
 imately focussed, or be lifted above the stage when the 
 slide is to be placed in position, so that the two may be in 
 
MICROSCOPICAL PRAXIS. 19 
 
 no danger of coming in contact, with the possible in- 
 jury of one or of both. 
 
 The rack should be as long as possible, so that the 
 body's movements shall be ample for all exigencies. 
 A length of from four and one-half to five inches is 
 none too great. And its motion should be, as some one 
 has said, "as smooth as oil." A coarse-adjustment 
 mechanism that is noisy when in action, that rattles 
 and gnashes its teeth, or one that makes the image 
 change its position by throwing the body out of centre, 
 should be rejected. The only place for such a thing is 
 a shelf in a museum of microscopical antiquities. The 
 action should be noiseless, perfectly smooth, and the 
 bearings so firmly in place, that the microscopist shall 
 have no fear that a heavy objective may force the body 
 to run downward by its weight and by the absence 
 of resistance in the coarse-adjustment mechanism, an 
 accident that has happened. This undue looseness, 
 however, may occur after constant and prolonged ser- 
 vice, and a remedy is usually provided by the optician, 
 who places screws in such a position on the arm or 
 elsewhere, that by tightening them the pinion-beariqgs 
 are tightened, and the trouble is corrected for a time. 
 Every stand, even the best, is liable to this annoyance 
 in a greater or lesser extent. 
 
 Some of the cheaper stands have no coarse adjust- 
 ment. The body is then encircled by a collar through 
 which it moves when actuated by the hand, the focus 
 being obtained by pulling and pushing on the tube. 
 This is a very inconvenient and undesirable arrange- 
 ment. It is awkward, since the friction is often so 
 great that the whole stand will move out of position 
 before the body will budge, and frequently, more 
 frequently than not, even when the foot is heavy 
 
2O MICROSCOPICAL PRAXIS. 
 
 enough to keep the instrument firmly on the table, 
 both hands are needed to manipulate the body. It is 
 dangerous, too, since under the circumstances, the body 
 has the obnoxious habit of suddenly slipping further 
 than the microscopist intends, stopping only when it 
 crashes againt the slide, where it usually grinds and 
 and crunches cover-glass and objective with apparently 
 fiendish glee. A stand without a coarse adjustment by 
 rack and pinion is a good stand to be permanently left 
 with the optician. No fine microscopical work can be done 
 with an instrument whose body slides through a friction 
 collar. That arrangement may be cheap, but it is also 
 a torment and a peril. 
 
 The Fine Adjustment. 
 
 When the objective has been imperfectly focussed by 
 the coarse adjustment, its position must be often 
 further changed until the image becomes clear and 
 bright, and the outlines as distinctly and sharply defined 
 as the lines in the best steel-engraving. This is accom- 
 plished by means of the fine-adjustment screw, which, 
 in the older stands, will be found on the lower end of 
 the body, at the front; on some of the oldest models 
 and on a few of the newest, it will be attached to the 
 stage, but in the best of the more recent it is at the 
 back of the arm. 
 
MICROSCOPICAL PRAXIS. 21 
 
 If the fine-adjustment screw is placed on the nose- 
 piece, the parts will sooner or later work loose, and 
 then every time the milled head is touched the body- 
 tube will wabble and the image dance. And the fine- 
 adjustment screw is touched very often. During an 
 observation the microscopist moves the stage with one 
 hand, and keeps the fingers of the other on the adjust- 
 ment screw continuously, constantly altering the focus 
 slightly, so as to judge of the structure of the object by 
 the changes in the appearance of the different optical 
 sections practically cut by the objective. 
 
 But the most serious objection to the older form, in 
 addition to what has been previously mentioned, is that 
 every movement of the fine-adjustment screw changed 
 the length of the body and altered the magnifying 
 power, which was therefore never the same for two 
 successive moments. With the lower powers this was 
 scarcely observable, but with high powers it became 
 almost conspicuously assertive. And during the use 
 of the micrometer for the measurement of the mi- 
 croscopic objects, it was a menacing danger, since the 
 value of the micrometer spaces was changed with every 
 turn of the fine-adjustment screw. With the mechan- 
 ism in the arm, all this annoyance is done away with, 
 since every movement of the screw moves the entire 
 body including the objective and the eye-piece. If 
 properly made it has no lost motion, and no side 
 movement. It responds immediately to a touch of the 
 finger, moving the body directly upward or downward 
 with no lateral play, so that the image, even under the 
 highest powers, does not seem to change its position 
 from side to side, a fatal defect if present. The screw 
 is usually and preferably placed vertically at the back 
 of the arm, within easy reach of the fingers as the mi- 
 
22 MICROSCOPICAL PRAXIS. 
 
 croscopist's elbow rests on the table; but some makers 
 place it under the front of the arm, at the side, or even 
 at the back but in a horizontal position or almost at 
 right angles to the optic axis, that imaginary line 
 drawn through the centres of the mirror, sub-stage, 
 stage, objective, body and eye-piece. 
 
 The milled head of the fine-adjustment screw on all 
 first class stands, and on some of the less expensive, 
 notably on Messrs. J. W. Queen & Co.'s Acme No. 3, 
 has the upper surface graduated for the measurement 
 of the thin glass always covering microscopic objects 
 when permanently mounted. This glass varies a good 
 deal in thickness, and since its presence influences the 
 action of the objective, it is often important to know 
 just what that thickness is. The value of each 
 graduated space on the wheel depends upon several 
 contingencies, seldom being the same in any two in- 
 struments by different makers. What that value is the 
 dealer will tell the purchaser if asked. On my own 
 stand the distance between two lines of the graduated 
 surface is equal to an elevation or depression of the 
 body-tube for the one one-thousandth of an inch. 
 
MICROSCOPICAL PRAXIS. 23 
 
 The Stage. 
 
 The stage should always be firm and steady under 
 pressure, but the pressure should be applied judiciously. 
 All microscope-stages, even those on the best stands by 
 Mr. Bulloch, Mr. Zentmayer and others, will respond 
 to the pressure of the thumbs, and be sufficiently de- 
 pressed to carry the object out of the focus of a medium- 
 power objective. The stage that responds the least is 
 the best, but perfection in this regard seems beyond 
 our reach. Neither is it absolutely essential. The 
 thumbs never press on the stage, unless they are de- 
 lirious. No heavy objects are placed there. No sane 
 microscopist would put a cobble-stone on his stage. 
 The optician may be trusted to give us the best that 
 the conditions of the problem will allow, and the ama- 
 teur purchaser need never test the stiffness of the stage 
 by the weight of his arms and shoulders transmitted 
 through his thumbs. If he does, he will deserve to test 
 the weight of the dealer's arms and shoulders trans- 
 mitted through a club. 
 
 The stage should be as thin as is consistent with the 
 proper stiffness and steadiness. This is needed to al- 
 low for certain effects of illumination. It sometimes 
 happens that an object must be studied by what is 
 called oblique light, that is, the mirror must be so ar- 
 ranged below and to one side of the object, that the re- 
 flected light shall impinge upon it obliquely, and oc- 
 casionally very great obliquity is needed, which can be 
 obtained only when the stage is thin, since a thick stage 
 and a consequently deep aperture in the centre, would 
 prevent very oblique rays from passing through to the 
 object. Many "Students'" stands are faulty in this 
 respect, the makers seeming to think that since oblique 
 light is needed only in somewhat advanced work, the 
 
24 MICROSCOPICAL PRAXIS. 
 
 beginner will not care for it. But I believe in offering 
 the beginner advantages which he may not at first ap- 
 preciate, but which he will at last live up to. The 
 aesthetic craze of striving to "live up to" a blue china 
 jug has happily passed, but the effect of living up to 
 one's privileges remains, and the effort should be even 
 the beginner's. 
 
 - On every stage there should be some kind of movable 
 plate on which the slide shall be placed, the whole 
 moving easily under the impulse of the fingers. Many 
 stands, the majority of so-called Students' stands, have 
 no such plate, spring clips being substituted, the slide 
 being placed under them and moved about by the fin- 
 gers. This arrangement answers well, provided the 
 clips will themselves remain permanently in position 
 when the slide is manipulated. The fingers are soon 
 educated to perform the most delicate movements, 
 guiding the slide by the gentlest pressure, and speedily 
 learning to keep even a living and lively microscopic 
 creature in the field; but to be able to do these things, 
 the spring clips must not press too heavily on the slide, 
 and they must especially be firmly or immovably fixed 
 in their sockets. As a rule, they are fitted so loosely 
 into the holes provided for them on the stage, that 
 scarcely more than a breath is needed to move them. 
 The result is, that during the constant manipulations 
 of the slide, they are gradually urged more and more to 
 one side or the other, until finally they strike the edge 
 of the cover-glass, push it out of place and, it may be, 
 ruin the object. The microscopist's eye is engaged at 
 the ocular, and his attention is concentrated on the 
 image, so that he cannot pay special heed to the spring 
 clips to see that they are not threatening his cover- 
 glass. The Griffith-Club stand is in this respect a model 
 
MICROSCOPICAL PRAXIS. 25 
 
 instrument. With it the spring clips are made after a 
 new and entirely original form, so that there is abso- 
 lutely no danger that they will turn and rend the cover. 
 If all stages having spring clips could have them after 
 Mr. E. H. Griffith's model the benefit would be great. 
 But if the dealer offers a stand with very loose clips, 
 reject it until he remedies the defect by fastening them 
 in their sockets. Then manipulations as delicate as any 
 to be made anywhere may be made with the slide under 
 them; but a stand with loose clips is a delusion and a 
 snare. 
 
 For special studies special stages are made. Warm 
 stages are used, the warmth being produced by 
 heated air, electricity, hot water or by heated metal 
 plates. Some complicated arrangements are described 
 which are often interesting and amusing, for at times it 
 would seem as if the inventors of these queer devices 
 put down on paper what they think should be useful, if 
 somebody could make them successful. They have dial 
 plates attached, and thermometers, and spirit lamps, 
 and electric batteries, and steam cylinders, and boilers 
 with a multiplicity of rubber tubes, all of which, with 
 many others for cooling objects, for subjecting them to 
 the influence of gases, and for other purposes, are 
 doubtless more or less useful in their particular depart- 
 ments, but they need not detain us now, as the beginner 
 will not need them, nor the advanced microscopist, 
 either, I imagine. 
 
 One of the most delightful of microscopical luxuries, 
 one which in some cases is an absolute necessity, is a 
 mechanical stage, provided it is the right kind. The 
 beginner will probably not buy a stand with a mechan- 
 ical stage, though he might do worse things, but if he 
 should even once perform any serious work with that 
 
26 MICROSCOPICAL PRAXIS. 
 
 device, he will, I am sure, never abandon it voluntarily. 
 A mechanical stage is a "thing of beauty," and if well 
 made the rest of that hackneyed quotation is descrip- 
 tive of it. 
 
 But what is it ? Only a stage so made that the 
 horizontal and the vertical motions are accomplished by 
 rank and pinion. The description is short, it seems a 
 small matter, but the stage is one of the most impor- 
 tant parts of the stand. An inconvenient stage means 
 an inconvenient stand. If properly constructed, a 
 mechanical stage is strong, light, firm, durable, desirable, 
 and unrelinquishable. If improperly made it may be 
 strong, firm, thin and light; it will be altogether abom- 
 inable. Scarcely any part of the stand sees so much 
 active service as the stage, unless it is the fine adjust- 
 ment, and scarcely any part must approach perfection 
 so closely as the stage unless, again, it is the fine-ad- 
 justment mechanism. To have this part loose and 
 wabbling, with the image dancing at every turn of the 
 screw, is as bad as having lost motion in the racks and 
 pinions of the mechanical stage, or to have the 
 parts loose and rattling, or to have them "lift," that 
 is, to be raised above the general plane surface when- 
 ever the milled heads are turned to bring the me- 
 chanism into action. Every movement should be 
 smooth, prompt, noiseless. Every slightest touch of 
 the milled heads should be followed by a movement of the 
 stage-plate that shall be visible through the microscope, 
 whatever it may be to the naked eye. To be forced 
 to turn the milled heads through part of a revolution 
 before the leaves of the pinion engage the teeth of the 
 rack, is something that will undermine the best dis- 
 position the microscopist is blessed with; and to feel the 
 pinion crunch against the rack, while the whole jolts 
 
MICROSCOPICAL PRAXIS. 27 
 
 and bounces along, and lifts the object out of focus, 
 will complete the ruin. Nothing of that kind is made 
 in the United States, but something very similar is 
 made outside of the United States. 
 
 The beginner, however, must understand that a me- 
 chanical stage, although one of .the most desirable of 
 microscopical luxuries, is by no means a necessity. 
 Work as good, important and valuable may be done 
 without it as with it. The convenience and accuracy 
 of its movements, its immediate response to every 
 touch, and the confidence that the microscopist 
 speedily learns to have in it, are among its attractive 
 qualities. 
 
 One great advantage in the use of the mechanical 
 stage has not been mentioned. This is the procedure 
 called "sweeping the field," or the act of searching 
 every part of the object field by field, so as to examine 
 every feature and to bring every part of a preparation 
 under the objective. With the ordinary stage as 
 moved by hand, it is easier to fail to bring a minute 
 point or minute object into the field than it is to find it. 
 The stage may carry the desired object in every direc- 
 tion except the right one, while with the mechanical de- 
 vice, the slide may be examined in every part and cor- 
 ner, with the certainty of finding what is sought. If 
 one field be carefully scanned as the stage, carries the 
 slide horizontally under the lens, and the specimen be 
 then moved forward to a distance equalling the width of 
 the field and again swept horizontally, the desired ob- 
 ject can scarcely fail to be finally caught. 
 
 Many practical microscopists have devised safety 
 stages to be used with very high-power objectives, so as 
 to prevent disaster, should the lens and the cover-glass 
 come in contact. They seem hardly necessary for the 
 
28 
 
 MICROSCOPICAL PRAXIS. 
 
 careful microscopist, who is usually so cautious in ap- 
 proximating objective and slide, that contact rarely 
 takes place. 
 
 But probably the simplest, and therefore the best, 
 among the many safety stages described by their in- 
 ventors, is the one designed by Mr. C. Stewart and 
 published in the "JOURNAL OF THE ROYAL MICROSCOPI- 
 CAL SOCIETY" of. London, for January, 1884. It is in- 
 tended, as Mr. Stewart says, to provide an economical 
 but effective arrangement for protecting slides from 
 breakage when being exhibited under high powers to 
 large classes of students, an intention which suggests 
 the usefulness of the device at microscopical soirees. 
 
 -77} 
 
 FIG. i. Stewart's Safety Stage. 
 
 It is shown in Fig. i. It consists of a wooden strip 
 about as long as the glass slide and somewhat wider, 
 with a central aperture, and two -wooden side-pieces 
 about one-quarter of an inch high. To each of the lat- 
 ter is fastened a thin strip of brass which projects be- 
 yond the ends of the wooden pieces. Across these 
 projecting ends two small rubber bands are stretched, 
 and rtie slide, passing between them, is delicately sus- 
 pended in the air, so that at the least touch of the ob- 
 jective it sinks, and warns the microscopist of the 
 danger. This effective device can be made at home by 
 any one, even the least ingenious. 
 
MICROSCOPICAL PRAXIS. 29 
 
 The Mirror-Bar. 
 
 Most of the smaller American instruments now-a- 
 days have the swinging mirror-bar which on them is 
 not objectionable, since they commonly have no pro- 
 vision for sub-stage apparatus, with the exception of 
 the diaphragm which is usually attached to the lower 
 surface of the stage. The mirror may therefore be 
 swung above the objective for its illumination, and so 
 fully supply the place, in this connection at least, of 
 the bull's-eye condensing lens. In use, the mirror-bar 
 is turned on its pivot until the mirror is above the level 
 of the stage, when the concave mirror is manipulated 
 until the light is thrown on the object, the parts being 
 returned to their former position at a moment's notice. 
 To obtain the effect of oblique light, the mirror and its 
 bar are swung to one side below the stage as far as 
 may be desired, and the light again arranged by alter- 
 ing the position of the reflector. 
 
 As a rule oblique light is used only in the study of 
 the striae of diatoms, and seldom for anything else. 
 Commonly the light is made as accurately central as is 
 possible, and here is one of the objectionable features 
 of this otherwise convenient, swinging mirror-bar as 
 applied to the cheaper stands. It often forces the stu- 
 dent to work unsuspectingly with oblique light. On 
 the best instruments a provision is made for clamping 
 these swinging part's in any position from "dead cen- 
 tral" to any angle of obliquity, and there is no danger 
 of involuntarily using what is not wanted. With the 
 advanced microscopist this danger is not great, since 
 he can tell at a glance, whether the light is properly 
 centred or not. With the beginner, however, it is dif- 
 ferent. His eye is not sufficiently educated to per- 
 ceive whether he is proceeding correctly or not, and if 
 
30 MICROSCOPICAL PRAXIS. 
 
 the mirror-bar has no clamping or centring adjust- 
 ments, it is as likely as not to be turned aside, giving 
 the observer undesirable shadows to look at, and to 
 disturb the corrections of his non-adjustable objectives. 
 On the best instruments then, these swinging parts are 
 an admirable convenience. On "Students' " stands 
 they may be an annoyance, unless the beginner is 
 warned in advance, and unless the manufacturers 
 would add some simple marks to show when the parts 
 are truly centred, as might easily be done at no extra 
 cost, and little extra labor. 
 
MICROSCOPICAL PRAXIS. 
 
 3 1 
 
 The Mirrors and their Use.* 
 
 When but one mir- 
 ror is supplied with 
 the stand it generally 
 is, and always should 
 be, concave. As a rule 
 however, two mirrors 
 are mounted on oppo- 
 site sides of the same 
 setting, one plane, the 
 other concave. The 
 size of the former does 
 not much matter, but 
 the latter should be as 
 large as possible, two 
 inches in diameter not 
 being too small for the 
 smallest. The surface 
 should be silvered, the 
 ordinary looking-glass 
 amalgam of mercury 
 and tin not having a 
 power of reflection 
 equal to that possessed 
 by a silvered surface, 
 and presenting a gray- The Acme Lam P of J w - Q ueen & Co - 
 ish appearance to the eye. The mirror-box turns so 
 that either the plane or concave surface may be used at 
 will, and it rotates on a stem connected directly with 
 the bar, or by means of a jointed arm. 
 
 The proper lighting of the object, and consequently 
 of the microscope, is one of the most important accom- 
 
 *In the preparation of this chapter the author has been greatly indebted to 
 an anonymous paper in " Science Gossip." 
 
32 MICROSCOPICAL PRAXIS. 
 
 plishments for the beginner to acquire, and he will 
 learn that there is a good deal of mathematics involved 
 in the study of the concave mirror. From the plane 
 surface the light is reflected unchanged, that is, the 
 rays are reflected as they are received, the angle of in- 
 cidence being equal to the angle of reflection. In cer- 
 tain circumstances impracticable in actual practice, the 
 plane mirror may focus the light of the sky. With this 
 almost impossible exception, the action of the plane 
 surface is entirely different from that of the concave 
 mirror. When the latter receives parallel rays it re- 
 flects them so that they are forced to converge and to 
 come to a focus, while the divergent rays are variously 
 affected according to certain optical principles. For 
 the mathematics of the action, and for the optical laws 
 governing them, the reader must consult the treatises 
 dealing with the science of optics. Yet the novice 
 should know the best position for the lamp and for the 
 mirrors, in order to obtain the most desirable effects. 
 
 The concave mirror need not necessarily be placed 
 at such a distance below the stage that parallel rays 
 will be focussed on the object, although it should be 
 movable in a vertical direction so that that focus may 
 be obtained if desired. It*is often necessary to have 
 something more of the effect of diffused light by lower- 
 ing the mirror, thus throwing a broader and less in- 
 tense illumination on the object. The beginner may 
 know when the mirror is focussed by noticing on the 
 slide the reflection of the window-frame by day, or of 
 the lamp-flame at night. And since the concave micro- 
 scope-mirror is always a part of the inner surface of a 
 hollow sphere, its focal distance may be readily learned, 
 as well as the size of the sphere of which it is a part. 
 
 To ascertain these, a simple experiment must be 
 
MICROSCOPICAL PRAXIS. 33 
 
 made with parallel rays of light. The sun is so far 
 from us that the rays are already practically parallel 
 and no apparatus is needed in the experiment, but if a 
 lamp be used, since its rays are divergent, it must be 
 placed as far as possible from the mirror, or the light 
 must be passed through a bull's-eye condensing lens. 
 In either case the mirror must be placed directly oppo- 
 site the source of light, the lamp-flame, if it be used, 
 being as nearly as may be on a level with its centre; 
 then by moving a white card back and forth between 
 the two, seek that point at which the reflected spot is 
 the brightest and the most distinctly defined, measure 
 the distance from the card to the mirror, and the prin- 
 cipal focus in inches has been obtained. This distance 
 in every concave mirror is one-half the radius of the 
 hollow sphere of which it is a part, so that by multiply- 
 ing the focus by two the radius will be known, and by 
 multiplying the radius by two the diameter of the 
 sphere will be obtained. The mirror on my own stand 
 has a principal focus of four inches, the radius of the 
 sphere is therefore eight inches, and the diameter six- 
 teen. 
 
 The angle at which the microscope slopes in relation 
 to the table-top; the angle at which the mirror slopes in 
 relation to the optic axis of the microscope; the dis- 
 tance of the mirror from the object, and the distance of 
 the lamp from the mirror, affect for the better or for 
 the worse the excellence and the desirability of the re- 
 sultant illumination, every change in any of these posi- 
 tions or distances having its immediate effect. For 
 every inclination of the microscope a calculation may 
 be made, whose result will give the proper inclination 
 of the mirror, the proper position of the lamp and of 
 the bull's-eye condenser. The beginner, if he wish 
 
34 MICROSCOPICAL PRAXIS. 
 
 to enter on the study of microscopical optics, will have 
 no trouble in finding accessible books for consultation. 
 The beginner who reads these chapters must accept the 
 somewhat dogmatic statements without any very ex- 
 tended explanatory reasons. 
 
 It has been found that about the best position for 
 the lamp and the mirror is such that the parallel 
 rays shall form an angle of ninety degrees with the 
 axis of the microscope, and forty-five degrees with the 
 axis of the mirror, or that imaginary line drawn through 
 the centre of the mirror and perpendicular to its sur- 
 face, and that the flame shall be on a level with the 
 centre of the mirror and the centre of the bull's eye 
 condensing lens, or the plano-convex lens forming the 
 front of the microscope-lamps supplied by the dealers. 
 The angle which, in this case, the rays form with the 
 axis of the instrument being ninety degrees, one-half of 
 that, or forty-five, is the angle of incidence, and the 
 mirror is also inclined at an angle of forty-five degrees 
 with the axis of the microscope. 
 
 The principal focus of the mirror and the angle of 
 incidence being known, the proper distance of the 
 former from the object may be ascertained by apply- 
 ing the following rule: "Multiply the cosine (to be 
 found in any book of mathematical tables) of the angle 
 of incidence by the principal focal distance, and the 
 product will be the required distance between the mir- 
 ror and the object." The mirror on my own stand hav- 
 ing a principal focus of four inches, the cosine of forty- 
 five degrees being 0.70711, the mirror should be about 
 two and eight-tenths inches from the object in order to 
 converge and to focus parallel rays upon it. 
 
 But we are talking about the use of parallel rays 
 when those from artificial light are divergent. How are 
 
MICROSCOPICAL PRAXIS. 35 
 
 they to be made parallel ? By the bull's-eye condenser, 
 either on a separate stand, as commonly supplied with 
 the instrument by the dealers, or on the front of the 
 Acme or of the Stratton microscope-lamp. The prop- 
 erty of this plano-convex lens is to change diverging 
 into parallel rays, or parallel into convergent, so that we 
 have only to place it between the lamp and the mirror, 
 at the proper distance from each, and the deed is done. 
 
 When the bull's-eye lens is used to illuminate the sur- 
 face of an opaque object, the question is often asked: 
 Which side should be turned toward the light, the plane 
 or the convex? The answer is that it depends upon 
 the intensity of the illumination desired. The plano- 
 convex lens of the bull's-eye has two principal foci, 
 their distance being somewhat different according to 
 which side is toward the lamp; with the convex surface 
 in that position, the bright spot of light at the focal 
 point is surrounded by a broad disk of fainter illumina- 
 tion, but with the plane surface toward the lamp the 
 focal distance is increased, the light condensed at the 
 focus is somewhat less brilliant, but the circle of fainter 
 illumination has almost disappeared. To get rid of 
 this weakening outer circle and to have, only the bright 
 spot, as well as to gain the convenience of the longer 
 focus, it is better to place the plane side toward the ob- 
 ject; fjut when the parallel rays are to be thrown on the 
 concave mirror to be thence converged to a bright 
 focus on the subject, it is better to place the convex 
 side toward the object. 
 
 The difference in the appearance may be observed, 
 and the distance of the two foci measured, by experi- 
 menting with a card in a way that needs no explanation. 
 But to obtain the best results, either when using the 
 bull's-eye with the concave mirror, or when using it as a 
 
36 MICROSCOPICAL PRAXIS. 
 
 direct illuminator for opaque objects, the flame should 
 always be placed on a level with the centre of the lens, 
 and as nearly as possible at. the focal point. 
 
 Unless we have a microscope-lamp like the Stratton 
 Illuminator, or the Acme, or some other special form, 
 it often happens that we wish to dispense with the use 
 of the bull's-eye condenser, which is always more or less 
 troublesome, and to take the light directly from the 
 lamp with the concave mirror. Here the conditions are 
 changed, the rays of light being divergent. The con- 
 cave mirror will focus them, but the mathematics of the 
 process are too abstruse to be entered into here. Still 
 the microscopist may readily find the proper distance 
 for the lamp from the mirror, or for the mirror from the 
 object, to obtain the best effects, provided he will make 
 an easy calculation, and refer to some treatise on micro- 
 scopical optics for the explanations. 
 
 If it is desired to know the proper distance at which 
 to place the mirror from the lamp, we must know the 
 radius of the mirror (twice the principal focus), the 
 angle of incidence and the distance of the mirror from 
 the object. If we then multiply the radius by the 
 cosine of the angle of incidence, and that product by 
 the distance of the mirror from the object, and divide 
 the result by twice the distance of the mirror from the 
 object, minus the product of the radius by the cosine 
 of the angle of incidence, the quotient will be the dis- 
 tance in inches for the lamp from the centre of the 
 mirror. This seems complex when expressed in words, 
 but in algebraic formula it is seen to be simple enough. 
 Let A represent the distance in inches between the 
 lamp and the mirror; </the distance from the mirror to 
 the object; R the radius of the mirror, and a the angle 
 of incidence, which in this case we will assume to be 
 
MICROSCOPICAL PRAXIS. 37 
 
 forty-five degrees. Then the algebraic formula will be, 
 
 . _d R cos a 
 2d R cos a 
 
 If the value of the symbols be a, 45;^,4 inches; ^?,8 
 inches; we shall have 
 
 8 8 X. 707 
 
 or about nine and one fourth inches (9.23), the position 
 of the lamp varying according to the value of the sym- 
 bols. 
 
 It may be more convenient to find the proper dis- 
 tance for the mirror from the object, that of the lamp 
 and the angle of incidence being known. In that 
 case the formula will be, 
 
 A R cos a 
 
 d 
 
 2A R cos a 
 
 Assuming the values to be; A, 10 inches; R, 8 inches; 
 and #, 45, we have 
 
 d 
 
 20 8 X. 707 
 
 or about four inches (3.95). 
 
 These superficial studies scarcely touch the subject 
 of the concave mirror, and the student who desires to 
 make a detailed examination must go elsewhere than 
 to these chapters. 
 
 The plane mirror is used only for the illumination of 
 very low power objectives and for certain special pur- 
 poses to be referred to hereafter. 
 
 As intimated, the bull's-eye lens is usually a trouble- 
 some piece of apparatus to use successfully. It is 
 
38 MICROSCOPICAL PRAXIS. 
 
 focussed with some difficulty, and unless correctly 
 focussed it is almost worthless, while it often happens 
 that after the microscopist has labored to get the awk- 
 ward thing into proper position, an accidental touch 
 disturbes it, and the careful adjustments must be 
 repeated, with much loss of time and of patience. Yet 
 a bull's-eye lens is essential, and the dealer often sup- 
 plies one with the stand. My advice to the beginner, 
 however, is to do without the bull's-eye in a separate 
 mounting, as supplied, and to substitute Messrs. J. W. 
 Queen & Co.'s Acme lamp, the Stratton Illuminator, 
 or some similar form. In these the bull's-eye is at- 
 tached to the front of the lamp, the flame being properly 
 focussed before the plane surface, while the whole 
 arrangement is one of the most convenient, useful and 
 manageable devices that microscopists can have on the 
 table, while the cost is but little more than that of a 
 first-class bull's-eye lens. 
 
 When using a concave mirror, the lamp, under all 
 circumstances, must be so placed that the flame is on a 
 level with the centre of the mirror. This is an essential 
 prerequisite to the attainment of the best results. Un- 
 der all circumstances, too, the broad side of the flame 
 must be kept away from the mirror. The only proper 
 portion of the flame ever to be employed is the narrow 
 edge. It may seem strange that this slender line of 
 light, as it appears to be, should have more intense 
 illuminating power than the whole wide front of the 
 flame, but such is the fact, and in all microscopical in- 
 vestigations it is the only portion that should be used. 
 It is said to have about eight times the intensity of the 
 broad side. 
 
 To the reader the use of daylight or of lamp-light 
 may seem one of personal preference only; but such is 
 
MICROSCOPICAL PRAXIS. 39 
 
 not the fact. My belief is that the majority of work- 
 ing microscopists never employ daylight when they can 
 command artificial illumination. The latter may be ob- 
 tained at almost any time and is entirely manageable. 
 It may be made into a blaze that shall almost blind the 
 user, or it way be subdued and softened until to see it 
 is a pleasure, its result a wonder and its effect beautiful. 
 
 The field illuminated by the soft, white, steady glow 
 of a good microscopical lamp is not only charming, but 
 useful; and it is beneficial to the eye, to which light is 
 a tonic and a healthful stimulus. The image produced 
 is the best the objective is capable of forming. It is 
 clear, sharply defined, sparkling and satisfactory, if all 
 the conditions are favorable. A better image may be 
 obtained from an inferior objective and lamp-light 
 properly employed, than from a good objective and 
 diffused daylight in whatever way the combination may 
 be used. On the eye the effect is not injurious even 
 after hours of prolonged investigation, provided, of 
 course, the light be properly modified. Only the eagle 
 can gaze directly at the sun, and even this favorite 
 simile of the poets forces the eagle to pose as a fraud, 
 for the bird protects its eye by the nictitating mem- 
 brane, the third eyelid, when gazing sunward. No 
 microscopist will gaze at the unmodified light of a 
 strong flame focussed by a concave mirror, or perhaps 
 intensified by the sub-stage condenser, without protect- 
 ing his eye by some device for reducing the blinding 
 glare. But when such modification is made, the effect 
 of artificial light is not harmful. 
 
 The image formed by an objective of a greater 
 amplification than that possessed by the one-half inch 
 or by the four-tenths, does not have by daylight that 
 exquisite sharpness of outline and brilliancy of aspect 
 
40 MICROSCOPICAL PRAXIS. 
 
 that it will have by artificial light. The general 
 appearance of things is watery and washed out, if the 
 light be taken from the blue sky, as must commonly be 
 done, or almost destructive of the eye if taken directly 
 from the sun. The only commendable light by day is 
 that reflected from a white cloud, or from a plane white 
 surface illuminated by the sun. Direct sunlight is 
 never used except at times in photomicrography. But 
 it is seldom that white clouds are sufficiently accommo- 
 dating to place themselves before the window and to 
 remain stationary and properly illuminated; and the 
 plane white surface is not often at one's command, 
 while a lamp is always ready. If the microscopist can 
 make a plate of plaster-of-Paris, and take the light 
 from it when illuminated by sunlight, the effect will be 
 good, almost as good as the effect of white cloud 
 illumination, but while the plaster-of-Paris plate in 
 diffused daylight gives a white field, it lacks, with high 
 powers, that soft intensity obtainable from the smallest 
 flame properly managed. On a rainy day the microscop- 
 ist would fare badly if he had no artificial light at his 
 command, and the microscopist that can work with the 
 instrument at night or not at all, would fare as badly if 
 lamp-light was as harmful and worthless as some 
 writers seem to think it is. 
 
 With daylight and no sub-stage condenser, use the 
 concave mirror; if the stand have a sub-stage condenser 
 then the plane mirror should be employed. 
 
 When artificial light is used, the table and the room 
 should be only faintly illuminated; there should be no 
 side lights to throw their reflections where they are not 
 wanted, and no extraneous light to interfere in any way; 
 there should be only the little spot of intense bright- 
 ness in the centre of the object, if the sub-stage con- 
 
MICROSCOPICAL PRAXIS. 41 
 
 denser be used, where it will do all that any one will 
 want done, if it be properly managed. My advice to 
 the beginner is to use the light of day in his microscop- 
 ical work as little as possible, and only with the lowest 
 powers, but to pay particular attention to the source of 
 his artificial supply, and to its manipulation. 
 
 Never take the light from a gas-flame. The quality 
 is not commendable, and the incessant flickering is per- 
 nicious in every way. Kerosene oil is the best illumi- 
 nant that can be used. The circular wick of the Ger- 
 man student-lamp gives a powerful light, but the flame 
 of a small flat wick is much to be preferred, since its 
 edge is intense and perfectly steady, the latter being of 
 prime importance, as a trembling light is one of the 
 worst things that the microscopist can use. If the be- 
 ginner does not care to buy a microscopical lamp, he 
 will find that a small hand-lamp, such as may be had 
 for twenty-five cents, will be all that he will need, es- 
 pecially if he use a bull's-eye lens, or a sub-stage con- 
 denser. 
 
 The light should be as white as possible. The yel- 
 low glare of the ordinary flame as seen through the mi- 
 croscope is hot and acrid. It should be cool, soft and 
 soothing. To make it so is not difficult. The older 
 microscopists were accustomed to produce the change 
 by filtering the light through a glass globe of water 
 colored faintly blue by sulphate of copper (blue vitriol,) 
 deepening the tint by the addition of strong ammonia- 
 water, thus obtaining the so-called monochromatic light, 
 or light of one color, often referred to in microscopical 
 literature. The result was that the blue water ex- 
 cluded the yellow rays, the light being consequently 
 cooled and softened in appearance. This, and indeed 
 all other methods of accomplishing the same end, some- 
 
42 MICROSCOPICAL PRAXIS. 
 
 what decreases the intensity of the illumination, but 
 enough will be left for all purposes, and it seems to 
 have the advantage of adding a desirable quality to the 
 light, even when the half-inch wick is used and turned 
 so low that the flame rises only just above the burner. 
 The device is troublesome, but the beginner may try it 
 by making a cell with parallel sides of glass, uniting 
 the parts with hot Canada-balsam or by a cement made 
 by melting together yellow wax and Canada-balsam. 
 The cell should be about one-eighth inch deep from 
 front to back, and filled with a saturated solution of the 
 copper-sulphate and ammonia, the color of which 
 should be as nearly sky-blue as possible. 
 
 As Dr. W. H. Dallinger has remarked, "True mono- 
 chromatic light really almost changes an achromatic 
 lens into an apochromatic one; but the great difficulty 
 has been hitherto how to produce monochromatic light 
 which should be absolutely such, and yet be within the 
 reach of all, and under control as to its measure of 
 intensity when employed with high powers." The 
 ammoniated solution of copper-sulphate is not truly 
 monochromatic, and experiments have frequently been 
 made to find an easily prepared and easily managed 
 solution whose effect should approach more nearly the 
 desired result. Such a solution has been obtained by 
 certain microscopists in Europe, and highly rec- 
 ommended as being practically monochromatic. I 
 have used it to a limited extent, but have not observed 
 that the effect is any better than with the use of the 
 properly colored glass as a light-modifier. Perhaps 
 with further study and experimentation the solution 
 might produce more satisfactory results. It is used in 
 the way already described for the copper-sulphate so- 
 lution, and gives with artificial illumination an orange 
 
MICROSCOPICAL PRAXIS. 43 
 
 light slightly tinged with green. It is made as follows: 
 Sulphate of copper, 2 ounces and i% drams; 
 Bichromate of potash, i dram and 2 scruples; 
 Sulphuric acid, 12 minims; 
 Water, 6^ ounces. 
 
 Similiar results may be attained by simpler means. 
 If the microscopist must use day-light, it will be of bet- 
 ter quality if he will fasten a pane of pale blue glass in 
 the window, and receive the light through it. At night 
 nothing mo're is necessary than to place one or more 
 small pieces of glass of a deeper blue above the ocular, 
 or in the sub-stage between the mirror and the object, 
 or between the mirror .and the light. In the latter 
 event a blue-glass chimney is all that is needed, if a 
 sub-stage condenser be not used. 
 
 None of these devices deprives the light of those 
 qualities demanded in what has been called "micro- 
 scopical gymnastics," or the use of first-class objec- 
 tives to show the finest striations on difficult diatoms, 
 or a series of lines closely ruled on glass plates, such 
 as those made by the late F. A. Nobert, of Germany, 
 and the late Charles Fasoldt, of Albany, N. Y. If the 
 reader desires to practise microscopical gymnastics, 
 however, he will find that the sulphate of copper or the 
 bichromate solution will be a rather better assistant 
 than the blue glass, since the former is said, by the late 
 Dr. J. J. Woodward, to increase the defining power of 
 first-class objectives when used in these gymnastic ex- 
 ercises, as the color may be controlled, while 
 glass of just the right sky-blue shade is not so 
 common as it should be. For all other purposes the 
 glass is amply sufficient, supplying the best obtainable 
 modification of the illumination. 
 
44 MICROSCOPICAL PRAXIS. 
 
 The centre is an important point in the microscope. 
 The optician takes the greatest pains to have the com- 
 ponent lenses of his objectives most accurately centred.. 
 The maker of the stand is careful to have all movable 
 parts act in a certain relation to the optical centre or 
 axis of the instrument; and the working microscopist is 
 always anxious. to have the illuminating beam strictly 
 central, except when he desires to use oblique light. 
 The centre of tne mirror is always in che optical axis 
 of the microscope, or should be, for ordinary work. 
 To accomplish this centralizing of the illumination Mr. 
 Edward Pennock has suggested an admirable method, 
 which the beginner is advised to adopt for all occasions. 
 Mr. Pennock's instructions are substantially as follows: 
 
 Having the object in place and lighted from the mir- 
 ror, the objective screwed on, and the eye-piece in the 
 tube, first focus. Then remove the eye-piece and ap- 
 plying the eye centrally to the end of the body-tube, 
 notice the spot of light'at the back of the objective. It 
 may appear as in A, figure 2, in which case move the 
 mirror or diaphragm or both until the illuminating beam 
 appears central, as in B. If now your lens is a good 
 one, properly adjusted, and the inner circle of B pre- 
 sents an evenly illuminated disk, you should obtain a 
 
 ABC 
 
 Fiji. -2. Fennock's method of centring the illumination. 
 
 good, sharply defined image. But there may not be 
 light enough or there may be too much, in which case 
 use a diaphragm of different size, or vary its distance 
 from the object, thus varying the angular size of the 
 
MICROSCOPICAL PRAXIS. 45 
 
 illuminating pencil. If the diaphragm be too large or 
 placed too near the object, it ceases to affect the an- 
 gular size of the illuminating beam, although it may act 
 in another way, and in this case the image of the mir- 
 ror is seen within the circle of the diaphragm, as at C, 
 if the objective is of sufficient aperture. 
 
 If lamp-light be used, the image of the flame may be 
 seen within the disk of the mirror and diaphragm, as at 
 D. This shows that the beam is not focussed upon the 
 object. This may be remedied by a bull's-eye con- 
 denser placed near the source of light, making parallel 
 the rays falling on the mirror, or simply by altering the 
 distance of the mirror from the stage. In some cases, 
 however, the mirror is not of the right focus and the 
 latter course cannot be adopted. The appearance 
 should be like B as nearly as possible. This is as use- 
 ful and successful when the sub-stage condenser is used 
 as it is with the mirror alone. 
 
 Reflected and transmitted light are terms which are 
 somewhat puzzling on first acquaintance, although they 
 are often met with in microscopical literature. They 
 refer to the illumination in connection with the object, 
 and not, as might be supposed, to the bull's-eye con- 
 densing lens or the mirror. A transparent substance is al- 
 ways examined by transmitted light, the illumination 
 being passed or transmitted through it from below, in 
 which event the mirror may be used, or the microscope 
 may be placed in a horizontal position and the light 
 taken directly from the lamp flame. Opaque objects 
 can not be studied by transmitted illumination. They 
 are restricted to reflected light, the illuminating beams 
 being thrown upon the surface, from which they are re- 
 flected. Most objects seen by the naked eye are seen 
 by reflected light. 
 
46 MICROSCOPICAL PRAXIS. 
 
 The Diaphragm. 
 
 The reader may suppose that there is but one form 
 of microscope-diaphragm, a flat disk of metal pierced 
 near its margin by apertures of various sizes. No idea 
 could be more erroneous. There are so many forms 
 that a plausible supposition is that whenever an optician 
 has a sleepless night, instead of tossing on his bed like 
 ordinary folks, he invents a new diaphragm. Then he 
 publishes it and nobody ever hears of it again; cer- 
 tainly nobody ever uses one out of a hundred of the 
 diaphragmatic monsters noted in the books. There are 
 the Iris, and the Spiral, and the Cylinder, and the Ca- 
 lotte, and the Spherical. There are oblong plates 
 pierced by all sorts of holes radiating in all sorts of di- 
 rections. They have horizontal slits, and vertical slits, 
 and oblique slits. They have big holes, and little holes, 
 and pin holes, and round holes, and oblong holes, and 
 square holes. They are concave and convex. They 
 are fastened below the stage, to its under surface, and 
 in a depression that brings them on a level with the up- 
 per surface. There is one form of two cylinders or 
 rollers revolved by a milled head, and encircled by two 
 conical grooves. They have special forms of holders. 
 They rotate on their centre, or they may be swung aside 
 on a movable arm attached to the stage. There are al- 
 ready so many that about the only kind remaining for 
 the reader to invent is one that shall be wound up and 
 go by clock-work and a spring, or by electricity, or 
 steam, or water-power or wind. 
 
 It has been said that the working microscopist can 
 get along with very few of the apparently desirable de- 
 vices offered by the dealers. That is true. Few things 
 microscopical are absolutely indispensible, and among 
 these I should not class, for general use, these refine- 
 
MICROSCOPICAL PRAXIS. 47 
 
 ments in the way of diaphragms. If the beginning mi- 
 croscopist is not careful he will be lost in a wilderness 
 of diaphragms. Yet the device in its simplest form is 
 essential to the proper action of the instrument, and to 
 the proper interpretation of the magnified image of the 
 object. Most young microscopists use too much light. 
 They seem to think that if a little is good, much is bet- 
 ter. The result is that the eye is exposed to risk, and 
 the finer details of the object are drowned in a flood of 
 glaring brightness. The field should be toned down to 
 a soft and pleasant glow, and to accomplish this, aside 
 from the use of the blue glass, the circular disk-dia- 
 phragm in the stage, or attached to it, is all that is 
 needed. This should be turned until the opening which 
 gives the proper effect is beneath the object, and no 
 aperture should be used because it seems to be the 
 right one. Try them in succession, and adopc the one 
 proper to the occasion. It is said that the opening to 
 be used should be the one which corresponds in size to 
 that of the front lens of the objective. This is true if 
 the diaphragm is a permanent fixture and on a level 
 with the upper surface of the stage, but if it can be 
 racked upward and downward, the light from the con- 
 cave mirror may be modified satisfactorily by this 
 movement. Few of the cheaper stands however, have 
 a diaphragm thus movable, and those adapted to the 
 use of a sub-stage condenser have the metal disk, or 
 other form, so arranged that it may be entirely re- 
 moved, and special diaphragms employed below, or 
 sometimes above, the lenses of the condenser, although 
 the last-mentioned position is not to be commended. 
 
 Dealers, and microscopists as well, differ in their 
 opinion as to the proper position of the diaphragm when 
 it is to be attached to the stage. Some place it half an 
 
48 MICROSCOPICAL PRAXIS. 
 
 inch or more below, others nearer on a level with the 
 upper surface. Its action differs according to its posi- 
 tion and the character of the light. As to its position 
 the student must usually abide by the decision of the 
 maker, unless he select a stand with the diaphragm on 
 the sub-stage, or one adapted to the use of a sub-stage 
 condenser. If it is attached to the stage, it should be 
 as close to the object as possible. 
 
 The reader will of course understand that parallel 
 rays cannot be affected in any way by the position of 
 the diaphragm in reference to the object. With con- 
 verging rays, however, the result is different. In this 
 case the lowering of the diaphragm will obstruct more 
 of the oblique rays the lower it is depressed, until it 
 comes in contact with the mirror itself. A similar 
 effect is obtained with converging rays by diminishing 
 the size of the aperture in a diaphragm immediately 
 beneath the stage, the smaller the opening the greater 
 the number of oblique rays obstructed. With parallel 
 rays, however, the depression of the diaphragm is with- 
 out noticeable effect, the amount of light being dimin- 
 ished by the use of decreased apertures in the dia- 
 phragm-plate, so that a diaphragm immediately below 
 the object can be used, by changing the size of the ap- 
 ertures, to modify either parallel or converging rays. 
 With the sub-stage condenser diminished illumination 
 is obtained by depressing the condenser, the focus of 
 the appliance thus being removed below and from the 
 object. As only parallel rays should be used with this 
 sub-stage accessory, their power and number are dimin- 
 ished by the use of diaphragms of different apertures, 
 as well as by the differing positions of the instrument 
 below the object, the diaphragms being below the con- 
 denser. The proper place then for the plate of dia- 
 
MICROSCOPICAL PRAXIS. 49 
 
 phragm-op^nings is immediately beneath the object 
 when the condenser is not used; when the condenser is 
 used the diaphragm should be as close as possible to its 
 back lens. Some stands have it attached to a cylindri- 
 cal box at some distance below the stage. Such stands 
 are good ones to be rejected. 
 
 The proper amount of light to be employed in ordi- 
 dary investigations cannot be taught in words. The 
 beginner must learn by experience, using his own best 
 judgment. Avoid a glare, and avoid semi-darkness. 
 There is a commendable medium, which the beginner 
 must find for himself, if he work alone. 
 
 The Eye-Piece, or Ocular. 
 
 This part of the microscope fits loosely into the 
 upper end of the body-tube, from which it may be 
 readily removed. Its function is to magnify the image 
 produced by the objective. In form it is a short tube 
 bearing a lens or a combination of lenses at each end 
 and varying in construction and in name almost as 
 greatly as does the diaphragm. The negative eye- 
 piece is the one commonly used. It is so named be- 
 cause its focus is within the tube, at a level with its 
 diaphragm. It cannot be used as a magnifying glass 
 without the objective. 
 
50 MICROSCOPICAL PRAXIS. 
 
 Eye-pieces should properly fit the booty-tube. A 
 tightly fitting ocular, and one that is too loose, are 
 equally annoying and inconvenient. It should not be 
 loose enough to shake about in the tube, although this 
 may be easily remedied by winding with paper; it 
 should be of a size sufficient to allow it to drop in by 
 its own weight, and out, too, if the microscope is turned 
 upside down. 
 
 The negative eye-piece is often called the Huyghen- 
 ian because the celebrated Dutch astronomer Huy- 
 ghens, who died in 1695, first used one of similar con- 
 struction on his telescope. Its focus, or the point at 
 which the image is formed, falls about midway between 
 its two lenses, while in the positive the focus is below 
 the lower glass. In the negative eye-pieces the lenses 
 are plano-convex and have their plane surfaces di- 
 rected upward, while a diaphragm with a single central 
 aperture is placed within the tube at a point corres- 
 ponding to the place where the image is formed. This 
 diaphragm serves to intercept all those marginal rays 
 not active in the production of the image, and partly to 
 correct the spherical aberration as well as the chro- 
 matic, the diaphragm being further utilized as a place 
 of support for the eye-piece micrometer used in the 
 measurement of microscopic objects. 
 
 The diaphragm is an important factor everywhere in 
 the microscope, but here it is especially useful, not only 
 because it acts as stated, but also because upon the size 
 of the aperture in its centre depends the size of the 
 field. By contracting that the field may be diminished 
 in extent. The diameter of the body-tube is not alto- 
 gether responsible for the size of the field, and neither 
 is the diameter of the eye-piece itself. The diaphragm 
 is here the important factor, the same sized opening 
 
MICROSCOPICAL PRAXIS. 51 
 
 being used in all negative eye-pieces of the same 
 power. 
 
 The size of the field appears larger to most persons 
 than it really is, and this apparent size is misleading to 
 many, whose estimates are often amusing and amazing. 
 There is a vast difference between the apparent and 
 the actual fields. The apparent is due to the eye-piece, 
 while the actual is represented by the actual amount of 
 surface shown at one time by the objective, this area 
 varying with every magnifying power and every objec- 
 tive. The actual field may be measured by placing a 
 stage micrometer below the objective and counting the 
 number of spaces included within the circular lighted 
 area, but to obtain the apparent diameter as produced 
 by the eye-piece, a simple calculation is necessary. 
 
 The rule is to multiply the diameter of the dia- 
 phragm-opening by the magnifying power of the eye- 
 lens. If the aperture is half an inch across, and the 
 eye-lens magnifies ten times, then the field is apparently 
 five inches in diameter. To me these matters, even so 
 simple a one as this, are more easily manageable if put 
 in the shape of an algebraic formula. In this instance, 
 then, if D represent the diameter of the diaphragm- 
 opening, P the magnifying power of the eye-lens, and 
 F the diameter of the apparent field, we will have 
 F=.DP, or, in the supposed case, ^=-|-Xio, or five 
 inches. 
 
 But the power of the eye-lens, how can that be ob- 
 tained ? This is one of those interesting little points 
 about which the inquiring student soon wants to know 
 something. The magnifying power of the eye-lens is 
 equal to ten inches (the arbitrary standard of distance 
 for distinct vision), divided by its focal length in inches, 
 so that it only remains to obtain that focal length, and 
 
52 MICROSCOPICAL PRAXIS. 
 
 both problems are solved. The formula here would be 
 P=j, t the focal distance of the eye-lens being ob- 
 tained in the manner described for ascertaining that of 
 the bull's-eye condenser or of the pocket-lens, except 
 that here the convex surface must always be turned 
 toward the light, since that is its^ position in the ocular. 
 In one of my own ocular's measured while writing this, 
 the focal distance of the eye-lens is ^ inches which, by 
 the formula P~% gives P=io-t-\fa or yf The 
 diameter of the diaphragm-opemng is by actual meas- 
 urement 0.7 inch. By the first-mentioned formula, J 7 
 DP, we have ^=7|x.7, or five inches, the apparent 
 diameter of the field, which is correct, as I have proved 
 by another, simpler, and on that account, probably a 
 better way. 
 
 This is to make the measurement by means of the 
 camera lucida, the latter being applied to the eye-piece 
 and the microscope placed in a horizontal position with 
 the camera lucida ten inches from the table, if the 
 Wollaston form of camera be used, the limits of the 
 field being marked by pencil and then measured by the 
 ordinary foot-rule. 
 
 If the reader make the experiment by measuring the 
 focal length of the eye-lens, he should take special care 
 to focus the lens as accurately as possible, since the 
 difference of a small fraction of an inch makes an 
 astonishing difference in the result. 
 
 The size of the actual field varies with the nominal 
 focus and with the power of the objective. The field 
 of Zeiss's variable A* when at its lowest power is, by 
 actual measurement, thirty twenty-fifths of an inch, 
 (1.2 in.), and, according to the approximate estimates of 
 Mr. Edward Bausch, a power of twenty-five diameters 
 will show a surface about one-fifth inch wide; fifty di- 
 
MICROSCOPICAL PRAXIS. 53 
 
 ameters will include an area one-tenth of an inch in 
 width; one hundred diameters, one-twentieth inch; five 
 hundred diameters, one one-hundredth, while one 
 thousand diameters will exhibits space only one two-, 
 hundredths of an inch wide. 
 
 At present eye-pieces of different powers are named 
 after the letters of the alphabet, the lowest magnifying 
 power being A, the next highest B, and so on down 
 the alphabet, the power increasing as the letters ap- 
 proach Z and ampersand. This method might not be 
 objectionable if it conveyed a correct idea of the power, 
 as it does not, some makers producing^ eye-pieces that 
 magnify as the B of other opticians, no two dealers hav- 
 ing precisely the same standard. As a rule, however, 
 the A ocular roughly approaches two inches in focal 
 length, with a power of five, and so continuing through 
 D, , F and others with a focal length of f , , -^ \, 
 inches, with powers varying from fifteen, twenty, 
 twenty-five to thirty, forty or more diameters. 
 
 A better plan would be to mark each with its focal 
 length, two inches, one inch, or whatever it may be, 
 and to have it understood that a certain length al- 
 ways represents a certain invariable power. Thus the * 
 one inch should possess a power of ten diameters; the j 
 two inch of five; the one-half by twenty; the one-fifth/ 
 by fifty. And if not only the focal length could be 
 stamped on the cap of the eye lens, but the power as 
 well, the change would be acceptable and the advan- 
 tages great. Eye-pieces are made varying in focal 
 length from two inches to one-sixteenth inch. The 
 power of the last should be one hundred and sixty di- 
 ameters. With it is I do not know, and do not care to 
 know, if that knowledge must be contingent upon per- 
 sonal ownership, for I would not accept such an ocular 
 
54 
 
 MICROSCOPICAL PRAXIS. 
 
 as a gift, if I were to be forced to use it. A more 
 worthless cumberer of the apparatus box I cannot im- 
 agine. No human eye could see anything satisfactorily 
 with such an eye-piece.. 
 
 The reader may desire to know how to measure the 
 power of his oculars and thus learn what the A, B and 
 C, or the 2^/2 or i inch represent in amplification. For 
 this purpose the late W. H. Bulloch, 
 of Chicago, has devised a simple ap- 
 paratus usable by anyone, and has 
 described an easy method whose re- 
 sults are at least approximately cor- 
 rect. The apparatus is shown in 
 figure 3. It consists of the micro- 
 scope B supplied with a two-inch 
 objective, and placed in a horizontal 
 position opposite to the tube G D 
 which carries at E F the eye-piece 
 to be measured, while at the end D is 
 a diaphragm pierced by an oblong 
 opening of known size, as at H. C 
 is a stationary stage bearing a mi- 
 crometer ruled to one hundredths of 
 an inch or to tenths of a milli- 
 metre. In Mr. Bulloch's instrument 
 the parts are all brass and fitted 
 with rack and pinion adjustments, 
 which are very well in their way, but 
 not essential to the working microscopist, who can 
 make the tube G D of stiff paper, and fasten it to the 
 top of a pile of books as the writer has done, using the 
 microscope for the other part of the instrument, the 
 diaphragm at D being also of paper, while the stage C 
 is that of the microscope. The distance from the di- 
 
MICROSCOPICAL PRAXIS. 55 
 
 aphragm D to the diaphragm within the eye-piece to be 
 measured should be ten inches, the arbitrary standard 
 of length for distinct vision. 
 
 Screw to the microscope the two-inch object- 
 ive, as it is the most convenient for this purpose, and 
 focus it on the lines of the micrometer at C. Then 
 bring the tube G D toward the micrometer until the 
 image of the aperture at H is focussed on the same 
 lines, presenting the appearance of H' in the figure. 
 Count the number of spaces occupied by the image, 
 and divide the number into the known size of the aper- 
 ture in the diaphragm at H. In Mr. Bulloch's case the 
 diaphragm-opening was 6.5 millimetres, which covered 
 eleven spaces of a micrometer ruled to tenths of a mil- 
 limetre. A simple formula and its application is, P the 
 power to be obtained; D the size of the diaphragm 
 opening, and m the number of micrometer spaces oc- 
 cupied; then P=.~ . In this case the value of D is 
 6.5 mm.; of m eleven-tenths mm, and the calculation, 
 in the form of a decimal fraction, is -f-:f = -f-f- or about six. 
 The reader of course understands that when his micro- 
 meter is ruled in parts of an inch, the diaphragm open- 
 ing atZ> in the figure, may be of any convenient size 
 provided it also is in parts of an inch. In the case of one 
 of Mr. Bulloch's eye-pieces belonging to my own stand, 
 and measured very hastily while waiting for the 
 dinner-bell to ring, being done, therefore, under very 
 adverse circumstances, the result was 4.4+, the 
 power marked on the cap of the eye lens being 4.5. 
 With greater care in reading the micrometer the re- 
 sult might have been actually that of Mr. Bulloch 
 although the difference is very slight. The tube G D 
 was a pasteboard cylinder into which the eye-piece 
 to be measured was wedged with a corner of the 
 
56 MICROSCOPICAL PRAXIS. 
 
 pocket handkerchief. The micrometer was ruled in 
 hundredths of an inch, and the diaphragm aperture was 
 two-tenths of an inch long (Z>), occupying four and 
 one half micrometer spaces (;;/). The calculation was, 
 
 = or or 
 
 However careful of his stand the microscopist may be, 
 the dust will collect on it and especially on the eye lens. 
 This when abundant is indistinctly visible and should be 
 removed. There is one right way to do this, and that is 
 not by a careless wipe with the pocket handkerchief, 
 as I have seen done. A particle of grit may be on the 
 glass or in the cloth, when a scratch will be made that 
 may be disastrous in its effects. To avoid such dan- 
 ger, blow away the dust with several short quick puffs 
 of the breath, then carefully wipe the surface with the 
 Japanese filter paper, as originally recommended by 
 Prof. S. H. Gage, of Cornell University. This paper 
 is extensively used by the dentists, and may be had 
 cheaply from dealers in dental supplies. It is soft and 
 thin, with no tendency to collect the dust, and with no 
 kaolin or gritty matter in its substance. It may there- 
 fore be used almost recklessly. Prof. Gage has also 
 recommended it for cleaning the fronts of objectives, 
 for which purpose it can not be excelled. A sheet of 
 it should be cut into small squares and kept in a 
 covered box on the microscopist's table, a piece being 
 used once only, to be thrown away after each 
 application to the objective or ocular. ^ 
 
 It is sometimes difficult to describe a special object 
 in the field, or some special part of an object so that 
 another person may recognize it. This is particularly 
 difficult if the other person has had no experience in 
 the use of the microscope. He will be as likely to 
 gaze with awe at an air bubble or a bit of colored 
 
MICROSCOPICAL PRAXIS. 57 
 
 
 
 wool, as at the specified object. To obviate this, a 
 pointer called an indicator is occasionally used. As 
 originally devised by Quekett it consists of a fine steel 
 hand so pivoted within the eye-piece, and near the dia- 
 phragm, as to be in the focus of the eye lens, and 
 capable of being swuhg forward to the centre of the 
 field so as to point to the special object, and then 
 swung back out of sight when not in service. It is not 
 much used now, but an anonymous writer describes a 
 simple form made by attaching a hair to the diaphragm 
 so as to extend half way across the field. He gums 
 the piece of hair to a bit of paper, the end projecting 
 to the proper distance beyond the edge. When dry 
 he cuts the paper to the right size, moistens the gum- 
 med side, and attaches the whole to the diaphragm. 
 He says that "This simple arrangement is entirely satis- 
 factory, and we have it now in constant use. The hair 
 is not objectionable in the ocular, as it appears as a 
 fine sharp line, and is quite overlooked when one gets 
 accustomed to its presence." To me personally the 
 presence of the fine line would be an annoyance. How- 
 ever, if the microscope is much used for showing ob- 
 jects to others, so simple an arrangement might be 
 commendable. 
 
58 MICROSCOPICAL PRAXIS. 
 
 The Micrometer. 
 
 The micrometer is an instrument for the measuring 
 of minute objects or of short spaces. In some form it 
 is in frequent use by the microscopist. It consists of a 
 series of fine lines ruled on glass, the distance between 
 the lines varying according to the wish of the microsco- 
 pist, or of the optician that makes the plate. These 
 bands of rulings are prepared by special and compli- 
 cated machines, some of which are so delicate and so 
 easily influenced for the worst, that they are allowed' to 
 work only in the darkness and the stillness of the night. 
 Nobert, of Prussia, was for many years the most noted 
 maker of fine rulings, some of his work having fora 
 long time served as tests for the best and highest-power 
 objectives. In this country we have had three micros- 
 copists who have turned their attention to the making 
 of fine micrometers and bands of closely ruled lines, 
 Prof. W. A. Rogers, who did splendid work but who has 
 now retired from this field arid transferred his ruling- 
 machine to Prof. M. D. Ewell of Chicago, the other 
 worker of note in this department being the late Charles 
 Fasoldt of Albany, N. Y., whose extravagant claims in 
 regard to his rulings detract from the estimation in 
 which he otherwise would have been held by intelligent 
 microscopists. His assertion that he had ruled a band 
 containing one million lines to the inch is known by 
 every well-informed microscopist to be an absurdity and 
 impossibility. No human eye has seen more than 120,- 
 ooo ruled lines to the inch, and no human eye probably 
 ever will. Professor M. D. Ewell is at present the only 
 person in this country ruling fine micrometers, and all 
 his work is commendable, while his prices are exceed- 
 ingly reasonable. 
 
 The microscopist should possess two forms of micro- 
 
MICROSCOPICAL PRAXIS. 59 
 
 meter, one for the stage, the other for use in the eye- 
 piece, where it is always employed without the stage- 
 micrometer, after the value of its spaces have been as- 
 certained. This is a great convenience over the old 
 and awkward, and often impossible, method of using 
 the stage-micrometer by placing the object above it and 
 focussing down through the thickness of both to see 
 how the lines stood in relation to the margins of the 
 specimen. This was oftener than not impossible for 
 use with high-power objectives, which always have a 
 short working-distance. 
 
 The two kinds of micrometers differ in form as well 
 as in position. That for the stage is mounted on a slip 
 of glass of the standard size, while that for the ocular 
 is a circular disk of thin glass made to rest on the dia- 
 phragm of the eye-piece tube. 
 
 The Stage-Micrometer. 
 
 The stage-micrometer is sometimes ruled directly on 
 the glass slip and used without a cover-glass; some- 
 times it is ruled on the cover itself which is then 
 mounted on the slip, the first-mentioned kind probably 
 being rather more easily used. The lines in either 
 case are* usually filled with graphite to make them more 
 distinct, for it has happened that although the micros- 
 
60 MICROSCOPICAL PRAXIS. 
 
 copist has known that the lines were there, they have 
 been so faintly ruled that they were invisible even 
 under the best objectives manipulated by an expert 
 microscopist. 
 
 Usually the lines will be in two narrow bands ruled 
 at the rate of one hundred and one thousand to the 
 inch. These are sufficient for most purposes; yet it 
 will be convenient for use with high-powers to have a 
 third band ruled at the rate of two thousand to the 
 inch, and this will not add much to the cost. If the 
 reader should prefer to have all his measurements 
 made in millimetres there will be no objection to it, but 
 in that case the micrometer may well be ruled to tenths 
 and hundredths of a millimetre. When ordering the 
 stage-micrometer therefore, it is well to give instruc- 
 tions on . hese points, otherwise the maker will use his 
 own judgment. For practical work nothing will be 
 needed less than the one two-thousandth of an inch or 
 the hundredth of a millimetre. 
 
 The stage-micrometer is rarely used for the direct 
 measurement of the object in the way already referred 
 to; the reasons for this have also been given. But 
 when this can be done with very low-power objectives, 
 the object is laid across the micrometer lines, and 
 measured much as a two-foot rule would measure it. 
 Thus, if the object extends across just two and one-half 
 of the one-hundredth inch spaces, it of course measures 
 two and one-half hundredths, or -J^.of an inch. 
 
 When the object extends into a space without en- 
 tirely crossing it to the next line, the portion of the 
 space occupied must be estimated by the eye, and here 
 enters a source of error against which the microscopist 
 must guard. The same holds true in the use of the 
 eye-piece micrometer when the object extends beyond 
 
MICROSCOPICAL PRAXIS. 6l 
 
 a line but not entirely across a space. The more ex- 
 pert the microscopist is in estimating distances, the 
 more nearly free from error will his measurements be 
 in such instances. At other times, if the micrometer be 
 correctly ruled, there will be little trouble in reading 
 the spaces, especially since every fifth line is, or should 
 be, longer than the remaining four, its end extending 
 above the general level of the others. This catches 
 the eye and enables the microscopist to read the lines 
 rapidly in groups of five. This direct method of meas- 
 uring, however, has been almost entirely superseded, 
 that by means of the camera lucida being more accur- 
 ate and better adapted to high-powers, if the microsco- 
 pist have no eye-piece micrometer. 
 
 By the camera lucida process just mentioned the 
 micrometer lines are focussed, after the microscope has 
 been placed in a horizontal position, with the eye-piece 
 about ten inches from the table-top. The camera 
 lucida is then attached and the lines drawn on the 
 paper upon the table. The micrometer is now to be 
 removed, the object substituted, and the number of 
 spaces its image occupies on the paper scale at once 
 read. Thus, if the one-thousandth inch spaces have 
 been sketched, and the image of the object, drawn with 
 the microscope in the same position and with the same 
 objective, occupies eight of those spaces, it of course 
 measures eight one-thousandths of an inch, or T -| T . A 
 scale should be made for every objective and eye-piece, 
 so that the image may be projected on the proper one 
 by the camera lucida, without the trouble of drawing 
 the micrometer lines every time this method is used. 
 But at best this is an awkw r ard way of accomplishing 
 what may be done with great ease and rapidity with the 
 eye-piece micrometer. 
 
62 MICROSCOPICAL PRAXIS. 
 
 Efforts have been made to -induce microscopists to 
 adopt some standard to which micrometers shall be 
 ruled, so that all measurements may be expressed in 
 the same terms. In this country and in England, parts 
 of an inch are used, unless the microscopist orders 
 otherwise; on the continent of Europe parts of a milli- 
 metre. The American Society of Microscopists and 
 others have endeavored to induce microscopists to 
 adopt the one one-thousandth of a millimetre, or about 
 one-twenty-five thousandth of an inch as the standard 
 for measurements, but thus far without much success, 
 although it will probably become common in time. 
 This one one-thousandth of a millimetre is to be known 
 as the micron, and to be represented by the Greek let- 
 ter //. In Europe it is frequently used, but in this 
 country writers in the 'scientific periodicals have not 
 employed it generally enough to make the majority of 
 readers of microscopical literature familiar with it. As 
 a unit of distance, the measure is in every way com- 
 mendable, and should be used whenever possible. 
 
 The Eye-piece Micrometer. 
 
 This, as its name indicates, is used only in the eye- 
 piece, where it is placed on the diaphragm. It is made 
 to fit in the tube, or in an adapter which then rests on 
 the diaphragm. It is used by rotating the eye-piece 
 
MICROSCOPICAL PRAXIS. 63 
 
 until the lines lie across the object to be measured and 
 at right angles to its length or breadth. The spaces 
 occupied are counted, and a simple calculation gives 
 the length or the breadth, when the value of each space 
 is known. 
 
 To ascertain this value, a stage micrometer is needed, 
 and when the latter has been used for this purpose it is 
 of no further importance unless the owner should de- 
 sire to ascertain the value of the spaces in another eye- 
 piece micrometer. The stage-plate may then be put 
 away, as the eye-piece disk is now all that will be 
 needed for all microscopical measurements. 
 
 Put the stage-plate in position and focus the lines. 
 Remove this eye-piece, insert the one that carries the 
 eye-piece micrometer, and again focus. The stage- 
 micrometer is then to be moved until one of its lines 
 exactly coincides with one of the lines in the eye-piece 
 micrometer, and as nearly as possible in the centre of 
 the field. Notice how many spaces in the eye-piece are 
 needed to fill one space of the stage-plate, and obtain 
 the value of one of the intervals in the former, by di- 
 viding the value of the single space on the stage-micro- 
 meter by the number of spaces it fills in the eye-piece 
 micrometer. Thus, if one of the one-hundredth inch 
 intervals on the stage-plate is just filled by three 
 spaces on the eye-piece micrometer, the value of one 
 space of the latter will be -^J-g- of an inch, or the yj-^ of 
 the stage-plate divided by the three divisions occupied 
 in the eye-piece micrometer. Or if ten spaces in the 
 eye-piece fill one of the 10 * 00 inch divisions on the 
 stage-micrometer, then the value of the one eye-piece 
 space is io ^ 00 inch, or -j-gVo divided by ten, and any ob- 
 ject that just fills one such space in the eye-piece 
 would be To ^ 00 inch in length; if it occupied five spaces 
 
64 MICROSCOPICAL PRAXIS. 
 
 it would be T^OTT or TsVo inch lon - The value of the 
 spaces must be ascertained in this way with every ob- 
 jective, and the measurement of the object should al- 
 ways be made under the same conditions as far as re- 
 gards tube-length and the position of the adjustment- 
 collar of the objective, any change in either at once al- 
 tering the value of the micrometer-spaces. 
 
 It sometimes happens that a certain number of di- 
 visions in the eye-piece cannot be made to fill exactly a 
 certain number on the stage-micrometer, a fraction of a 
 space being left over. For instance, thirteen divisions in 
 the eye-piece may perhaps occupy two and one-half of the 
 one one-hundredth inch spaces on the stage-plate. To 
 simplify the calculation in such a case, the draw-tube 
 may be extended until a certain number of eye-piece 
 spaces will exactly fill a certain number on the stage- 
 micrometer, when the position of the tube must be re- 
 corded, and all subsequent measurements made with it 
 at that place. Or, if the microscopist have an eye sen- 
 sitive to small distances, he may estimate the part of a 
 space occupied. 
 
 The position of the adjustment-collar of the objective 
 also alters the value of the spaces, since any movement 
 towards closed point or bringing the component lenses 
 closer together, increases the magnifying power, and 
 decreases it when made toward open point. With ad- 
 justable objectives, therefore, the value must be ascer- 
 tained for every position of the collar, unless the mi- 
 croscopist is willing to place the adjustment-ring at 
 open or at closed, and to allow it always to remain 
 there when measurements are to be made. 
 
 The eye-piece also affects the value of the spaces, 
 since they are magnified by the eye-lens whose power, 
 of course, varies in oculars of different focal lengths 
 
MICROSCOPICAL PRAXIS. 65 
 
 It is best, indeed, always to employ the same ocular, 
 allowing the micrometer to remain on the diaphragm, 
 using that eye-piece for measuring-work only. The 
 one-inch, or the one and one-half inch ocular is a de- 
 sirable one for the purpose. 
 
 The value of the spaces with each objective should 
 be recorded for ready reference. The reader will prob- 
 ably prefer to devise his own method of doing this, but 
 the following is the writer's tabular form, given only to 
 show the manner of preparing the record in this partic- 
 ular instance. 
 
 Value of Micrometer Divisions with each Ob- 
 jective. 
 
 Objective. 
 
 Adjustment. 
 
 Inch 
 (fractions.) 
 
 Inch 
 (decimals.) 
 
 Micron /* 
 
 D-tube. 
 
 I-IO 
 
 Closed. 
 
 i -6000 
 
 0.00016664- 
 
 4 
 
 In. 
 
 The number of intervals on the eye-piece micrometer 
 is of no importance, provided they are equally spaced. 
 The ruling may therefore be safely left to the maker. 
 The late Dr. J. J. Woodward, however, thought other- 
 wise, for he has said that, "So long as the English 
 microscopists continue to express the results of their 
 measurements in decimals of an English inch, there 
 will be American microscopists who will do the same, 
 
66 MICROSCOPICAL PRAXIS. 
 
 either for all purposes or for particular work, and of 
 course it is very desirable that the measurements also 
 should be accurate. The stage-micrometers on this 
 system in the market are usually ruled in hundredths 
 and thousandths of an inch. The latter divisions are 
 too wide to give values to the eye-piece micrometer with 
 the higher powers, while the five-thousandths and ten- 
 thousandths and even finer divisions ruled also on some 
 of these micrometers, are inconveniently close. I would 
 advise the makers to rule such micrometers four-tenths 
 of an inch long, divided into hundredths of an inch, one 
 of the hundredths being divided into ten, another into 
 twenty-five. These latter spaces, each representing 
 one twenty-five-hundredth of an inch, sufficiently ap- 
 proximate the hundredth of a millimetre to be used 
 with equal convenience with the higher powers. The 
 scale on the glass eye-piece micrometer, used with these 
 stage-micrometers, should be, if specially made for the 
 purpose, four-tenths of an inch long, divided into one- 
 hundred parts, each one two-hundred-and-fiftieth of an 
 inch; but these divisions would so closely approximate 
 those of the metric eye-piece micrometer proposed, 
 that it might be used without inconvenience instead." 
 
MICROSCOPICAL PRAXIS. 67 
 
 To Ascertain the Comparative Enlargement of 
 a Drawing. 
 
 It is sometimes desirable to learn how much the 
 drawing exceeds the size of the object, especially when 
 drawings are to be made to a certain scale. To do this, 
 divide the length of the drawing by the actual length of 
 the object as ascertained by the micrometer, and the 
 quotient will be the enlargement of the sketch. Thus, 
 if the drawing is five-tenths of an inch long, and the ob- 
 ject one one-hundredth of an inch, the sketch is en- 
 larged fifty diameters. 
 
 How to Care for the Instrument. 
 
 When the microscope is received from the dealer it 
 will be in a case, with a lock and key which are often 
 mentioned in the catalogues as if they were rare and 
 unfamiliar things. After the box has been lifted to the 
 table by the brass handle at the top, the door is opened 
 and the owner glances within, his heart beating a little, 
 faster, and agreeable anticipations bringing a pleasing 
 expression to his face. The instrument will probably 
 have the front toward the back of the case, therefore 
 turned away from the microscopist. At first acquaint- 
 ance it will turn its back on him in more senses than 
 one. 
 
68 MICROSCOPICAL PRAXIS. 
 
 I have seen men clutch the upright stand by the body 
 and the milled heads of the coarse adjustment, drag out 
 the unresisting thing and set it down on the table with 
 a bang. Such men are not fit to possess a microscope. 
 The instrument may be strong and well made, but as 
 some one has said, it is never necessary to brutalize it. 
 If it be supplied with a base-board as it should be, 
 gently slide it out of the case by pulling board and all 
 toward you, and as gently place it on the table. If it 
 be not attached in any way to the case, carefully lift it 
 out by means of the arm. If you treat the instrument 
 kindly it will repay you a thousand fold. If you at- 
 tempt to coerce it, a rebellion will be speedy and your 
 downfall sure. 
 
 Sometimes one eye-piece will be found in the body 
 tube, sometimes in a side box or drawer, according to 
 the size and style of the case and the stand. In any 
 event the eye-piece is to be gently dropped into the top 
 of the body-tube as the stand rests vertically on the 
 table. The microscopist seats himself on a chair and 
 in any position that he may find comfortable. Every 
 observer will form habits of his own in reference to his 
 position before the instrument, and will have his own 
 ideas as to the proper size and style of his work-table, 
 and perhaps even to the number of legs that the table 
 should have. Some writers have advised that there 
 shall be three legs to the microscopical table so that it 
 may be steady on an uneven surface. There is no ob- 
 jection to three legs, if the microscopist wants them. 
 He may also sit on a three-legged stool, if he should de- 
 sire to do so. But since the floors of modern houses 
 are seldom uneven enough to disturb the equanimity of 
 a quadrupedal table, that seems to be the preferable 
 form, the great desiderata being firmness and solidity. 
 
MICROSCOPICAL PRAXIS. 69 
 
 I remember that the ladies in my family were once at- 
 tacked by the aesthetic notion that if I could be induced 
 to put the microscope on what they called a "Tea-poy" 
 table, and under a glass shade, it would look well. The 
 table had four filamentous legs, and a shelf half way 
 between the floor and the top, the whole being a silly 
 invention of some frivolous mind. The thing trembled 
 at a touch, the shelf scraped my shins, the microscope 
 danced, the lamp wabbled, and the deluded victim ex- 
 pressed his opinion. ^Esthetics are well enough, but 
 they should be looked for in the object under the lens 
 rather than in the table. I now use a strong, substan- 
 tial, four-legged pine table that cost less than three dol- 
 lars, and I would not change it for a Louis XIV. or for 
 a Chippendale. All that is needed is that it shall be 
 solid and firm, with an abundance of top space, and a 
 drawer or two to hold the many "traps" and "dodges" 
 that soon accumulate. 
 
 Seated before the stand, incline it at a convenient an- 
 gle, the stage, the mirror, that is, the front of the mi- 
 croscope, of course being turned from you. When in- 
 clining the instrument, do so if possible by means of the 
 arm; at any rate do nothing to bring a strain on the 
 coarse or on the fine adjustment. All the least costly 
 stands will remain in an inclined position, held there by 
 the friction within the joint at the top of the pillar; in 
 first-class instruments the trunnions carry tightening or 
 binding screws, so that the wear that sooner or later be- 
 comes noticeable in the former can be taken up in the 
 latter. 
 
 With the eye-piece in place, and the body inclined, 
 attach the objective. To do this, rack up the body un- 
 til there is no danger that the front of the lens will 
 come in contact with the stage. Unscrew the top of 
 
70 MICROSCOPICAL PRAXIS. 
 
 the brass box containing the objective and tip the latter 
 out into the palm of the left-hand, supporting it with 
 the ringers. Take it up with the right-hand, and turn- 
 ing the screw end upward, screw it to the lower end of 
 the body-tube. It is unnecessary to caution the reader 
 in regard to crossing the threads of the screws. If that 
 be done and the objective wedged into the nose-piece, 
 the owner of that stand may have a sad experience. 
 Mr. Wm. Wales relates an instance of this kind where 
 the objective could not be removed by hand, so the 
 wise owner used a heavy pair of gas-fitter's pliers, and 
 succeeded in damaging the instrument to the amount of 
 forty-five dollars, pulling out the entire fine-adjustment, 
 which in this case was on the lower end of the body. 
 It is often useful to rotate the objective backward for a 
 short distance, until the threads are felt to slip into 
 place, when the lens may then be screwed home by gen- 
 tle forward turns. If it does not move easily and 
 smoothly, something is wrong, and no force should be 
 applied, but the objective must be removed and the 
 difficulty discovered and corrected. 
 
 If the microscope is to be used by day-light a posi- 
 tion near a north window is the best, as the light from 
 the northern sky is the most uniform. A white cloud 
 illuminated by the sun is the most desirable light by 
 day, but it can seldom be obtained. Most microsco- 
 pists have some favorite position before the window, 
 many preferring the stand so arranged that the mirror 
 shall face the window and the sky; others place it so 
 that the window shall be at the side of the instrument. 
 This is entirely a personal preference, nothing, so far as 
 I know, being gained or lost in either position. If, 
 however, daylight is used with a sub-stage condenser, 
 the plane mirror should be turned toward the sky. If 
 
MICROSCOPICAL PRAXIS. 7 I 
 
 no sub-stage condenser is employed, then the concave 
 surface may be directed upward. 
 
 The use of lamp-light has already been referred to, 
 and the employment of some special form of microscope- 
 lamp recommended. Its position in relation to the mir- 
 ror may, if desired, be ascertained by the formulae given 
 in a preceding chapter, but I think that most microscop- 
 ists, for ordinary work, do not enter into the niceties 
 of such an arrangement, reserving these refinements for 
 some very special and delicate investigations. ' The 
 reader will find however, that any attention paid to 
 these points, even for every-day observations, will re- 
 sult in good. Usually the lamp is placed somewhat in 
 advance of the microscope, and always on the left-hand 
 side. This gives plenty of room on the table, so that 
 the lamp is in no danger of being overturned. 
 
 Do not handle the mirror too daintily; if well mounted 
 there is little danger of injuring it, and a firm grasp 
 makes it more easily manageable. At first there will 
 probably be some difficulty in illuminating the field as 
 it should be illuminated, but a little practice will ac- 
 complish it. The entire circular space called the field 
 should be evenly lighted; there should be no shadows 
 nor faintness in the glow near the edge. Some writers 
 recommend that a piece of tissue paper should be 
 placed over the stage-opening and .the mirror manip- 
 ulated until the light is thrown exactly in its centre; it 
 is then removed and the light of course passes 
 through the objective and up the body-tube. This is 
 a good plan if the reader has trouble in seeing where 
 the reflection is thrown, but usually the light may be 
 observed on the front of the objective. The field can 
 scarcely be illuminated while the eye is at the eye-piece; 
 the illumination may then be completed, but not begun- 
 
72 MICROSCOPICAL PRAXIS. 
 
 The only plan is to use the paper on the stage, or to 
 observe when the front lens receives the light. Then 
 apply the eye and gently manipulate the mirror, trying 
 to improve matters; but even now the best can not be 
 obtained. This must wait until the slide is on the 
 stage, when the body is carefully racked down and the 
 focus obtained as previously directed. It is possible 
 that the field may then be evenly illuminated, but too 
 faintly to show the object properly, or oblique shadows 
 may be thrown across it, or only one little space at the 
 side may be bright while all the rest is semi-obscure. 
 This must be remedied by gently moving the mirror 
 while eye is at the ocular. When all the circular region 
 is lighted as well as seems possible, remove the eye- 
 piece and centre the illuminating beam by the method 
 suggested by Mr. Edward Pennock and described on 
 a preceding page. Then with the eye-piece again in 
 position, the field should appear brightly lighted in 
 every part. It will probably be too bright, and the ob- 
 server must either rack down the condenser, if he uses 
 one, until the desirable softness of illumination is ob- 
 tained, or the same result is to be worked for by rota- 
 ting the diaphragm. 
 
 With very low-power objectives where the actual 
 field is comparatively large, the apparent field cannot 
 be evenly lighted by either mirror. With all low 
 powers, and without a sub-stage condenser to modify 
 the intensity, it is better to use the plane mirror. This 
 will sufficiently illuminate the field of all below the one 
 inch, until we reach those very low powers now used, 
 such as Zeiss's variable A* which may be used as a 
 three or a five inch, with all intermediate amplifications, 
 by simply turning an adjustment collar. The field of 
 these lenses cannot be lighted throughout its whole ex- 
 
MICROSCOPICAL PRAXIS. 73 
 
 tent by either mirror. The concave gives only a cen- 
 tral bright spot while the plane surface does little better. 
 To light this large region fully, the plane mirror should 
 be covered with a disk of white card-board, and parallel 
 rays thrown on it by the bull's-eye condensing lens. 
 The mirror and the card are then manipulated as the 
 mirror alone should be. Or a more successful result 
 may perhaps be attained by taking the light with the 
 mirror from the brightly illuminated inner surface of a 
 white lamp-shade. With the two-inch or the one-inch 
 the plane mirror alone is sufficient. All higher powers 
 demand the use of the concave surface, unless the bull's- 
 eye lens is placed between the lamp and the mirror, 
 with a sub-stage condenser between the mirror and the 
 object; or unless some form of microscopical lamp is 
 used with a permanent bull's-eye lens, in which cases 
 the plane surface should be turned toward the con- 
 denser. 
 
 It sometimes happens that if the microscope has 
 travelled for some distance by railroad, the owner will 
 have trouble to get rid of a broad, crescentic shadow in 
 an otherwise well-lighted field. No manipulation of 
 the diaphragm or other sub-stage appliance serves to 
 remove the annoyance; it remains partly eclipsing the 
 field by its region of darkness, and the beginner 
 may think that something serious has happened., 
 The trouble is caused by the jarring of the dia- 
 phragm in the body or in the draw-tube, the journey 
 having shaken it too far downward. Remove the body, 
 first of course taking off the objective and the eye-piece, 
 and push the diaphragm up a little way, repeating the 
 operation if not at first successful. 
 
 It is well for the observer to keep both eyes open 
 when using the microscope, and if he begin with this 
 
74 MICROSCOPICAL PRAXIS. 
 
 plan he will be doing well, for it is somewhat of a 
 trouble at the start to see anything in the microscope 
 with both eyes open, as the unemployed organ seems 
 to dominate. At first the images of the object on the 
 table and of the object under the microscope will 
 mingle, but as combination is impossible, the result will 
 be amusing and annoying. At one moment the magni- 
 fied image will have the mastery, at the next the micro- 
 scopic objects will dominate the view, and at times the 
 eye will fail to take cognizance of anything. But with 
 a little practice the result is entirely satisfactory, and 
 the brain will finally take notice of the magnified image 
 only. 
 
 It has been suggested that instead of allowing the 
 unoccupied eye to roam about aimlessly as it does, and 
 as may be noticed when another person is at the micro- 
 scope with both eyes open, it would be better to give 
 it a dark surface to gaze at, or as some recommend, a 
 white surface. Consequently many forms of eye-shades 
 have been devised. They are all applied to or near the 
 eye-piece, a projecting arm carrying a disk for the pro- 
 tection of the unoccupied eye. Dr. L. B. Hall, of 
 Philadelphia, has described a device for this purpose 
 which may be made by the microscopist himself. 
 
 It consists of a small, opaque disk supported by a 
 wire extending from its outer edge downward to a point 
 on the body-tube low enough to be out of the way of 
 the nose, then bent upward parallel to the tube, but 
 not touching it, and attached to a ring near the top. 
 Dr. Hall's- own eye-shade was made of No. 18 brass 
 wire about twenty inches long, with a loop about one 
 and three-quarter inches in diameter made at one 
 end and covered with black paper to form the disk. 
 The other end of the wire was made into a ring to fit the 
 
MICROSCOPICAL PRAXIS. 75 
 
 body-tube, and the intermediate portion bent as de- 
 scribed. The ring about the body-tube may be 
 covered with chamois-skin if desired, to protect the lac- 
 quer. 
 
 To return the microscope to the case demands move- 
 ments the reverse of those used to take it out. The 
 body is racked up, the objective unscrewed (taking 
 care not to drop it or to strike it against any hard sub- 
 stance), then gently replaced in its brass box and the 
 lid screwed on. The slide is taken from the stage, and 
 if permanently mounted, is returned to its proper cabi- 
 net; the mirror is turned at right-angles to the optic 
 axis of the instrument; the eye-piece is removed and 
 deposited in the receptacle prepared for it; the body is 
 racked down, and placed in a vertical position, and the 
 stand is lifted into the case, where it remains with its 
 front looking at the back wall of the box. The door is 
 then closed and locked. If there are children around 
 that cannot be controlled, the key would better be car- 
 ried in the owner's pocket. Two minutes' brutal usage 
 will do more injury to a good stand than months of 
 proper treatment. No one but the microscopist should 
 ever touch the objectives, and he should carefully avoid 
 the contact of his fingers with the lens. Optical glass 
 is soft and easily injured. The owner would do well to 
 act accordingly, for a scratched or broken objective is 
 a ruined one. 
 
 Dust is an insidious and dangerous enemy to the mi- 
 croscope. It gets into the bearings and the movable 
 parts, and harms them. The microscopist should 
 therefore often wipe the stand with a soft old handker- 
 chief or a fine chamois-skin. The latter is the better, 
 since the fibres from the handkerchief often become a 
 source of -trouble. No liquid should be used; alcohol 
 
76 MICROSCOPICAL PRAXIS. 
 
 should especially be avoided as it will remove the 
 lacquer. The rack and the pinion-bearings may be oc- 
 casionally and sparingly touched with the porpoise-oil 
 used by jewellers, or with very superior sewing- 
 machine-oil. Exceeding small quantities must be used, 
 and the parts at once wiped almost dry, otherwise the 
 oily surfaces will collect the dust, and the last state of 
 that microscope will be worse than the first. The 
 dealers use soft and thick grease for lubricating pur- 
 poses, or a refined tallow. The parts of a well-made 
 stand need lubricating only at very long intervals. 
 They will work well if kept perfectly clean and free 
 from dust. 
 
 The microscopist will sometimes be annoyed by one 
 or more indefinite specks or spots apparently in the 
 field, but whether on the eye-piece or on the objective 
 he cannot decide. That they are not on the object or 
 on the cover-glass is determined by moving the slide; 
 if the specks remain stationary they are either on the 
 ocular or on the objective. To discriminate between 
 these, rotate the ocular; if the offending particles move 
 they are on it; if not, they are on the objective. If on 
 the former, the two lenses may be unscrewed and care- 
 fully wiped by the Japanese filter-paper; if upon the 
 objective, the front lens may be guardedly touched with 
 the Japanese paper, or the back combination very 
 cautiously swept with a soft camel's-hair brush. This 
 must be carefully done, and the brush must be scrupu- 
 lously clean, otherwise the glass may be scratched. In all 
 these operations the breath is a useful and harmless 
 assistant. 
 
 There are often found on a slide three objects that 
 perplex the beginner. These are air-bubbles, oil-drops 
 and that quivering and apparent dancing of minute 
 
MICROSCOPICAL PRAXIS. 77 
 
 particles called the Brownian movement, or pedesis, or 
 sometimes the pedetic motion. Air-bubbles have been 
 described as wonderful things. I remember to have 
 once shown a slide of urinary deposit to a physician, 
 who immediately cried out that the patient must be in a 
 dreadful condition, for those big, round, black-bordered 
 things surely must be deadly. Like the majority of 
 persons unaccustomed to microscopical investigation, 
 he had gazed at the most prominent object in the field, 
 and not at what I wanted him to see, for he had looked 
 at two or three air-bubbles, that are of a truth rather 
 frightful to the uninitiated. In such cases an indicator 
 in the eye-piece is a useful contrivance. 
 
 Air-Bubbles and Oil-Bubbles. The Brownian 
 Movement. 
 
 Rather than take up space by an attempted descrip- 
 tion of air-bubbles, I recommend the reader, if he be 
 not already familiar with them, to follow the plan sug- 
 gested by Prof. S. H. Gage in his "Notes on Micro- 
 scopical Methods," and make bubbles for examination. 
 
 Air-bubbles and oil-drops both appear with a bright 
 central spot and a broad dark border. To obtain the 
 former, Prof. Gage suggests that a drop of mucilage 
 shall be placed on a glass slip and beaten with a broad 
 blade until it looks milky on account of the inclusion 
 
78 MICROSCOPICAL PRAXIS. 
 
 of air. Put on a cover glass but do not press it down. 
 With this under the objective, focus upward and down- 
 ward, noticing that in focussing up, the central bright 
 spot becomes very clear and the black ring very 
 sharply defined, until the whole is dimmed by being far 
 beyond the focus. As Prof. Gage also says, the air- 
 bubble is one of the most useful means for ascertaining 
 whether or not the illumination is strictly central; if it 
 is not, the bright spot will not be in the centre of the 
 bubble, as it will be if the light is strictly axial. And if 
 the mirror be swung to one side so as to make the illu- 
 mination oblique, the bright spot will appear on the 
 side of the bubble away from the mirror. This is an 
 important experiment to make whenever in doubt in 
 this connection, as precisely the opposite effect obtains 
 in the oil-drop, the bright spot, when oblique light is 
 used, then being on the same side with the mirror. Oil- 
 drops may be prepared for examination by beating 
 together a drop of mucilage and one of clove oil. An 
 air-bubble has been described as a cancer-cell, and as the 
 microbe of la grippe. 
 
 The reader may have more trouble in experimenting 
 with oblique light than with central. About all that I 
 can tell him is to swing the mirror to one side, usually 
 toward the right-hand as being more convenient, and 
 then to manipulate it until the light is properly re- 
 flected on the object, the degree of obliquity of course 
 varying with the position of mirror-bar, the mirror or 
 both. Frey in his work entitled "The Microscope and 
 Microscopical Technology," says that "Considerable 
 practice is requisite with oblique illumination. The 
 aperture of the stage must be freed from diaphragms, 
 or any other apparatus that may be under the stage, 
 and the various positions of the mirror are to be tried 
 
MICROSCOPICAL PRAXIS. 79 
 
 while the eye is looking into the microscope. Truly 
 diabolical illumination is thus sometimes obtained, 
 which, however, shows many fine details in an astonish- 
 ing manner." Oblique light, as previously remarked, 
 is chiefly used in the resolution of the fine lines on the 
 surfaces of diatoms, these little ridges not being too 
 minute to cast a shadow on the side opposite to that 
 from which the light is received. Oblique light is oc- 
 casionally used to produce delicate shadows when the 
 microscopist is studying other objects, yet the effect is 
 rarely employed. In such cases however, when the 
 mirror is swung far to one side, and the objective is 
 not properly corrected, the result is as Dr. Frey calls it, 
 truly diabolical. 
 
 The Brownian movement, or pedesis, is that continu- 
 ous quivering or dancing common to all minute parti- 
 ticles when suspended in water. It is not an evidence 
 of life, and should not be mistaken, as it is apt to be 
 mistaken by the beginner, for minute living creatures. 
 It may be seen to good advantage by rubbing up a 
 little gamboge or India ink in water, allowing the larger 
 parts to settle, and then examining a drop of the super- 
 natant liquid with a high-power objective. The field 
 will be full .of dancing and trembling particles, moving 
 irregularly, but as if endowed with life. Similar move- 
 ments are beautifully visible within certain desmids and 
 algae, especially if they are not in a healthy condition, 
 the minute black granules there hovering together, 
 swinging and quivering like a swarm of microscopic 
 bees. It is also noticeable within the little sacs near 
 the base of the spinal nerves of the common frog, and 
 in almost any place where finely divided matters are in 
 suspension in a thin liquid. The cause of the move- 
 ment is not known. It has been supposed to be pro- 
 
8o MICROSCOPICAL PRAXIS. 
 
 duced by currents of heat or of electricity, but the sub- 
 ject is still without a satisfactory explanation. How 
 long it lasts is also unknown. One writer claims to 
 have prepared a slide which he examined at the end of 
 seven years, and found the particles as active as at first. 
 Soap and water are said to produce an energetic pede- 
 sis, and it is claimed that our hands are cleansed as 
 effectually by the violent Brownian movements of the 
 soap;. as by its chemical action. In any event, do not 
 mistake this uncertain dancing, as seen under the mi- 
 croscope, for the quivering of minute bacteria or for 
 any other living plants or animals. 
 
 In addition to the three foregoing microscopical bug- 
 bears, which cease to terrify when well known, the 
 student should make himself familiar with the appear- 
 ances of starch-granules, and with cotton, woolen and 
 linen fibres, especially when colored, as woolen fibres 
 are likely to be if the work-room is carpeted, as it should 
 not be. I can scarcely imagine an object any more as- 
 tonishing on first acquaintance than a purple fibre from 
 the carpet or elsewhere. These, with cotten and linen, 
 are likely to be found in any preparation; even in 
 mounted slides they are common, having fallen into the 
 mounting medium or been entangled in the object it- 
 self. The student may mistake them for something 
 more important, unless he makes himself familiar with 
 them at the beginning of his studies. Cotton fibres 
 have been mistaken for casts from the uriniferous 
 tubules of the human kidney. 
 
 The difference in the appearance of convex and con- 
 cave bodies is also important and useful. Microscopi- 
 cal and ordinary vision differ so widely from each 
 other, that it is often impossible to decide whether a sur- 
 face is convex or concave, especially if the object be 
 
MICROSCOPICAL PRAXIS. 8l 
 
 uniformly covered with markings that may be bosses or 
 depressions; sometimes the same trouble is experienced 
 by the naked eye. Grooves belong in the same cate- 
 gory with depressions. When the objective is racked 
 upward, a convex surface will appear lighter; a concav- 
 ity will appear lighter when the objective is focussed 
 downward. 
 
 It is often difficult to find a small object and bring it 
 into the field. This is especially true with high powers, 
 the trouble increasing with the increase of magnifica- 
 tion, because the actual field of high-power objectives is 
 so small that the chances of escape for the small 'object 
 are much greater than are the microscopist's for cap- 
 turing it, especially if the mechanical stage be not used. 
 The only recourse is to remove the high-power object- 
 ive, substitute a lower power, find the object, place it in 
 the centre of the field, and then to re-attach the high- 
 power lens, when the object sought should be some- 
 where within the illuminated space. 
 
 Thin Glass. 
 
 Before thin glass was obtainable as easily and cheaply 
 as at present, microscopists used very thin pieces of 
 mica, and for use with exceedingly high-power object- 
 
82 MICROSCOPICAL PRAXIS. 
 
 ives whose working-distance is excessively short, it is to 
 a certain extent still used. The older microscopists 
 preserved their objects between two pieces of window- 
 glass. 
 
 The method of manufacturing this thin microscopi- 
 cal glass is a secret known only, I believe, to the 
 Messrs. Chance of Birmingham, England. Some 
 time ago the report passed the rounds of the journals to 
 the effect that the method of making it had been dis- 
 covered in Germany, and that it would soon be sup- 
 plied very cheaply, but nothing further has been heard 
 from it of late. All that is used in this country is im- 
 ported in sheets and cut by the dealers into circles or 
 squares of various sizes. A suggestive remark in this 
 connection is made by Dr. S. Czapski when writing of 
 the peculiar cover-glass needed for use with Zeiss's 
 latest apochromatic objective of 1.63 N. A. He says, 
 "The production of these cover-glasses in the usual way 
 by blowing in a furnace was forbidden by their sub- 
 stance." Further than this the method of manufacture 
 has not been explained. 
 
 The thin sheets of glass, so brittle that they break 
 almost at a word or a look, are .cut with a diamond. 
 The circles are made by placing on the glass a plate 
 pierced with holes somewhat larger than the disks de- 
 sired, and a diamond is then run around inside these 
 openings until the entire surface of the glass sheet is 
 covered with the scratched circles. To attempt to 
 break these out would be followed by disastrous conse- 
 quences, but if the glass be laid aside for a day or two, 
 the disks will fall out of their own accord. The squares 
 awre cut with a diamond and a ruler, after the thin sheet 
 has been attached by water to a flat surface, usually of 
 plate glass. After the cutting the squares are easily 
 
MICROSCOPICAL PRAXIS. 83 
 
 broken off by sliding the sheet to the edge of the sup- 
 port. 
 
 When the glass comes from the dealer it is never 
 clean enough to be used for microscopical purposes, 
 but to clean it sufficiently nothing more is usually 
 needed than a careful wiping between two surfaces of 
 soft, old muslin. I have never found it necessary to 
 use any of the chemical mixtures recommended by some 
 authors. 
 
 Several mechanical cleaning-devices have been de- 
 scribed, but nothing is better than two smooth, wooden 
 blocks, each with a surface tightly and smoothly cov- 
 ered with soft, thick chamois-skin. The cover-glass is 
 placed on one block while the other rubs the surface 
 until it is clean, when the whole is turned over and the 
 other block rubs the other side of the glass. With this 
 simple contrivance it is hardly possible to break the 
 thinnest cover. 
 
 After being cleaned the glass should be kept 
 clean, as well as free from scratches. It is ex- 
 posed to the latter if thrown loosely into a box, 
 where a particle of hard dust may do mischief. 
 
 Mr. C. E. Hanaman of Troy, N. Y., has described a 
 method of keeping the covers protected from accident 
 and from dirt. He places them in drawers or in boxes 
 filled with narrow strips of new white blotting-paper, 
 between which they are stood on edge. This method, 
 Mr. Hanaman says, not only preserves them from break- 
 age and enables him readily to pick them out when 
 wanted for use, but also assists him to select, for special 
 preparations, those of the most desirable thickness, as, 
 by holding the drawer or the box between the eye and 
 the light, it is easy to select the thickest or the thinnest. 
 
 The glass varies a good deal in thickness even in the 
 
84 MICROSCOPICAL PRAXIS. 
 
 same lot as supplied by the optician. Its thickness may 
 be most conveniently measured by some sort of microm- 
 eter-gauge, of which there are several in the market 
 for the use of machinists. Carl Zeiss makes one espec- 
 ially for the microscopists, but either of the three man- 
 ufactured in this country, especially that of Bausch and 
 Lomb, will answer equally well. It is not only conven- 
 ient but important to have covers of different thick- 
 nesses sorted out, so that almost any kind may be had 
 at a moment's notice, and the micrometer-gauge is 
 slightly more accurate in its results than the fine-ad- 
 justment screw, with which the thickness may also be 
 estimated. An experiment made while writing this 
 gives the thickness of a cover as measured by a gauge 
 to be one two-hundredth inch, while the fine-adjust- 
 ment screw gives 0.0045, or about -g^-g- inch. 
 
 To use the former, place the cover between the jaws 
 of the instrument, close them and read the thickness in 
 hundredths and thousandths of an inch. To use the 
 fine-adjustment screw for the purpose, if the milled 
 head be graduated and the value of the spaces known, 
 it is only necessary to count the number of divisions 
 employed in focussing from one surface of the glass to 
 the other. A few particles of dust will answer as ob- 
 jects on which to focus, or a minute drop of ink on 
 each side of the cover, and so close as to be together 
 in the field of a high-power objective, but not close 
 enough to overtop each other. In the experiment re- 
 ferred to, the number of the divisions on the milled 
 head used in focussing from the lower to the upper sur- 
 face 'of the cover, were four and one-half, and as each 
 division corresponds to a movement of the body-tube 
 of one one-thgusandth inch, the cover was four and one- 
 half thousandths, or ^4^ inch thick. 
 
MICROSCOPICAL PRAXIS. 85 
 
 The most desirable as well as the cheapest and best 
 micrometer-gauge now in the market is made by 
 Messrs. Bausch and Lomb of Rochester, N. Y. This 
 instrument possesses several commendable features 
 which may become of great importance to the working 
 microscopist. 
 
 If the reader have no graduated milled-head to his 
 fine-adjustment screw, and no micrometer-gauge in his 
 possession, he may still measure his thin glass, although 
 the process is not as convenient as with these appli- 
 ances. 
 
 Place the cover upright on the stage so that its edge 
 shall present toward the objective. A narrow slit in a 
 cork or even in a slide of soft wood will hold it in posi- 
 tion, and enough light will be reflected from the edge 
 of the glass to supply the demand. Focus the object- 
 ive on the glass, place the microscope in a horizontal 
 position, apply the camera lucida to the eye-piece, and 
 draw two lines to represent the borders of the magni- 
 fied edge of the thin cover. Remove the glass, for it 
 substituting a stage-micrometer, and draw one or more 
 of the micrometer spaces on the paper. As the value 
 of these spaces will, be known, a comparison between 
 them and the magnified width of the glass may be easily 
 made, the two having been enlarged to the same degree 
 by the same objective and eye-piece. If the width of 
 the glass as represented on the paper occupies one-fifth 
 of the micrometer-space as shown on the paper, the ac- 
 tual thickness of the glass will of course be one-fifth of 
 whatever may be the value of that special space on the 
 micrometer. If the distance for the glass should equal 
 the distance of the one one-hundredth inch space on 
 the micrometer, of course the thickness of the glass is 
 one one-hundredth of an inch. 
 
86 MICROSCOPICAL PRAXIS. 
 
 Dr. S. Czapski reports the following original method 
 for determining the thickness of the cover-glass over 
 mounted preparations, a measurement which it is often 
 important to know for high-power work. The proced- 
 ure presupposes the possession of some cover-glasses, 
 the thickness of which is known, and that the head of 
 the fine-adjustment screw is divided by radial lines. 
 
 The upper and lower surfaces of the cover are fo- 
 cussed with central illumination, and the amount of 
 turn given to the fine-adjustment screw noted for each 
 cover-glass, it being unimportant whether the exact 
 value of the screw is known or not; the reader therefore 
 having it in his power to make his own radial lines on a 
 disk of paper to be pasted on the milled head of the 
 fine-adjustment screw. If the surfaces of the cover- 
 glass do not present any obvious marks to focus on, 
 an artificial one, such as dust or scratches, must be sup- 
 plied. If the numbers thus obtained be compared with 
 the known real thickness of the covers, a reduction- 
 factor is obtained from their quotients, which is avail- 
 able for determining measurements of a similar kind, 
 that is to say, for measurements of other cover-glasses 
 with the same objective, ocular, diaphragm and tube- 
 length. The focussing differences are always to be 
 multiplied by this factor in order to obtain the true 
 thickness of the layer. 
 
 As an example: Objective DD Zeiss, diaphragms 
 mm. in diameter; tube length 155 mm.; and four cover- 
 glasses, the thickness of which, already ascertained, are 
 0.146, 0.168, 0.187, 0.22. The focussing differences 
 marked by the head of the fine-adjustment screw were 
 35, 40, 45, 52 divisions. Then the reduction-factors in 
 i/ioooyu are ^ _ 4>I7; y_8 = 4 . 2o; yy = 4-l6; % o 
 4.23; or on the average 4.19, say 4.20. If the thick- 
 
MICROSCOPICAL PRAXIS. 87 
 
 ness of these cover-glasses had not been known, but the 
 focussing difference had been obtained and multiplied 
 by 4.2, the results would have been 0.147, 0.168, 0.189, 
 0.218, instead of 0.146, 0.168, 0.187,0.22. Differences 
 of -f- o.ooi, o.o, -f- 0.002, 0.002; a result more than 
 sufficiently accurate for the purpose. 
 
 Another method of measuring the cover-glass of a 
 permanently mounted preparation by means of the ad- 
 justment-collar of the objective, has been suggested by 
 Professor C. K. Wead. This also necessitates the poss- 
 ession of a cover whose thickness is accurately known. 
 With the adjustment-collar placed at zero (the open 
 point), dust, finger-marks or other minute objects are 
 focussed on the lower surface of the cover-glass whose 
 thickness is known, and the collar is turned until simi- 
 lar objects are in focus on the upper surface. This 
 should be repeated several times and a record kept of 
 the number of spaces through which the collar has been 
 turned; take the average number of spaces and divide it 
 into the known thickness of the cover. The quotient 
 will be the number which should be used as a multi- 
 plier of the spaces on the collar when it is used in the 
 foregoing way to ascertain the thickness of a cover- 
 glass on a mounted slide. 
 
 An example will make this plain. Suppose the 
 known thickness to be 0.0058 inch and several readings 
 of the collar to be 3. 6, 3. 75, &c., the average of all the 
 experiments being 3.56. Dividing 3.56 into 0.0058 (the 
 known thickness of the cover used to obtain the value of 
 the spaces on this special objective), the quotient is 
 0.001629, which is hereafter to be used as the multiplier 
 of the spaces utilized over some cover of unknown 
 thickness. If we assume, for the sake of the example, 
 that the number of spaces is 7.50, multiplying this by 
 
88 MICROSCOPICAL PRAXIS. 
 
 the decimal fraction mentioned, we have 0.0124175, the 
 thickness sought for the cover of the mount. 
 
 This convenient method may be verified by the grad- 
 uated milled-head of the fine-adjustment screw. 
 
 It is exceedingly important to know the thickness of 
 the cover when using non-adjustable objectives. These 
 lenses are corrected by their makers for a certain thick- 
 ness, and as the adjustment cannot be changed, and 
 since the microscopist, until recently, has not known 
 for what cover-thickness the objectives were intended, 
 he was at the mercy of circumstances, and forced to ac- 
 cept and be content with whatever image he could get. 
 Now we are able to use covers of at least approximate 
 correctness with our non-adjustable objectives, thanks 
 to Prof. S. H. Gage, of Cornell University, who has in- 
 vestigated the matter. Prof. Gage submitted certain 
 questions in relation to the subject to the various opti- 
 cians of the world, and from his published results I 
 take the following, so that the reader, if he possess 
 non-adjustable objectives by any of these makers, may 
 use covers of the proper thickness to obtain the best 
 results, provided his microscope body-tube is of the 
 standard length. 
 
 f J. Grunow, New York. 
 fifa mm. } H. R. Spencer and Smith, Buffalo, N. Y. 
 
 ( Powell & Lealand, London. 
 1 6.-15 mm Ross & Co., London. 
 
 mm. Bausch & Lomb, Rochester, N. Y. 
 
 mm. Carl Zeiss, Jena. 
 T t.6_ mm. Zeiss for his Apochromatic oil immersions. 
 ^ j The Gundlach Optical Co., Rochester, N.Y. 
 
 ( R. & J. Beck, London. 
 11=1.7 mm. J. Zentmayer, Philadelphia. 
 ^ mm. Nachet et Fils, Paris. 
 
MICROSCOPICAL PRAXIS. 89 
 
 mm. Swift & Son, London. 
 
 mm. C. Reichert, Vienna. 
 The glass, which is said to be crown-glass, from 
 which the covers are made, is brittle, and until the 
 microscopist becomes somewhat of an expert he will 
 break them with amazing facility. The skill needed in 
 handling the fragile things is easily acquired, but 
 reasoning from the number of devices recommended 
 for the purpose, their inventors have despaired of ac- 
 quiring that skill themselves, and have judged others 
 by their own standard. Many and peculiar cover-glass 
 forceps are obtainable, all more or less useful, perhaps, 
 but I have never tried them, having always relied on 
 my fingers alone. The simplest device, and one readily 
 made by any novice, is the following, recommended by 
 Mr. J. C. Douglas, who says that he long wanted "a 
 simple appliance for picking covers out of the liquid in 
 which they may be soaking, selecting them from their 
 box,, placing them flat upon the object to be examined 
 or mounted, and picking them off the slide when neces- 
 sary after examining the object covered. Forceps and 
 needles have grave inconveniences. Chase's mounting 
 forceps simply drop the cover, and are inferior, both 
 in simplicity and utility, to the following plan: Cut a 
 piece of suitable size from a flat rubber ring; fix this, 
 by a large-headed pin cut short, on to the end of a ce- 
 dar stick, driving the head of the pin so as to form a 
 depression in the rubber; wet the rubber, and on press- 
 ing it against a cover-glass it will adhere to it, and the 
 glass may be manipulated as desired. To disconnect 
 the rubber from the glass, it is merely necessary to in- 
 cline the stick so as to detach the rubber at one edge, 
 when the adhesion ceases at once. The apparatus is 
 more durable if a little cementing material be used on 
 
90 MICROSCOPICAL PRAXIS. 
 
 the stick, as the pin sometimes draws through the rub- 
 ber." 
 
 Personally I prefer to get along without any other 
 help than a fine needle in a match handle, using the 
 needle to lift the cover so as to take it in the fingers, 
 and also as a means to lower it slowly over the object 
 after having placed one edge against the slide, support- 
 ing the opposite margin by the needle. In this way 
 the cover may be gradually depressed or as slowly 
 raised. 
 
 The Objective. 
 
 The objective is the combination of lenses applied to 
 the lower end of the body-tube, and so named because, 
 when in use, it is near the object to be examined. It 
 produces an enlarged image of the object, which is in 
 its turn enlarged by the eye-piece, the optical combin- 
 ation thus forming the compound microscope. 
 
 When the optician has, with infinite labor, accurately 
 ground the minute bits of glass which he is forced to 
 use in his high-power objectives, when he has care- 
 fully placed them in the brass mounting so that the 
 centre of each lens shall be precisely over the centre of 
 all the other lenses below it, and when he has corrected, 
 as well as he. may, the chromatic and the spherical 
 aberrations, he finds that the cover-glass which the mi- 
 croscopist places over his objects, is the cause of 
 further trouble. 
 
MICROSCOPICAL PRAXIS. 91 
 
 Correcting the Objective for the Cover-glass. 
 
 An objective that will produce a fine image of the 
 object without a cover-glass, that is, one corrected 
 for an uncovered object, will give an imperfect image 
 when used over covered specimens. This effect was 
 first noticed by Andrew Ross, one of the older British 
 opticians, who also discovered the means of correcting 
 it, this being done by separating or approximating the 
 front lens and the back systems of lenses which to- 
 gether form the objective. The possession of this means 
 for cover-glass. correction makes the difference between 
 an adjustable and a non-adjustable objective. To the 
 former the optician adds a rotating collar, by the move- 
 ment of which the microscopist alters the position of 
 the front lens, or of the back systems, and thereby ad- 
 justs the objective for the thickness of the cover. The 
 non-adjustable objectives are without the rotating col- 
 lar or other means for altering the fixed position of the 
 lenses, and these forms are therefore corrected by the 
 maker for a certain thickness of cover, and are 
 immovably adjusted at that point, so that 
 the use of a thinner or of a thicker glass interferes with 
 the corrections and produces an imperfect image. The 
 thickness for which certain opticians adjust these kinds 
 of objectives has already been given, as ascertained by 
 Prof. S. H. Gage, and to that list the reader is referred, 
 so that with his non-adjustable objectives he may use 
 the cover-glass of the thickness recommended by the 
 manufacturer. 
 
 Low-power objectives, such as the one-inch and 
 lower, never have means of correcting them for 
 the influence the cover-glass, and such high-powers 
 as the one-fourth and the one-fifth are often without it, 
 especially if the angular aperture is small. The best 
 
92 MICROSCOPICAL PRAXIS. 
 
 high-power objectives always have the 'correction adjust- 
 ment, the collar in the cheaper forms moving the front 
 lens, whilst in first-class objectives it acts on the back 
 systems, the front lens being stationary. The effect is 
 the same in either case, but the last-mentioned method 
 is the better, since it obviates all danger of bringing 
 the front lens in contact with the cover, to the proba- 
 ble detriment of one or of both, and, which is as im- 
 portant, prevents the possibility of throwing the lenses 
 out of centre as may sometimes through rarely 
 happen when the adjustment is made by the front lens. 
 
 In most adjustable objectives the collar is so gradu- 
 ated that when the adjustment has been obtained for a 
 certain cover-thickness it may be recorded, and re- 
 peated in the future without loss of time. When the col- 
 lar is at zero the lenses are as wide apart as possible, and 
 the adjustment is said to be open, or at the open point, 
 and it is then corrected for an uncovered object. When 
 turned as far as possible in the opposite direction the 
 lenses are brought nearer together and are then said to 
 be closed, and the objective is corrected for the thick- 
 est glass over which it can be used. Some cheap Brit- 
 ish objectives are adjustable by the sliding back and 
 forth of an outer tube carrying the front lens, two 
 marks on the brass mounting being labelled "covered" 
 and "uncovered." But this method is very inelegant, 
 uncertain and inaccurate. 
 
 The influence of the cover is scarcely noticeable even 
 with moderately high powers of small or of medium an- 
 gles of aperture. With wide angles it becomes a dis- 
 turbing element of importance, at least to the accom- 
 plished microscopist. By the novice the effect would 
 probably be overlooked even when using the best of 
 wide-angled objectives, yet like so many other points 
 
MICROSCOPICAL PRAXIS. 93 
 
 in use of the microscope, as one's knowledge increases 
 and as the eye becomes educated and drilled in the per- 
 formance of optical feats new to it at first, the observer 
 will begin to perceive deficiences in the image, espec- 
 ially if he read of the fine action of similar objectives 
 in the hands of others, or sees that action through the 
 microscope of his expert friends. 
 
 The microscopist must teach himself by experiment 
 and by study how to adjust his objective. He will 
 often go wrong if he be working alone, for the proper 
 results can be recognized only after much experience. 
 How best to make the adjustment must be worked out 
 by the solitary student slowly and painfully; it can be 
 taught by personal intercourse and demonstration; it 
 cannot be taught on paper. The last mentioned 
 method has been tried, and the failure was a dismal 
 one. Yet there are certain suggestions which are val- 
 uable and helpful, and may be used as crutches until 
 the novice is able to walk alone. If all opticians would 
 graduate the adjustment-collar, as some now do, so that 
 the proper thickness of cover-glass should be indicated 
 by the figures on the brass mounting, the difficulty 
 would be lessened, as we could then measure our thin 
 glass and adjust accordingly. 
 
 The image should be clear, distinct and brilliant. It 
 should not be misty, semi-obscure or dull. The mar- 
 gins, as some one has said, should be as sharply defined 
 as are the finest lines in the best steel-engravings, and 
 the narrow boundary lines should be the narrowest pos- 
 sible and perfectly black. These appearances depend 
 greatly on a quality of the objective called its power of 
 definition, and for this the optician is responsible. But 
 in reference to these delicately defined outlines a writer 
 has said, that when a good objective is correctly ad- 
 
94 MICROSCOPICAL PRAXIS. 
 
 justed, they should be about -n^Vo <r of an mc ^ m width. 
 
 Every movement of the adjustment-collar necessi- 
 tates a change in the focus, so that while adjusting the 
 objective the microscopist keeps one hand at the fine- 
 adjustment screw, and the other on the milled ring of 
 the collar, following the movement of the latter by a 
 counteracting movement of the fine adjustment, the 
 collar, at the beginning of the work, usually being 
 placed at zero, the open point of the lenses. The ob- 
 jective is then focussed, and the collar turned toward 
 the closed point until the best definition is obtained, 
 the microscopist altering the position of the adjustment 
 toward open and closed even after he has what seems 
 to be a good result, so that he may improve it if possi- 
 ble. 
 
 There are two suggestions in this connection that 
 are exceedingly useful to the novice and to the ama- 
 teur. The one is made by Mr. W. H. Wenham, and is 
 that the microscopist shall select any dark speck or any 
 opaque portion of the object, and bring the outlines 
 into perfect focus; then place the finger on the fine-ad- 
 justment screw and move it briskly backwards and for- 
 wards in both directions from the first position. Ob- 
 serve the expansion of the dark outlines of the object, 
 both when within the focus, that is, when the object is 
 nearest to the objective, and when it is without the fo- 
 cus, or furthest from the objective. If the greater ex- 
 pansion, or "coma," is when the object is without the 
 focus, the lenses must be placed further asunder, or to- 
 ward the mark "uncovered," that is, toward the open 
 point or zero. If the greater expansion is when the ob- 
 ject is within the focus, the lenses must be brought 
 closer together, or toward closed point. When the ob- 
 jective is in proper adjustment, the expansion of the 
 
MICROSCOPICAL PRAXIS. 95 
 
 outline is exactly the same, both within and without the 
 focus. When the scales of the insect called the Podura, 
 and also when diatoms are used as the test objects, if 
 the dots or other markings on the surface have a tend- 
 ency to run into lines when the object is placed with- 
 out the focus, the lenses must be brought closer to- 
 gether; but on the contrary, if the lines appear when 
 the object is within the focus, the lenses should be 
 further separated. 
 
 This is an exceedingly difficult method in actual prac- 
 tice, demanding much experience and more skill than 
 is possessed by even accomplished amateurs. The nov- 
 ice will soon observe that for his purposes the sugges- 
 tion is worthless. Yet is a valuable method, and in the 
 hands of an expert microscopist, capable of satisfactory 
 results. Still, even for the expert, there is a better 
 method, that of adjusting for the best image, and it is 
 doubtful if the accomplished microscopist ever thinks 
 of using any other. 
 
 Another, and for the beginner, much simpler method 
 is that recommended, I think, by Dr. Lionel S. Beale. 
 Here, with the adjustment-collar at zero, the objective 
 is focussed on the object, and the collar turned until a 
 particle of dust, a striation, or any minute inperfection 
 on the upper surface of the cover is brought sharply 
 into focus, when the objective will be corrected for 
 that particular glass and preparation. The microsco- 
 pist then re-focusses with the fine-adjustment screw, 
 and proceeds with the investigation. This however, is 
 applicable only to objectives whose adjustment-collar 
 moves the front lens. 
 
 The Podura already referred to, and whose scales are 
 so useful to the microscopist for testing certain, quali- 
 ties of his objectives, is an agile little insect to be found 
 
96 MICROSCOPICAL PRAXIS. 
 
 under stones and sometimes in damp cellars, where it 
 jumps about with surprising alacrity. Its scales should 
 be mounted dry, but it is much the better plan to buy 
 a slide of them from the dealer, since the little creature 
 is difficult to capture, and is in addition, not very com- 
 mon. 
 
 The scales are minute and have given the opticians 
 and the microscopists a lively time in the discussion as 
 to the character of the markings which the microscope 
 reveals. These markings have the form of exclamation 
 points from which the dot or period at the base has been 
 removed. The discussion, which is even now unsettled, 
 is whether the exclamation points are spines projecting 
 from the surface of the scale and from which they may 
 be removed by electricity, as some contend, or whether 
 they are simply elevated folds or ridges of that surface. 
 But whatever may be their character, they form excel- 
 lent objects over which to study the adjustment of the 
 objective. When the objective is properly adjusted 
 and focussed, for the focussing is here of great impor- 
 tance, the exclamation marks should stand out, each 
 one distinct from its neighbor on each side, and the 
 lower part should not trail off into the head of the 
 mark below it, but should end clearly and sharply, with 
 a well-defined space of varying extent between its 
 termination and the rounded head or upper region 
 of the next exclamation point below it. 
 
 Messrs. R. and J. Beck, of London, issue a circular 
 descriptive of their "National Stand/' in which the 
 appearances of the Podura scale are shown under the 
 correct and incorrect adjustments of the objective, 
 within the proper focus and without it. These are here 
 reproduced in Figs. 4-9. In Fig. 4. the scale is shown 
 as it appears when the objective is correctly adjusted 
 
MICROSCOPICAL PRAXIS. 
 
 97 
 
 Fig. 4 . 
 
 Fix. 5- 
 
 Fig. 6. 
 
 Fig. 7 . 
 
 Fig. 8. 
 
 Fig. 9. 
 
 Podura Scale. 
 
 Fig. 4, as the scale should appear, the objective being correctly adjusted and 
 focussed. Fig. 5, the same as it appears on each side of the focus, when the 
 objective is correctly adjusted. Fig. 6, here the adjustment is correct, but the 
 focus is slightly wrong. Fig. 7, the objective is incorrectly adjusted, but properly 
 focussed. Fig. 8, the same beyond the focus. Fig. 9, the same within the focus. 
 
98 MICROSCOPICAL PRAXIS. 
 
 and focussed; in Fig. 5 as it appears on each side of the 
 proper focus; in Fig. 6 the adjustment is correct, but 
 the focus is slightly altered one way or the other, the 
 change being as little as possible. In Fig. 7 the objec- 
 tive is incorrectly adjusted, but properly focussed, while 
 Fig. 8 is the same as seen beyond the focus, and Fig. 9 
 the same when too near the objective, or within the fo- 
 cus. 
 
 The late Dr. Allen Y. Moore, writing about cover- 
 correction says: Every objective has a certain color 
 with which it shows best, and there is probably no ob- 
 ject better adapted to the purpose of determining this 
 color than the Podura scale. By examining the scale 
 with a first-class one-fourth or higher power of medium 
 or of wide aperture, it will be seen that the "exclama- 
 tion marks" are more or less colored. Pay no atten- 
 tion to this at first, but carefully turn the collar back 
 and forth until the marks appear sharpest and smallest. 
 That will be the point of best correction, and now the 
 color of the markings should be noticed. Having care- 
 fully determined the exact tint of best correction, throw 
 the objective a little out of proper adjustment by turn- 
 ing the collar toward open point, or zero. This over- 
 corrects it and at the same time a change in the color 
 is noticeable. The markings seem to expand, becom- 
 ing hazy. Now turn the collar towards closed until 
 the point of best correction is passed; here the same 
 thing is seen in regard to expansion and haziness, but a 
 different tint seems to make its appearance. By at- 
 tending very closely to this color (which is the secon- 
 dary spectrum), the proper correction can easily be 
 made. A certain one-fifteenth inch objective when 
 correctly adjusted, shows the markings of a brilliant 
 ruby-red (and most of the finest objectives which I 
 
MICROSCOPICAL PRAXIS. 99 
 
 have seen show best with this color); by turning the 
 collar toward zero they become greenish, while, if 
 turned toward closed, they become pink. Hence, at the 
 first trial of any such object, should it appear green the 
 collar must be turned toward closed until the ruby tint 
 appears, and if too pale a red or pink, the collar should 
 be turned toward zero. By a little practice the micros- 
 copist can tell at a glance which way to turn the collar. 
 
 When using this plan the reader must remember that 
 all objectives do not correct in the same color. What 
 the special tint may be, the microscopist must ascertain 
 by noting the color of the markings on the scales when 
 the objective is at the point of best correction; he may 
 thereafter correct the lens by means of its color (its sec- 
 ondary spectrum) for other thicknesses of the cover. 
 To do this however, demands an eye very sensitive to 
 color and to slight changes in tint. 
 
 Mr. Edward Bausch, in his little book on "The Ma- 
 nipulation of the Microscope," prefers the diatom called 
 Pleurosigma angiilatum as an object over which to make 
 cover-corre-ction. He recommends that the objective 
 be placed at the open point as usual, and focussed on 
 the diatom. If no lines are to be seen, turn the collar, 
 and focus above and below the plane of the diatom so 
 that this shall be indistinct, and look for the lines. 
 Possibly, after a little, they will begin to appear faintly; 
 if not, continue to turn the collar toward the closed 
 point. The lines must now soon begin to make their 
 appearance, and when they do so, they will probably 
 seem to be above the plane of the diatom. This shows 
 that the objective is approaching its correction for the 
 cover. Keep the lines in focus while the collar is being 
 gradually turned, until they and the outlines of the dia- 
 tom lie in one plane, when the objective will be cor- 
 
100 MICROSCOPICAL PRAXIS. 
 
 rected for that cover. Record the number, and since 
 at the beginning the adjustment was at the open point, 
 now close it, and again adjust by turning the collar the 
 other way. The graduations should be at the same de- 
 gree in both cases, or at least within two degrees of 
 each other. If there is a greater discrepancy, Mr. 
 Bausch states that it is due to a want of the faculty of 
 perception; the microscopist's eye not yet having 
 been sufficiently trained. The reader must remember 
 however, that the proper objective is to be used with 
 the Pleurosigma, or with any other test-object. With 
 this special diatom a good one-fourth or one-fifth inch 
 or higher-power lens will be needed. To attempt to 
 resolve the lines with a one-inch objective would be 
 absurd. 
 
 The striae are more difficult to see if the diatoms are 
 mounted in Canada balsam. When dry under the 
 cover, they may appear almost conspicuous, and their 
 markings be easily observed with the appropriate ob- 
 jective, while if the same specimens be mounted in bal- 
 sam, they seem to fade away, and must be carefully 
 sought for with a low-power objective before a high- 
 power can be used for the resolving of the lines, or for 
 experimenting with the adjustment-collar. Before I 
 had used the microscope much, and when I was grop- 
 ing about alone and without help except my own awk- 
 ward failures, I received as a present a slide of Pleuro- 
 sigma angulatum with a cracked cover-glass. The dia- 
 toms were exquisitely clean and beautiful, but I thought 
 to improve the appearance of the slide by applying a 
 new cover, which I did with Canada balsam, the mount 
 being originally a dry one. When I looked for those 
 diatoms with their sides curved into the line of beauty, 
 they had disappeared, and in my ignorance I threw 
 
MICROSCOPICAL PRAXIS. IOI 
 
 away the slide, thinking that the diatoms had fallen off 
 and been lost. I have often wished for it since, for I 
 now know that it was a valuable thing. 
 
 An objective may be adjusted for the cover by means 
 of the draw-tube. The effect of lengthening or of 
 shortening the tube is not so noticeable with low 
 powers as with high, yet the same result may be at- 
 tained with it as with the adjustment-collar, but to use 
 this method the body-tube must be divided. Shorten- 
 ing the draw-tube in this case has the same effect as 
 closing the lens-systems, the objective then being cor- 
 rected for thicker covers, while by lengthening the 
 tube it is corrected for thinner glass, or the systems are 
 in effect opened. Mr. Edward Pennock has called at- 
 tention to this fact, stating that objectives of high- 
 power (% inch and upward), may thus be used with a 
 different length of tube from that for which they are 
 corrected, by using a thicker or a thinner cover-glass. 
 For example, to use an objective on a long tube when 
 it has been corrected i^ fl snjQJ^" hp j ng^a thirmpr 
 cqy^r-gTass. But to use an objective on a sliort LUDe when 
 it has been corrected for a long tube, use a thicker 
 cover-glass. To use .with a thinner cover an objective 
 corrected for a certain thickness of cover-glass, 
 lengthen the tube of the microscope. To use with a 
 thicker cover an objective that has been corrected for a 
 certain thickness of cover-glass, shorten the tube of the 
 microscope. 
 
 All non-adjustable objectives are corrected by their 
 manufacturers for a certain definite length of body- 
 tube, and any change in that length results in an 
 injurious change in the adjustments and consequently 
 in the perfect action of the lens. What that 
 special length might be microscopists have had 
 
102 
 
 MICROSCOPICAL PRAXIS. 
 
 no means of knowing until recently, when Prof. S. H. 
 Gage of Cornell University, placed 
 them under renewed obligations by 
 ascertaining and publishing the vari- 
 ous lengths employed by the promi- 
 nent opticians of the world. To 
 learn these Prof. Gage sent the dia- 
 gram, Fig. 10, to the opticians, ask- 
 ing them to give him the length for 
 which they correct their objectives, 
 and to indicate the points between 
 which the measurements are made. 
 Some measure from the top of the 
 eye-piece, to the lower end of the 
 body, which has been suggested as 
 a standard, since it is readily ascer- 
 tained by anyone, and having the 
 length once determined, says Prof. 
 Gage, it would not need to be 
 changed when an objective of dif- 
 ferent length of setting was used. 
 Others measure from the top of the 
 eye-piece to the front of the front 
 lens of the objective. The follow- 
 ing is the table of lengths as given by Prof. Gage, 
 with references to the diagram so that the points and 
 the distances may be seen at a glance. 
 
 Fig. 10. Tube-length 
 
 used by various 
 
 opticians. 
 
MICROSCOPICAL PRAXIS. 103 
 
 Pts. included in 
 Tube-lengths. 
 
 See Diagram Tube-length in Millimetres. 
 
 Fig. 10. 
 
 f Grunow, New York 203 mm. 
 
 , J Nachet et Fils, Paris, 146 or 200 mm. 
 
 1 Powell and Lealand, London. . . 254 mm. 
 
 [_C. Reichert, Vienna 160 to r8o mm. 
 
 f Bausch & Lomb Op. Co. .Rochester, 21 6 mm. 
 Bezu, Hausser et Cie., Paris . . . 220 mm. 
 
 11 J Klonne und Miiller, Berlin .... 160-180 or 254 mm. 
 
 1 1 W. & H. Siebert, Wetzlar .... 190 mm. 
 
 | Swift & Son, London 228*4 mm. 
 
 [_C. Zeiss, Jena 160 or 250 mm. 
 
 a-g Gundlach Optical Co., Rochester, 254 mm. 
 
 c-d Ross & Co., London, 254 mm. 
 
 c-e R. &. J. Beck, London 254 mm. 
 
 c-g H. R. Spencer & Co., Geneva, N.Y*., 254 mm. 
 
 c-e E. Leitz, Wetzlar ) 125-180 mm. 
 
 Oil immersions ) 160 mm. 
 
 Some objectives have the collar so marked and ar- 
 ranged by the optician that by it alone^without examin- 
 ing the image, the adjustment for cover may be made, 
 provided the thickness of the cover-glass be known. 
 If. the cover is, for instance, 0.15 mm. thick, the collar 
 should be placed at 15; if the cover is 0.25 mm., set the 
 collar at 25. This .method has been adopted in this 
 country by Messrs. Bausch & Lomb, and by Carl 
 Reichert and others in Europe. 
 
104 MICROSCOPICAL PRAXIS. 
 
 Chromatic Aberration. 
 
 With the exception of Zeiss's apochromatic objectives 
 none is perfectly corrected for chromatic and for 
 spherical aberration. There is in all a remnant of color 
 and a remnant of spherical trouble which it is impossi- 
 ble to dispose of, except by using Prof. Abbe's apo- 
 chromatic devices. In what is called the chromatic or 
 the color-aberration of the objective, the different 
 colored rays are not all brought to the same focus, and 
 in his efforts to remedy these defects the optician does 
 either too much or too little. When the violet rays are 
 focussed before the red, the objectives is said to be 
 under-corrected;- when the red light is focussed before 
 the violet, the lens is over corrected. 
 
 Mr. Edward Bausch, the optician, in his little work 
 on "The Manipulation of the Microscope," says that 
 the amount of color depends somewhat upon the power 
 of the eye-piece, becoming more conspicuous as the 
 power is increased, and that the color outside of the 
 secondary spectrum is not always prejudicial. The 
 microscopist must depend upon the optician for the ex- 
 cellence of 'these corrections, and the optician will 
 serve him well, but it is always interesting, and often 
 useful, to be able to test the perfection of the maker's 
 results. So far as chromatic aberration is concerned 
 this may be done in several ways. 
 
 If an organic object is to be employed as a test, the 
 most useful are the Podura scale and a diatom. Mr. 
 Edward Pennock has published a convenient table of 
 color corrections which I quote as follows: 
 
MICROSCOPICAL PRAXIS. 
 
 Table of Color Corrections. 
 
 Within the focus. Without the focus. 
 
 Under correction, . . 
 
 Slightly under, but a^| 
 large number of the I 
 finest lenses have this f 
 color, J 
 
 Nearly colorless, shows 
 the secondary spec- 
 trum 
 
 Over correction, . . 
 
 Brick red 
 
 Claret 
 
 Lilac 
 
 Blue 
 
 Greenish blue. 
 
 Light green. 
 
 Paler green. 
 
 Yellow. 
 
 To make the appearances conspicuous the mirror 
 should be swung to one side and the colored fringes 
 (the complimentary colors of the secondary spectrum), 
 at the margins of the object be noticed, and the object, 
 should, as in Mr. Pennock's table, be examined both 
 within and without the focus, the first mentioned 
 method by central illumination being the best for the 
 learned optician whose eye has been acutely trained to 
 observe slight chromatic variations. When the object 
 is viewed under oblique light from the mirror swung 
 toward the right-hand side, the left-hand margin of the 
 object will be bordered with violet, and the right-hand 
 side by yellow if the objective is over corrected; when 
 under-corrected the left-hand margin will be yellow and 
 the opposite edges blue or violet, a low-power eye- 
 piece being used. 
 
 When an inorganic substance is used as the test, al- 
 most any small object orbit of dirt will answer the pur- 
 pose, and in every instance that object should be as 
 nearly as possible in the centre of the field. 
 
 Professors Naegeli and Schwendener however, 
 recommend as the simplest and most trustworthy 
 mode of testing the chromatic aberration, that one-half 
 
IO6 MICROSCOPICAL PRAXIS. 
 
 the surface of either the front or of the back lens of the 
 objective be covered with tin-foil or with black paper, 
 so that only one-half shall remain optically effective. If 
 a minute aperture in a blackened plate then be ex- 
 amined, it will appear colorless if the objective is per- 
 fectly achromatic, as no ordinary objective can be; or 
 the margins will be colored as described in the foregoing 
 experiments with oblique light. It is better, however, 
 not to meddle much with the glass surfaces of the ob- 
 jective. And this method has also some other objec- 
 tionable features. 
 
 With high-powers these methods are sufficient, but 
 with low-powers the opticians recommend the use of a 
 minute globule of mercury intensely illuminated by re- 
 flected light. This is the artificial star so often referred 
 to in microscopical literature. It is made by beating 
 with a broad blade a globule of mercury until it is as 
 fine as dust. These minute globules are then used as 
 reflecting surfaces, and are preferably mounted on a 
 slip of black glass, as recommended by Mr. W.'H. Wen- 
 ham. 
 
 If the objective is well corrected the colored fringes 
 will be pale green when the globule is without the fo- 
 cus, and pale violet when it is within. If the lens is 
 under-corrected, the green will become bluish or violet, 
 and when within the focus the color may be bright red. 
 If over-corrected, the colors are yellow and blue, when 
 without and within the focus respectively. 
 
MICROSCOPICAL PRAXIS. 107 
 
 Spherical Aberration. 
 
 Of the two troubles, chromatic and spherical aberra- 
 tion, the latter is the more important to be entirely 
 overcome; yet, except in the Zeiss apochromatics there 
 is, even in the best objectives, some of it left, to which 
 the same expressions of under and over-correction are 
 applied. When the central rays meet before the per- 
 ipheral ones, the objective is said to be over-corrected; 
 when the peripheral rays meet first, the lens is under- 
 corrected. 
 
 In the combination pocket-lens the spherical as well 
 as the chromatic aberration is partly corrected by a dia- 
 phragm which obstructs the passage of the peripheral 
 rays. For this reason the object appears more dis- 
 tinct, but the size of the field and the amount of light 
 are much reduced. In some objectives this plan is 
 adopted when the corrections are not made by the com- 
 ponent lenses themselves, with much the same effect as 
 in the case of the combination pocket-lens. 
 
 As a test for spherical aberration the optician makes 
 use of the artificial star. This is done by examining it 
 within and without the focus, and studying the coma, 
 or the expansions of the margins. If the expansion is 
 greater when the distance between the objective and 
 the star is increased, the objective is under-corrected; 
 if the expansion is greater when the reverse obtains, 
 the objective is over-corrected. 
 
 A more accessible test for the general reader is that 
 made by coating a slip of glass with a thick layer of 
 India ink, which will crack into minute fissures when 
 dry, or in which a fine needle-point may make delicate 
 marks for the transmission of light. In using this and 
 all other tests for the same purpose, with low-powers, the 
 diaphragm must be removed and the mirror be brought 
 
I08 MICROSCOPICAL PRAXIS. 
 
 nearer, so that the cone of rays shall fill the whole 
 aperture of the objective; and with high-powers a sub- 
 stage condenser should be used for the same purpose. 
 The objective is sharply focussed on the edges of the 
 fissures, and if these appear with a blue fog the aberra- 
 tion is not sufficiently corrected. They should be seen 
 without a halo, clearly and distinctly. If the foggy ap- 
 pearance increases within the focus, the objective is 
 over-corrected; when it increases without the focus the 
 lens is under-corrected. 
 
 For testing both forms of aberration, Prof. Abbe has 
 devised a test-plate consisting of a series of cover- 
 glasses ranging in thickness from 0.09 mm. to 0.24 mm., 
 their lower surfaces being coated with a silver film 
 through which are ruled groups of lines varying in dis- 
 tance from 3-J-g- to -j-g^g- inch. These covers are ce- 
 mented to the slip by Canada balsam. To examine an 
 objective of large aperture, the inventor says in his in- 
 structions accompanying the plate, the disks must be 
 focussed in succession, observing in each case the 
 quality of the image in the centre of the field, and the 
 variation produced by using alternately central and 
 very oblique illumination. When the objective is per- 
 fectly corrected for spherical aberration for the partic- 
 ular thickness of cover-glass under examination, the 
 outlines of the markings in the centre of the field will 
 be sharp by oblique illumination, and without any neb- 
 ulous doubling or indistinctness of the minute irregul- 
 arities of the edges. If, after exactly adjusting the ob- 
 jective for oblique light, central illumination is used, no 
 alternation of the focus should be necessary to show 
 the outlines with equal sharpness. 
 
 If an objective fulfills these conditions with any one 
 of the disks it is free from spherical aberration when 
 
MICROSCOPICAL PRAXIS. IOQ 
 
 used with cover-glasses of that thickness. On the 
 other hand, if every disk shows nebulous doubling or 
 an indistinct appearance of the edges of the lines with 
 oblique illumination, or if the objective requires a dif- 
 ferent focal adjustment to get equal sharpness with 
 central as with oblique light, then the spherical correc- 
 tion of the objective is more or less imperfect. 
 
 Nebulous doubling with oblique illumination indi- 
 dicates over-correction of the marginal zone, indistinct- 
 ness of the edges without marked nebulosity indicates 
 under-correction. 
 
 The test for chromatic aberration is based on the 
 character of the color-bands which are visible with 
 oblique illumination. With good correction the edges 
 of the lines in the centre of the field should show only 
 narrow color-bands in the complimentary colors of the 
 secondary spectrum, namely on one side yellow-green 
 to apple-green, and on the other violet to rose. The 
 more perfect the correction of the spherical aberration 
 the clearer this color-band appears. 
 
 For the examination of objectives of smaller aper- 
 ture (less than 40 or 50), we may obtain all the necessary 
 data for the estimation of the spherical and the chro- 
 matic corrections by placing the concave mirror so far 
 laterally that its edge is nearly in the line of the optic 
 axis, the incident cone of rays then filling only one- 
 half of the aperture of the objective, by which means 
 the sharpness of the outlines and the character of the 
 color-bands can be easily estimated. Differences in the 
 thickness of the cover-glass within the ordinary limits 
 are scarcely noticeable with such objectives. 
 
 It is of fundamental importance in employing this test- 
 plate to have brilliant illumination and to use an eye- 
 piece of high power. With oblique illumination the light 
 
110 MICROSCOPICAL PRAXIS. 
 
 must always be thrown perpendicularly to the direc- 
 tion of the lines. 
 
 When, with practice, the eye has learned to recog- 
 nize the finer differences in the quality in the outlines 
 of the images, this method of investigation gives very 
 trustworthy results. Differences in the thickness of 
 cover-glasses of o.oi to 0.02 mm. can be recognized 
 with objectives of two or three mm. focus. 
 
 Objectives are commonly more nearly free from 
 spherical aberration in the centre than at the peri- 
 phery. It is for this reason that when a minute object 
 is to be examined wit]? a high-power, the advice always 
 given is to bring it to the centre of the field. This 
 must be specially remembered when measuring objects 
 that extend too near the sides of the field, since the 
 spherical aberration of the lateral portions of the lens 
 has the effect of increasing or of dimnishing the am- 
 plification, and the measurement will not be correct if 
 the aberration is at all marked. This result of spheri- 
 cal defect is usually very noticeable in what is called 
 flatness of field. 
 
 Flatness of Field. 
 
 Occasionally the field, instead of appearing as a flat 
 or level plane, seems to be like the inside or the outside 
 of a shallow cup, and although this is one result of 
 
MICROSCOPICAL PRAXIS. Ill 
 
 spherical aberration, the latter alone cannot be justly 
 blamed. In some objectives the curvature is so great 
 that the focus must be changed to a very perceptible 
 extent before marginal objects can be clearly seen, after 
 which the central portions become indistinct. This 
 curvature should be as slight as possible, although, 
 since it cannot be entirely eliminated, except by the 
 use of apochromatic glass and of special eye-pieces it 
 must be endured. 
 
 It may, as stated by Mr. Edward Bausch be due to a 
 defective eye-piece. This may be determined by observ- 
 ing whether it shows equally with different objectives, 
 and by using different eye-pieces, but of the same fo- 
 cal length, with the same objectives. And Mr. Bausch 
 gives good advice when he says that in two objectives, 
 if the predominant feature of one be resolving power, 
 and of the other flatness of field, select the former. 
 
 A simple method of testing the flatness of the field 
 is to strew starch-grains over a slide, and to observe 
 whether the centre and the periphery of the field of 
 view are in exactly the same focus at the sime time. 
 If the centre is focussed and the margin appears hazy 
 and indistinct, as it probably will appear even with the 
 best objectives, the field is not perfectly flat. Similarly 
 minute substances, as minute bacteria, strewn over the 
 lower surface of the cover-glass will tell the same story 
 with high-power objectives. Perfect flatness of field, 
 even with the best of lenses, does not exist. 
 
 Professors Naegeli and Schwendener give the follow- 
 ing method for testing this feature in objectives. A 
 cover-glass with a straight edge is placed upon the dia- 
 phragm of the eye-piece (the eye lens having been re- 
 moved), so that the edge shall appear in the circular 
 opening of the diaphragm as a chord of arc. The real 
 
112 . MICROSCOPICAL PRAXIS. 
 
 image of another straight line, which is viewed as an 
 object, is made to coincide with this chord precisely as 
 the adjustment of a particular division-line to the mar- 
 gin of an object is effected in micrometric measure- 
 ments. If a complete coincidence takes place 
 whether or not they appear straight or curved in the 
 virtual image then the real image is free from distor- 
 tion; in all other cases it is distorted. 
 
 Angular Aperture. 
 
 The older opticians defined angular aperture to be 
 the angle at the apex of a tri-angle whose base is repre- 
 sented by the width of the front lens of the objective, 
 its two sides being the most oblique rays of. light which 
 can pass through that objective from a point in the 
 object when focussed, that point being at the apex of 
 the triangle. But as the result of the investigations 
 made by Professor Abbe of Jena, objectives are now 
 rated according to what he has named the Numerical 
 Aperture (N. A.). This is one-half of the sine of the 
 angular aperture multiplied by the refractive index of 
 the medium in which the objective is used, which may 
 be air, as it must be for a dry objective, water, glycer- 
 ine or some homogeneous-immersion fluid. 
 
MICROSCOPICAL PRAXIS. 113 
 
 The refractive index of air is taken to be i.ooo; of 
 water it is 1.333; f glycerine, 1.475; f ^ f cedar, 
 1.510; of Canada balsam, 1.540; of styrax, 1.582; of 
 crown glass, 1.51 to 1.53. 
 
 The angular or the numerical aperture may be ob- 
 tained by the simple methods described in another 
 place, or the N. A. may be ascertained from the follow- 
 ing table when the' angular aperture has been learned 
 from the optician. The list is taken from "The Journal 
 of the Royal Microscopical Society," for which it was 
 prepared by Mr. J. W. Stephenson, to whom microsco- 
 pists owe the use of homogeneous-immersion objec- 
 tives, as he was the first to suggest a practicable means 
 of utilizing the optical principle involved in their con- 
 struction and employment. 
 
 A glance at the table will show how to use it to 
 ascertain the N. A. If a dry objective have an angular 
 aperture of 125 (in the column headed "Air"), it should 
 correspond in resolving power and in some other good 
 qualities, with a w T ater-immersion having an angular 
 aperture of 84, and with a homogeneous immersion of 
 71, while its N. A. will #"0.89. 
 
MICROSCOPICAL PRAXIS. 
 
 APERTURE TABLE. 
 
 Numerical i . . 
 Aperture. 
 
 Water. 
 
 Homogeneous 
 Immersion, 
 
 1 -5 2 
 
 
 
 180 o' 
 
 1.51 
 
 . . 
 
 . . 
 
 166 51' 
 
 1.50 
 
 . . 
 
 , . 
 
 161 23' 
 
 i.49 
 
 
 
 157 12' 
 
 1.48 
 
 
 . 
 
 153 39' 
 
 1-47 
 
 
 . 
 
 150 3 2 ' 
 
 1.46 
 
 
 . . 
 
 147 42' 
 
 1-45 
 
 . 
 
 . . 
 
 145 6' 
 
 i.44 
 
 . . 
 
 . . 
 
 142 39' 
 
 i-43 
 
 
 . . 
 
 140 22' 
 
 1.42 
 
 
 . 
 
 I 3 8 12' 
 
 1.41 
 
 
 . . 
 
 136 8' 
 
 1.40 
 
 
 
 134 10' 
 
 1-39 
 
 
 . . 
 
 132 16' 
 
 1.38 
 
 . . 
 
 
 130 26' 
 
 1-37 
 
 
 
 128 40' 
 
 1.36 
 
 
 
 126 58' 
 
 1-35 
 
 
 . . 
 
 125 18' 
 
 
 . 
 
 . . 
 
 123 40' 
 
 1-33 
 
 . . 
 
 1 80 0' 
 
 122 6' 
 
 1.32 
 
 . . 
 
 165 56' 
 
 120 33' 
 
 I-3 1 
 
 
 160 6' 
 
 119 3' 
 
 1.30 
 
 
 155 38' 
 
 "7 35' 
 
 1.29 
 
 
 151 5o' 
 
 116 8' 
 
 1.28 
 
 . 
 
 148 42' 
 
 114 44' 
 
 1.27 
 
 
 145 27' 
 
 I 13 21 
 
 1.26 
 
 
 142 39 
 
 in 59' 
 
 I - 2 5 
 
 
 140 3' 
 
 no 39' 
 
 1.24 
 
 , 
 
 i37 36' 
 
 109 20' 
 
 1.23 
 
 
 135 i7' 
 
 108 2' 
 
 1.22 
 
 
 133 4' 
 
 106 45' 
 
 1. 21 
 
 
 130 57' 
 
 105 30' 
 
 1.20 
 
 
 128 55' 
 
 104 15' 
 
 I.I9 
 
 t 
 
 126 58' 
 
 103 2 
 
 1.18 
 
 
 125 3' 
 
 101 50' 
 
 I.I7 
 
 
 123 13' 
 
 100 38' 
 
 1.16 
 
 
 121 26' 
 
 99 29' 
 
MICROSCOPICAL PRAXIS. 
 
 APERTURE TABLE Continued. 
 
 Numerical 
 Aperture. 
 
 Air. 
 
 Water. 
 
 Homogeneous 
 Immersion. 
 
 IS 
 
 
 119 41' 
 
 9 8 20' 
 
 1 4 
 
 . . 
 
 118 o' 
 
 97 ii' 
 
 *3 
 
 . . 
 
 116 20' 
 
 9 6 2' 
 
 .12 
 
 . . 
 
 114 44' 
 
 94 55' 
 
 .11 
 
 
 H3 9' 
 
 93 47' 
 
 .10 
 
 
 ni 36' 
 
 92 43' 
 
 .09 
 
 
 110 5' 
 
 9i 38' 
 
 .08 
 
 . 
 
 108 36' 
 
 90 34' 
 
 .07 
 
 . . 
 
 107 8' 
 
 89 30' 
 
 .06 
 
 . . 
 
 105 42' 
 
 88 27' 
 
 5 
 
 . . 
 
 O s I 
 
 104 16 
 
 87 24' 
 
 .04 
 
 
 102 53' 
 
 86 21' 
 
 03 
 
 
 101 30' 
 
 8 5 19' 
 
 .02 
 
 
 100 10' 
 
 84 1 8' 
 
 .01 
 
 
 98 50' 
 
 83 17' 
 
 .00 
 
 1 80' o' 
 
 97 3i' 
 
 82 17' 
 
 0.99 
 
 163 48' 
 
 96 12' 
 
 81 17' 
 
 0.98 
 
 157 2' 
 
 94 5 6 ' 
 
 80 17' 
 
 0. 9 7 
 
 '51 52' 
 
 93 4o' 
 
 79 18' 
 
 0.96 
 
 147 29' 
 
 92 24' 
 
 7 8 20' 
 
 -95 
 
 143 36' 
 
 91 10' 
 
 77 22' 
 
 0.94 
 
 140 6' 
 
 89 56 
 
 76 24' 
 
 -93 
 
 136 52' 
 
 88 44' 
 
 75 27' 
 
 0.92 
 
 133 5'' 
 
 87 32' 
 
 74 30'- 
 
 0.91 
 
 131 o' 
 
 86 20' 
 
 73 33' 
 
 0.90 
 
 128 19' 
 
 85 10' 
 
 72 3 6 ' 
 
 0.89 
 
 125 45' 
 
 84 o' 
 
 7i 40' 
 
 0.88 
 
 123 17' 
 
 82 51' 
 
 70 44' 
 
 0.87 
 
 120 55' 
 
 81 42' 
 
 69 49' 
 
 0.86 
 
 118 38' 
 
 80 34' 
 
 68 54' 
 
 0.85 
 
 116 25' 
 
 79 37' 
 
 68 o' 
 
 0.84 
 
 114 17' 
 
 78 20' 
 
 67 6' 
 
 0.83 
 
 112 I2' 
 
 77 i4 
 
 66 12' 
 
 0.82 
 
 110 10' 
 
 76 8' 
 
 65 18' 
 
 0.81 
 
 108 10' 
 
 75 3' 
 
 64 24^ 
 
 0.80 
 
 106 16' 
 
 73 58' 
 
 63 3i' 
 
 0.79 
 
 104 22' 
 
 72 53' 
 
 62 38' 
 
n6 
 
 MICROSCOPICAL PRAXIS. 
 
 APERTURE TABLE Continued. 
 
 Numerical 
 Aperture. 
 
 Air. 
 
 Water. 
 
 Homogeneous 
 Immersion . 
 
 0.78 
 
 102 3 l' 
 
 7i 49' 
 
 61 45' 
 
 0.77 
 
 100 42' 
 
 70 45' 
 
 60 52' 
 
 0.76 
 
 98 56' 
 
 69 42' 
 
 60 o' 
 
 0-75 
 
 97 n' 
 
 68 40' 
 
 59 8' 
 
 0.74 
 
 95 28' 
 
 67 37' 
 
 58 16' 
 
 0-73 
 
 93 46' 
 
 66 34' 
 
 57 24' 
 
 0.72 
 
 92 6' 
 
 6 5 32' 
 
 56 32' 
 
 0.71 
 
 90 28' 
 
 64 32' 
 
 55 4i' 
 
 0.70 
 
 88 51' 
 
 63 3i' 
 
 54 50' 
 
 0.69 
 
 87 16' 
 
 62 30' 
 
 53 59' 
 
 0.68 
 
 85 41' 
 
 61 30' 
 
 53 9 
 
 0.67 
 
 84 8' 
 
 60 30' 
 
 52 18' 
 
 0.66 
 
 82 36' 
 
 59 3o' 
 
 51 28' 
 
 0.65 
 
 81 6' 
 
 58 30' 
 
 50 38' 
 
 0.64 
 
 79 36' 
 
 57 3i' 
 
 49 48' 
 
 0.63 
 
 78 6' 
 
 56 32' 
 
 48 58' 
 
 0.62 
 
 76 38' 
 
 55 34' 
 
 48 9' 
 
 0.6 1 
 
 75 10' 
 
 54 36' 
 
 47 19' 
 
 0.60 
 
 73 44 
 
 53 38' 
 
 46 30' 
 
 0-59 
 
 72 18' 
 
 52 4 o| 
 
 45 4o 
 
 0.58 
 
 7: 54; 
 
 5! 4 2 ' 
 
 o / 
 
 O / 
 
 44 5 1 
 
 O / 
 
 0-57 
 
 09 30 
 
 5o 45 
 
 44 2 
 
 0.^6 
 
 68 6' 
 
 49 48' 
 
 43 14 
 
 -55 
 
 66 44' 
 
 49 Si' 
 
 42 25' 
 
 o.54 
 
 65 22' 
 
 47 54 
 
 O f 
 
 4i 37, 
 
 0-53 
 
 64 0^ 
 
 46 58' 
 
 40 48' 
 
 0.52 
 
 62 40' 
 
 46 2 
 
 40 o' 
 
 Q-5 1 
 
 61 20' 
 
 45 6' 
 
 39 o 12' 
 
 0.50 
 
 60 o' 
 
 44 10' 
 
 38 24 ; 
 
 0.48 
 
 57 22' 
 
 42 18' 
 
 36 49' 
 
 0.46 
 
 54 47' 
 
 40 28' 
 
 351 is; 
 
 0-45 
 
 53 3o' 
 
 39 33' 
 
 34 27 
 
 0.44 
 
 52 13' 
 
 38 38' 
 
 33 4 
 
 0.42 
 
 O f 
 
 49 4 
 
 36 49' 
 
 3 2 5 
 
 0.40 
 
 47 9 
 
 35 o' 
 
 3 3 1 ' 
 
 0.38 
 
 O f 
 
 44 4 
 
 33 12' 
 
 28 57' 
 
 0.36 
 
 42 12' 
 
 31 24' 
 
 O ? 
 
 27 24 
 
MICROSCOPICAL PRAXIS. 
 
 117 
 
 APERTURE 
 
 Numerical 
 Aperture. 
 
 Air. 
 
 Water. 
 
 Homogeneous 
 Immersion. 
 
 -35 
 
 40 58' 
 
 30 30' 
 
 26 3 8' 
 
 0-34 
 
 39 44' 
 
 29 37' 
 
 25 51' 
 
 0.32 
 
 37 20' 
 
 27 5l' 
 
 24 1 8' 
 
 0.30 
 
 34 56' 
 
 26 4' 
 
 22 4 6' 
 
 0.28 
 
 32 32' 
 
 24 18' 
 
 21 I 4 ' 
 
 0.26 
 
 30 10' 
 
 22 33' 
 
 19 42' 
 
 0.25 
 
 28 58' 
 
 21 4 0' 
 
 18 56' 
 
 0.24 
 
 27 46' 
 
 20 4 8' 
 
 18 10' 
 
 0.22 
 
 25 26' 
 
 I 9 2' 
 
 16 38' 
 
 0.20 
 
 23 4' 
 
 17 18' 
 
 i5 7' 
 
 0.18 
 
 20 44' 
 
 15 34' 
 
 i3 36' 
 
 o.i 6 
 
 18 24' 
 
 13 5o' 
 
 12 5' 
 
 0.15 
 
 17 14' 
 
 12 58' 
 
 n 19' 
 
 0.14 
 
 16 5' 12 6' 
 
 10 34' 
 
 0.12 
 
 13 47 ' 
 
 9 4' 
 
 0.10 
 
 12 29' 8 38' 
 
 7 34' 
 
 0.08 
 
 O / 
 
 9 ii 
 
 ' 6 54' 
 
 6 3' 
 
 0.06 
 
 6 53' 5 10' 
 
 4 32' 
 
 0.05 
 
 5 44' 4 18' 
 
 3 46' 
 
 To Measure Angular Aperture. 
 
 The optician usually engraves the angle of aperture 
 on the mounting of the best objectives, and with the 
 immersion lenses this will be the numerical aperture (N. 
 A.). For the correctness of the measurement the mi- 
 croscopist may trust the optician, yet he may readily 
 
Il8 MICROSCOPICAL PRAXIS. 
 
 measure the angle for his own satisfaction, by methods, 
 which, although perhaps not as accurate as those of the 
 manufacturer, are sufficiently so for the needs of the 
 amateur. 
 
 If the microscope-stand have a graduated, revolving 
 platform beneath the pillars, nothing else will be needed 
 except a candle which should be placed at one end of 
 the table, and so that the flame shall be about on a 
 level with the objective to be measured when the mi- 
 croscope is in a horizontal position. Remove the mir- 
 ror and all sub-stage apparatus, place the ocular in po- 
 sition, and when the field has been evenly lighted, ro- 
 tate the horizontal instrument to one side until one-half 
 the field is dark, the line of demarkation between the 
 light and the dark halves being made as nearly central 
 and perpendicular as possible. The graduation on the 
 base of the stand will be one-half of the angle of aper- 
 ture (not the numerical aperture), and may be verified 
 by rotating the microscope to the other side, and again 
 bisecting the field with the shadow, when the reading 
 should give the same degree. 
 
 The value of the reading has been said to be one- 
 half of the angle of aperture, but this is contingent upon 
 the arrangement of the graduations. If the instrument 
 was at zero at the beginning of the experiment, this 
 holds good; but if the platform is so graduated, as it is 
 with some stands, that the starting point cannot con- 
 veniently be at -zero, then the instrument must be ro- 
 tated in both directions, and the difference therefore of 
 the graduations will be the angular aperture. On my 
 own stand the graduations begin at zero and advance 
 both ways, so that a low-power objective just measured, 
 gives 5 when rotated to one side, the aperture conse- 
 quently being ten degrees. For the sake of verifying 
 
MICROSCOPICAL PRAXIS. 1 19 
 
 this result, the base was so turned that the rotation be- 
 gan at 50. When one-half the field was bisected, the 
 graduation marked 56; when rotated in the other direc- 
 tion, the instrument stood at 46, the difference giving 
 the same angle as in the former experiment. 
 
 If the stand have no graduated platform, the entire 
 instrument, in the same conditions and in the same po- 
 sition, may be rotated. On a sheet of paper draw a cir- 
 cle whose diameter is equal to the greatest length of 
 the foot of the stand. Rotate the microscope within 
 this circle until the field is exactly bisected, and mark 
 the paper against one side of the foot; rotate it in the 
 opposite direction, and when the other side of the field 
 is again bisected, make against the same side of the 
 foot another mark on the paper. Extend these two 
 lines until they meet, and measure the included angle 
 with the common protractor. This will give the angu- 
 lar aperture of that objective in air. 
 
 The accuracy of all these measurments will depend 
 entirely upon the experimenter's carefulness in bisect- 
 ing the field of the objective, and in measuring the angle 
 with the protractor. Slight errors at the beginning 
 will have a painfully conspicuous appearance at the 
 end. 
 
 Should the reader unfortunately have a stand with- 
 out a joint for inclination, he may still measure the 
 angles of his dry lenses by a method suggested by an 
 anonymous writer. With the microscope placed in a 
 vertical position on a table, preferably a table with a 
 dark-colored cover, the eye-piece, the mirror and all 
 the sub-stage apparatus are to be removed. Rack 
 down the body-tube until the objective passes as far as 
 possible through the stage-opening. Place two pieces 
 of white card one on each side of the instrument, and 
 
120 MICROSCOPICAL PRAXIS. 
 
 while looking down the body-tube, move them back 
 and forth until both can be just seen at the opposite 
 and outer edges of the field of the objective. Measure 
 the distance from the inside margin of one card to the 
 inside margin of the other, and the distance of the 
 front of the lens from the table. Draw the first men- 
 tioned distance as a horizontal base-line, and the latter 
 as a perpendicular to its centre. Connect both sides 
 of the base with the top of the vertical line, and the 
 angle at the apex, or more correctly the two angles 
 formed with the perpendicular at the apex of the 
 triangle, will represent the aperture of the objective, 
 and may be measured with the protractor. 
 
 The adjustment-collar usually has an influence in 
 this connection. When the objective is to be measured 
 for angle, the adjustment should be at or near closed, 
 as that is usually the point of maximum aperture. 
 
 For the purpose of measuring angular aperture the 
 optician and the students of microscopical optics 
 employ a special apparatus termed an apertometer, of 
 which there are several forms in use and with which 
 the end desired may be attained with great accuracy. 
 
 The Abbe Apertometer. 
 
 The simplest, perhaps the best apertometer, at least 
 the best-known, is that devised by Professor Abbe, of 
 Jena, and made by the accomplished optician, Carl 
 Zeiss and by -his successors. It consists of a heavy, 
 
MICROSCOPICAL PRAXIS. 121 
 
 semi-circular disk of glass graduated on its bevelled 
 edge, and bearing two movable metal clips, one on 
 each side, which mark the limits of the aperature in a 
 way similar to that already described in the simple 
 methods for measuring aperture referred to on a pre- 
 ceding page. A low-power objective accompanies the 
 apparatus. The disk is placed on the horizontal stage 
 of the microscope, the objective to be measured being 
 focussed over a central spot on its surface, when the 
 metal clips are to be moved on each side essentially 
 as already described, and the angle is read off between 
 the clips. The apparatus is useful if the microscopist have 
 many objectives to measure, as a manufacturing optician 
 or a dealer might have, but to the general reader it 
 would be merely an optical luxury, to be possessed if he 
 be wealthy, but not to be specially desired if he must be 
 a little economical. The angular aperture may be 
 ascertained by any of the simple methods already de- 
 tailed, and the numerical aperture read off from the 
 table given on another page. 
 
 Many objectives, perhaps the majority of even first- 
 class objectives, admit rays of light around and from 
 near the margins of the lens which have no part in the 
 formation of the image, except in some cases to produce 
 an imperfect result by their interference with the rays 
 which enter into and emerge from the more nearly cen- 
 tral area of the lens. These oblique rays do not reach 
 the same focus which the more central rays reach, the 
 result being a possible deterioration in the perfect 
 action of the objective. Yet when measuring their 
 lenses for angular aperture, the majority of opticians 
 measure not only the more central, but also the mar- 
 ginal rays which may have a bad influence on the ob- 
 jective's action. This is not quite fair to the purchaser 
 
122 MICROSCOPICAL PRAXIS. 
 
 yet it is commonly done. It is done by all opticians, 
 so far as I have been able to learn, except by the late 
 Robert B. Tolles, of Boston, and by Mr. Herbert Spen- 
 cer of the Spencer and Smith Optical Company, of 
 Buffalo, N. Y. These two accomplished opticians 
 measure only the rays which are truly image-forming. 
 This method of ascertaining the angular aperture of an 
 objective originated with the late R. B. Tolles, and is 
 an eminently honest and praiseworthy manner of deal- 
 ing not only with the objective, but with the purchaser 
 who puts his trust in the manufacturing optician, and 
 has no kind feelings toward that man when he is deceived. 
 The majority of objectives when measured by the image- 
 forming rays only, will be reduced in angle in a way that 
 will be painful to the unsuspecting owner. The usual 
 method, the one which has thus far been been described 
 in the preceding pages, measures not the true angular 
 aperture, although it is always called by that name, 
 but it measures the angle of the field, quite another 
 thing, because the field will include all the rays that 
 pass through the objective, the extreme marginal ones 
 as well those that are more central and image-forming. 
 Thus a good objective in my possession, claiming an 
 angular aperture of 135, which it has when it is meas- 
 ured for what is practically the angle of the field, has, 
 when measured for the image-forming rays, an angle of 
 only about 96 ! Still, it is an excellent objective and 
 may be recommended; yet it might be as confidently 
 commended if the maker had told the truth about it, or 
 at least had given the real facts in the case. 
 
 The method of measuring this true angle of aperture 
 does not materially differ from that already described. 
 The only essential difference is that the objective is 
 focussed on an object while it is being measured, and 
 
MICROSCOPICAL PRAXIS. I 2 "$ 
 
 the angle is read as soon as the image begins to be 
 changed for the worse. The procedure demands 
 rather more care and skill than that already described, 
 but the owner of what claim to be wide-angled objec- 
 jectives will be rewarded if he teach himself the neces- 
 sary skill for his own personal satisfaction. The mat- 
 ter has been especially studied by Dr. George E. Black- 
 ham, of Dunkirk, N. Y., who has called the attention of mi- 
 croscopists to the subject, and has published the de- 
 tails of his method of making the measurement. The 
 following paragraphs are chiefly from Dr. Blackham's 
 paper. 
 
 The objective to be measured is attached to the 
 microscope with the eye-piece in place, exactly as for 
 ordinary work, and is focussed on some suitable object 
 in the centre of the field, the correction collar being 
 used if necessary to get the best image the objective is 
 capable of giving. The object should be a transparent 
 one, the resolution of which is a fair test of the powers 
 of the lens. After these arrangements have been com- 
 pleted, the body of the microscope is turned to a hori- 
 zontal position, the mirror swung out of the way, and 
 the object illuminated from below the stage by a nar- 
 row radiant, such as the flame of a toy candle. The 
 source of illumination is then moved to the right and 
 the left in succession, till either the centre of the field 
 becomes darkened or the image is spoiled. The 
 angular value of this distance through which the source 
 of illumination can be moved before this takes place, is 
 the available angular aperture of the lens; that is, the 
 useful aperture for definition. In some lenses the 
 image is spoiled long before the centre of the field 7s 
 darkened, so that the aperture, for the mere transmis- 
 sion of light, is much greater than that for definition. Such 
 
124 MICROSCOPICAL PRAXIS. 
 
 lenses are imperfectly corrected for the marginal rays and 
 their performance can be improved by cutting off these 
 aberrant marginal rays by means of diaphragms, and so 
 reducing their angular aperture (for transmission). 
 
 Instead of moving the candle as Dr. Blackham sug- 
 gests, the microscope may be turned on the graduated 
 base which some stands have, or the angular value may 
 be ascertained by some of the other methods previously 
 described, and which are rather easier for most experi- 
 menters than to make the estimates and calculations 
 demanded by Dr. Blackham's method. Or if the 
 mirror-bar be graduated and capable of being swung to 
 one side, as Dr. Blackham says it should be (in which 
 we all agree with him), the microscopist has a means of 
 reading the obliquity to which it is swung by substitut- 
 ing a toy candle for the mirror and rotating it to one 
 side instead of moving the candle alone as in the other 
 plan, or of turning the microscope on its base. 
 
 If greater accuracy be desired, it can be secured by 
 the use of an opaque slide with a transparent line 
 across it, in which the objects are mounted, and this 
 line'can be so placed on the stage as to bisect the field 
 of view very accurately in a vertical direction, and the 
 exact moment at which the centre of the field is 
 darkened can thus be determined with greater pre- 
 cision. Such a slide can readily be made by cutting a 
 3x1 inch slip from a photographic plate, exposing it to 
 the full sunlight, developing and fixing it, and then 
 drawing a sharp knife across it on the film side. In 
 the transparent line thus made, diatoms or any other 
 objects may be mounted in balsam and covered in the 
 usual way. 
 
 The plan of measurement here described, if used 
 without any devices below the stage, can measure aper- 
 
MICROSCOPICAL PRAXIS. 125 
 
 tures only below i.oo numerical aperture; that is to 
 say, angular apertures of 180 air, of 97 31' water, or 
 of 82 if in glass or homogeneous immersion-fluid, 
 and must always give the results in terms of the equiva- 
 lent air-angle. Hence when we have occasion to 
 measure the aperture of a lens exceeding, or indeed 
 closely approaching i.oo N. A., it becomes necessary to 
 use below the stage some device like the little hemis- 
 pherical lens, which being attached to the under side of 
 the slip by an immersion contact, allows the rays to 
 pass into the slide without refraction, and consequently 
 gives the angle in terms of the glass, or the homogene- 
 ous angle. 
 
 It not rarely happens that the manufacturing optic- 
 ian does not mark on the mounting of his objectives 
 the aperture that the purchaser thinks should be there, 
 after the purchaser has measured it. This refers, of 
 course, to the aperture that includes all the entering 
 light, the marginal as well as the image-forming rays. 
 The optician will never use half a degree, as there is no 
 reason why he should; and he does not seem to care 
 much about odd numbers, as shown in the following 
 table of angular and numerical aperture of the objec- 
 tives mentioned, which were all measured with an Abbe 
 apertometer in the expert hands of Prof. M. D. Ewell, 
 of Chicago, from whose report the list is taken. 
 
126 
 
 MICROSCOPICAL PRAXIS. 
 
 Table of the Aperture of Certain Objectives. 
 
 Maker. 
 
 Bausch & Lomb, 
 
 u a 
 
 Beck, 
 Crouch, 
 
 Grunow, 
 
 Gundlach, 
 Hartnack. 
 Leitz, 4 
 
 Spencer & Co., 
 
 II U 
 
 
 
 Spencer & Smith, 
 
 Description. 
 
 Professional i in. 
 /4 in. 
 
 2 in., first class. 
 ^ in., students. 
 -J- in., students. 
 ^ in., first class. 
 -fa in., first class. 
 ^ in., horn. imm. 
 \ in., first class, 
 i in. 
 
 Aperture 
 claimed 
 
 Aperture 
 as 
 
 by maker. 
 
 measured. 
 
 36 
 
 3 
 
 60 
 
 61 
 
 22 
 
 21 
 
 75 
 
 7 
 
 110 
 
 95/2 
 
 140 
 
 114 
 
 ucT iu~ 
 
 N. A. 1.43 N. A. 1.28 
 100 98 
 
 25 24 
 140 \ ^closed 
 ( 147 open. 
 
 N. A. 1.41 N. A. 1.39 
 
 f 
 
 J-, horn. imm. 
 
 i in. 40 47 
 
 18 mm., No. 3, dry. N. A. 0.28 N. A. 0.28 
 32 mm., No. 7, dry. N. A 0.85 N. A. 0.88 
 -fa in., oil imm. N. A. 1.30 N. A. 1.28 
 
 J in., students. 
 -fa in., horn. imm. 
 
 -h in -> " 
 
 " " 
 
 94 
 
 B. A. 130 N. A. 1.36 
 " B. A. 125 -N. A. 1.32 
 
 -fa, first class. 130 126 
 
 J, first class, dry. 150 167 
 
 in, horn. imm. B. A. 138 N. A. 1.41 
 
 Tolles, 
 
 Zeiss, 
 
 i in. 
 
 33 32i 
 
 i jj-j 
 
 70 71 
 
 " ^ horn. imm. 
 
 N. A. 1. 00 N. A. 0.97 
 
 -J-, horn. imm. 
 
 N. A. 1.32 N. A. 1.31 
 
 2 in. 
 
 Unknown i2\ 
 
 y^in, water imm. 
 
 u j 0.99 closed 
 | 0.90 open 
 
 1, A A. 
 
 36 3 i 
 
 i, c c. 
 
 o o 
 90 102 
 
 Y^-g, horn. imm. 
 
 N. A. 1.27 N. A. 1.27 
 
MICROSCOPICAL PRAXIS. 127 
 
 To Measure Numerical Aperture (N. A.). 
 
 For the measurement of numerical aperture, the late 
 Dr. Allen Y. Moore used a method whose application 
 was probably original with himself. The following is 
 his description. A low-power objective (a three-inch 
 or a four-inch is convenient for the purpose), is at- 
 tached to the microscope, and the latter placed in a 
 horizontal position. The objective whose numerical 
 aperture is to be measured is screwed into the sub- 
 stage with its front facing toward the mirror which 
 should be turned as far as possible to one side, or 
 preferably removed from the stand. If now the micro- 
 scope be properly focussed, a bright disk of light will 
 be seen, and will represent the acting diameter of the 
 back lens of the objective in the sub-stage. The 
 camera lucida should be attached to the eye-piece, and 
 the diameter of this disk marked on paper. A hemi- 
 spherical lens is now to be applied to the front of the ob- 
 jective to be measured, and should have immersion 
 contact with it, that is, should be attached by means of 
 a drop of glycerine or of homogeneous-immersion fluid. 
 This will enlarge the bright circle to the full aperture 
 of the objective, and this bright disk should also be 
 drawn on the paper. Remove the objective from the 
 sub-stage, and on the microscope-stage place a micro- 
 meter ruled to thousandths of an inch, and with the 
 camera lucida project these lines upon the two circles 
 on the paper, thus measuring their diameter. If the 
 diameter of the larger circle be divided by that of the 
 smaller, the quotient will be the numerical aperture 
 This applies however, only to objectives whose N. A. is 
 greater than i.oo. 
 
128 MICROSCOPICAL PRAXIS. 
 
 Definition. 
 
 The definition, or the defining power, of an objective, 
 is its ability to produce an image of the object ex- 
 amined that shall be clear, sharp, crisp and true. The 
 outlines of the image, as has already been remarked, 
 should be so distinctly defined that they shall be as 
 sharply marked as the lines of a copper-plate engrav- 
 ing, and the structure of the object must at the same 
 time be as clearly defined as is the contour. 
 
 The greater the angular aperture the more perfect 
 and satisfactory will be the definition. Small-angled 
 objectives have some qualities that make them more 
 easily manipulated by the novice, but the exquisite 
 definition of the wide-angled glasses is not one of them; 
 and the wide-angled lenses lack the element of penetra- 
 tion, or depth of focus, possessed by the smaller-angled 
 objectives and valued to a certain extent in some in- 
 vestigations. 
 
 There are many tests for the definition of low and of 
 medium powers of small angle. For the former, Mr. 
 E. M. Nelson, an accomplished British microscopist, 
 suggests the flattened proboscis of the blow-fly mounted 
 in Canada balsam, and for the latter the minute hairs on 
 the same object. Under medium powers of high 
 angle, he recommends stained bacteria, and the diatom 
 Pleurosigma formosa mounted in balsam. 
 
 According to Prof. Abbe, small fragments of the dia- 
 tom Pleurosigma angulatum may be used to advantage 
 as tests of even the best and widest angled immersion 
 objectives. In connection with the illumination during 
 the testing, he recommends that the mirror should be 
 so arranged that the light shall just graze the centre of 
 the front surface of the objective, the striae of the dia- 
 tom being at right angles to the general direction of 
 
MICROSCOPICAL PRAXIS. I2Q 
 
 the illumination; this tests the marginal zone of the 
 lens. The mirror is then to be shifted laterally so as to 
 produce the most oblique illumination. In both cases 
 the outlines and the structural striae should not only 
 appear equally sharp, but should coincide without dif- 
 ference o( level and without lateral displacement. If 
 an objective fulfills these requirements, at least in the 
 centre, says Professor Abbe, it may be depended upon 
 to produce accurate images. An intermediate position 
 of the mirror would furnish additional proof that the 
 different zones co-operate simultaneously in the pro- 
 duction of the images. 
 
 Resolving Power. 
 
 The resolution, or the resolving power, of an object- 
 ive is its ability to separate fine and close markings or 
 lines, such "as the striae on the diatoms, or those closely 
 ruled lines on the artificial test-plates of various 
 makers. This ability depends upon certain qualities of 
 the lens, especially upon the numerical aperture, and not 
 upon magnifying power. In the separating of very 
 fine and close lines it is customary to use high-power 
 eye-pieces, not because the increase in magnifying 
 power increases the resolving power of the objective, 
 but because it separates the lines a little wider so that 
 the eye may more easily see them. 
 
 10 
 
130 MICROSCOPICAL PRAXIS. 
 
 The reader should remember that to do much in the 
 resolving of diatoms or of fine rulings, the eye must be 
 specially educated for the work. Whilst the micros- 
 copistmay be expert in the use of his objectives and il- 
 luminating apparatus, and whilst he may be a learned 
 histologist, or an accomplished investigator in some 
 other department of science, he may not be able to ex- 
 hibit fine lines or diatom-striae, either to himself or to his 
 friends. This is aspecial department in which special eye- 
 training is demanded. And whilst it is well to be able 
 to resolve the most difficult diatoms, so as to be able to 
 know positively by one's own experience what a certain 
 objective will do, it should not be the chief aim in the 
 microscopist's life. The resolving of diatoms is useful 
 in studying the manipulation' of the objective, and im- 
 portant in comparing the action of different objectives 
 from different makers, and with different angles of 
 aperture, but there are many things more important 
 than these "microscopical gymnastics." It is exceed- 
 ingly important that the owner of good objectives 
 should study them to learn what they will do, and how 
 he may obtain the best results from them, but to know 
 no more of microscopical research than is contained in 
 an attempt to force one objective to resolve a few lines 
 more than another, seems a poor compensation for the 
 time, labor and expense, when the world is teeming 
 with innumerable things about which we know nothing, 
 and when even one little corner of one little field of in- 
 vestigation, well cultivated with the microscope, will 
 bring the cultivator fame, and the scientific world in- 
 crease of happiness through increase of knowledge. 
 
 The resolving power of a well-corrected objective de- 
 pends upon its numerical aperture. A lens of wide 
 aperture will show finer and closer lines than will a small- 
 
MICROSCOPICAL PRAXIS. 131 
 
 angled glass, the resolution increasing and diminishing 
 in a certain proportion as the numerical aperture is en- 
 larged or contracted, all objectives that have the same 
 numerical aperture having the same resolving power. 
 
 The character of the illumination, whether central or 
 oblique, is an important element. Oblique light will 
 bring out markings not visible with central illumina- 
 tion, and the objective that will resolve Pleurosigma 
 angulatum with central light, other things being equal, 
 is a better lens than the one that resolves it only with 
 oblique illumination. 
 
 The adjustment of the objective is also of great im- 
 portance in this connection. And the medium in 
 which the test is immersed has great influence. A dia- 
 tom mounted dry is more easily resolved than the same 
 diatom in Canada balsam. The character of the lines 
 or of the dots is important to the success of the work. 
 Two diatoms may have the same number of lines to the 
 fraction of an inch, yet one may be easily resolved, 
 whilst the other may tax the resources of the best ob- 
 jectives and all the microscopist's skill, because the 
 striae on one are strongly developed, while those of the 
 other are faint, low'and delicate. The same holds true 
 to a certain extent with ruled lines. The stronger and 
 deeper, and the better filled with graphite these cuts 
 in the glass may be, the more easily are they resolved 
 by the proper objective, or at least, the more easily are 
 they seen by the eye. 
 
 Although diatoms of the same species differ to some 
 extent in the character and often in the number of their 
 striae, they are the most accessible tests for resolu- 
 tion that we have. They are used by all microscopists 
 and by the opticians, as they form a ready means of 
 comparison. A convenient collection is Moller's or 
 
132 MICROSCOPICAL PRAXIS. 
 
 Thum's test-plate (Probe Platte) consisting of a slide of 
 twenty diatoms arranged in a line according to the dif- 
 ficulty of their resolution, beginning with Triceratium 
 favus and ending with Amphipleura pellucida. The slides 
 may be had balsam-mounted or dry. These prepara- 
 tions are somewhat costly, and the reader may as read- 
 ily and at less expense supply himself with several 
 slides of different tests, with the advantage of having 
 many of the same form on the slide in many different 
 positions, and probably of different degrees of fine stri- 
 ation. 
 
 The microscopist that desires to test his objectives, 
 and to train his eye to see fine lines and dots, should 
 purchase balsam-mounted slides of Pleurosigma angula- 
 tum, Frustulia Saxonica, Surirella gemma, Amphipleura 
 pellucida, or, if he studies convenience rather than ex- 
 pense, a test plate by Moller or by Thum. But with 
 the diatoms mentioned he will have enough to test the 
 best and highest-angled objectives. Pleurosigma angu- 
 latum may serve as a test for the one-fourth or the one- 
 fifth inch objective, which should resolve the transverse 
 lines with central light, and three sets, one transverse 
 .and two oblique in opposite directions, under the proper 
 illumination. A four-tenths inch by Mr. Tolles in my 
 possession will resolve this diatom in beads when 
 mounted in styrax, as well as exhibit the transverse 
 striae on a dry Surirella gemma. 
 
 Frustulia Saxonica is more difficult, that is, the striae 
 are finer and much closer 'together than those of P. an- 
 gulatum, demanding a high-power of wide angle to re- 
 solve them. A one-sixth or one-eighth of fine quality 
 will be needed to conquer them. 
 
 Amphipleura pellucida is the most difficult of the 
 known diatom tests. Photography has recently resolved 
 
MICROSCOPICAL PRAXIS. 133 
 
 it into dots or beads, but to show the exceedingly fine 
 and delicate transverse striae only, is a labor for the 
 best of ordinary modern objectives, although the ma- 
 jority of the recent lenses of wide aperture will do the 
 work easily, if properly treated. The diatom has been 
 resolved into beads by Powell and Lealand's -J- inch ob- 
 jective, and by the remarkable new apochromatic y 1 ^, 
 1.63 N. A. by Zeiss, in the hands of Dr. H. Van Heurck. 
 
 Oblique light is always necessary for this kind of 
 work; it is often needed so oblique that the upper edge 
 of the mirror, when swung, to one side, is only a little 
 below the surface of the stage. The concave mirror 
 should be carefully focussed on the object, and the dia- 
 tom so placed that the striae desired to be shown shall 
 be at right angles to the direction of the illumination, 
 and the position of the adjustment-collar must be care- 
 fully and frequently changed until the best results are 
 attained. Daylight is not adapted to suoh work. Di- 
 rect sunlight should never be used. 
 
 In the study of the striae of diatoms the mirror alone 
 is often not sufficient. In all such work the hemispher- 
 ical lens is a useful accessory, concentrating the light 
 on the diatom, and adding somewhat to its intensity. 
 This lens is simply a little button of glass about half an 
 inch in diameter, and of the proper curvature to focus 
 the light upon the object. It is attached to the lower 
 surface of the slide by a drop of water, of glycerine or 
 of homogeneous-immersion fluid. If too much liquid 
 be used the lens will slip out of position when placed 
 on the inclined stage. Use only enough therefore to 
 hold it firmly and to expel all the air from between it 
 and the slide to which it is attachedr 
 
 The following table gives a list of the diatoms on 
 Holler's test-plate, with the number of striae to the 
 
134 MICROSCOPICAL PRAXIS. 
 
 inch. Eupodiscus argus begins and ends the line so that 
 it may be readily found. 
 
 Diatom. Striae in i-iooo inch. 
 
 1. Triceratium favus Ehr. 3.1 to 4.0 
 
 2. Pinnularia nobilis Ehr. 11.7 to 14.0 
 
 3. Navicula lyra Ehr. var. 14.5 to 18.0 
 
 4. Navicula lyra Ehr. 23.0 to 30.5 
 
 5. Pinnularia interrupta Sm. var. 25.5 to 29.5 
 
 6. Stauroneis Phcenicenteron Ehr. 31.0 to 36.5 
 
 7. Grammatophora marina Sm. - 36.0 to 39.0 
 
 8. Pleurosigma Balticum Sm. - 32.0 to 37.0 
 
 9. Pleurosigma acuminatum (Kg.) Gr. 41.0 to 46.5 
 
 10. Nitzchia amphioxys Sm. 43.0 to 49.0 
 
 11. Pleurosigma angulatum Sm. 44.0 to 49.0 
 
 12. Grammatophora oceanica Ehr. 60.0 to 67.0 
 
 13. Surirella gemma Ehr. - 43- to 54.0 
 
 14. Nitzchia sigmoidea Sm. - 61.0 to 64.0 
 
 15. Pleurosigma fasciola Sm. var. 55.0 to 58.0 
 
 16. Surirella gemma Ehr. - 64.0 to 69.0 
 
 17. Cymatopleura elliptica Breb. - 55.0 to 81.0 
 
 1 8. Navicula crassinervis Breb. - 78.0 to 87.0 
 
 19. Nitzchia curvula Sm. - 83.0 to 90.0 
 
 20. Amphipleura pellucida Kg. - 92.0 to 95.0 
 The following is the list of the diatoms used on 
 
 Thum's test-plate. 
 
 1. Triceratium favus Ehr. 
 
 2. Navicula nobilis Kutz. 
 
 3. Navicula lyra Ehr. var. 
 
 4. Navicula lyra Ehr. 
 
 5.- Pleurosigma attenuatum Sm. 
 
 6. Stauroneis Phamic enter on Ehr. 
 
 7. Grammatophora marina Sm. 
 
 8. Pleurosigma Balticum Sm. 
 
 9. Pleurosigma acuminatum (Kg.) Gr. 
 
MICROSCOPICAL PRAXIS. 135 
 
 10. Plettrosigma angulatum Sm. 
 
 11. Nits chia sigma Sm. 
 
 12. Surirella gemma Ehr. 
 
 13. Nits chia sigmodea Sm. 
 
 14. Nitschia obtusa var. Schweinfurthii Gr. 
 
 15. Cymatopleura nobilis Hantz. 
 
 1 6. Nitschia linear is Sm. 
 
 17. Grammatophora subtilis Bail. 
 
 1 8. Surirella gemma Ehr. 
 
 19. Frustttlia Saxonica Rahb. 
 
 20. Amphipleura pellucida Kg. 
 
 In addition to diatoms,- microscopists are in the 
 haoit of using artificial tests in the form of exceedingly 
 fine and close lines ruled on glass by a special machine. 
 The most celebrated of these test-plates, those of the 
 late F. A. Nobert, of Prussia, have already been referred 
 to, the best known and rnqst important being his slide 
 bearing nineteen bands of lines, the number of lines in 
 each band increasing and the spaces decreasing up to 
 the last or nineteenth. The other plates by the same 
 maker are seldom heard of at the present day. 
 
 The Lepisma saccharina, the markings on whose 
 larger scales are as close together as the lines on the 
 first band of this plate, is a common little insect often 
 seen in old books or running actively in dark places. 
 It is so elongated and flattened, and so silvery-gray in 
 color, that it is frequently called the "silver-fish insect." 
 The body has three bristle-like radiating caudal fila- 
 ments and two thread-like antennae, and when full- 
 grown, may be fully an inch long. It is covered with 
 scales which at one time were used as tests, and are 
 still commendable for certain purposes. They should 
 be mounted dry. 
 
136 MICROSCOPICAL PRAXIS. 
 
 The following is Naegeli and Schwendener-'s list of 
 diatoms whose lines correspond in number to the num- 
 ber of lines in each band of Nobert's nineteen-band 
 plate. 
 
 Band. Lines to o.ooi in. Lines to mm. Approximate equivalent. 
 
 1. 11.26 443 Lepisma saccharina; large scales. 
 
 2. 16.89 665 Pinnularia viridis. 
 
 3. 22.52 886 
 
 4. 28.13 4108 
 
 5- 33-78 1329 Pleurosigma Balticum. 
 
 6. 39.41 1550 attenuatum. 
 
 7. 45.04 1773 
 
 9.' 56.*30 22?6 \Ptiurosigma angulatum. 
 
 10. 61.93 2439 Grammatophora marina. 
 
 11. 67.56 2653 Nitzchia lima ris. 
 
 12. 73.19 2870 \Navicularhomboides. 
 
 13. 78.82 3105 \ Grammatophora subtilissima. 
 
 14- 84.45 33 22 
 
 15- 90-08 3546 \ Frustulia Saxonica. 
 
 16. 95-7 1 3768 r 
 
 17. 101.34 3989 ) Amphipleura pellucida. 
 
 18. 106.97 4211 
 
 19. 112.60 4433 
 
 In "The English Mechanic," Mr. G. D. Hirst de- 
 scribes a method, not original with him, of intensify- 
 ing the resolving power of an objective when used 
 over close-lined tests. He says: Take, for instance, 
 the diatom Amphipleura pellucida, and having got the 
 best results obtainable with the illuminating apparatus 
 at one's disposal, let the analysing prism of the polari- 
 scope be placed over the eye-piece, and rotated until 
 it darkens the field, which it will do although not to the 
 same extent a's when used with the polarizing 
 
MICROSCOPICAL PRAXIS. 137 
 
 prism. On carefully focussing the diatom, the lines 
 will show themselves with an extraordinary increase of 
 definition. Specimens that without the aid of the 
 prism show only a washy sort of resolution, will now 
 show the lines as black as the bars of a gridiron. 
 
 The application of the prism will of course not make 
 an objective resolve a test beyond the reach of its aper- 
 ture. 
 
 Penetration. 
 
 The penetrating power of an objective is its ability 
 to show the structure of an object below the plane 
 for which it is focussed; that is, the objective can pene- 
 trate to a certain distance into the object without being 
 lowered by the focussing mechanism. It is of very 
 little importance, and is incompatible with resolving 
 power. It depends chiefly upon the numerical aper- 
 ture, like so many other qualities of the objective, de- 
 creasing in a certain proportion with the increase of the 
 aperture, and increasing as that diminishes. 
 
 One of the advantages of objectives with wide aperture 
 (N. A.) is, that while they show the most delicate de- 
 tails of the surface for which they are focussed, there is 
 no confusion by the introduction of a somewhat indistinct 
 view of structure below that focal plane, yet the objec- 
 
138 MICROSCOPICAL PRAXIS. 
 
 tive may, in certain conditions, be depressed for the 
 satisfactory examination of an inferior plane of the ob- 
 ject, with little interference with the superior surface 
 just examined and left. With certain wide-angled 
 glasses it is possible to produce optical sections of the 
 object, by which the relations of structure may be satis- 
 factorily demonstated. 
 
 Working-distance. 
 
 The working-distance of an objective is the distance 
 between the lower surface of the front lens, when the 
 objective is focussed, and the upper surface of the ob- 
 ject, the practicable distance being reduced by the pres- 
 ence of the cover-glass. It varies according to the 
 magnifying power and to the angular aperture, depend- 
 ing chiefly upon the latter. The optician can control it 
 to a certain extent, making it, within certain limits, long 
 or short at will, since it depends not only on the power 
 and on the aperture, but upon the thickness of the 
 lenses composing the objective, upon their curvature 
 and their number. 
 
 Objectives are not named according to their work- 
 ing-distance. A one-inch objective will not have a one- 
 inch working-distance, and the- latter will not be one- 
 tenth of an inch with a one-tenth inch microscope-lens 
 Objectives are rated according to their power and their 
 
MICROSCOPICAL PRAXIS. 139 
 
 focal length in comparison with single lenses. A one- 
 inch objective is supposed to have the same magnifying 
 power as a simple lens of one-inch focus, and a one- 
 fifth inch objective to be the same in power as a simple 
 lens having a focal length of one-fifth inch. 
 
 Working-distance is a convenient quality, since an 
 objective of long working-distance is more easily used 
 than one that focusses so close to the cover as to be 
 almost in contact with it. In some cases however, it is 
 an obstacle, especially in certain immersion-objectives 
 where the optician has so increased it that the capillary 
 attraction is scarcely sufficient to hold the immersion 
 liquid in place between the lens and the cover. But 
 this is not a common occurence. 
 
 To Measure Working-distance. 
 
 If the stand have a scale and vernier at the side of 
 the body, and the front lens of the objective is flush 
 with the mounting, the working-distance may be 
 readily measured by carefully bringing the objective in 
 contact with the slide, after which it is focussed on any 
 imperfections or small particles on the surface, and the 
 distance to which it has been raised is then to be read 
 on the scale. In high-power objectives the distance 
 may be ascertained by the fine-adjustment screw, if the 
 milled-head is graduated and the lens flush with the 
 
140 MICROSCOPICAL PRAXIS. 
 
 mounting, as it often is not. Yet the distance between 
 the front of the mounting and the front of the lens is so 
 short that it may be disregarded in the lower powers. 
 In the higher, if a more accurate measurement is de- 
 sired, it may be made by using two objectives. 
 
 In such cases, place the lens whose working distance 
 is to be ascertained, in the substage so that the front 
 faces upward, and arrange an object behind it until the 
 small image is distinct at its focus. With a low-power 
 objective on the microscope, focus on the face of the 
 objective to be examined, and measure the distance 
 through which the body-tube must be raised in order to 
 get a distinct view of the image formed by the objec- 
 tive whose working-distance is to be ascertained. 
 
 When the working-distance of an adjustable objec- 
 tive is to be ascertained, the collar should be at or near 
 closed point, unless the microscopist desires to know it 
 both for covered and for uncovered objects, as it varies 
 slightly under these conditions in which case he must 
 measure it with the collar at both adjustments. 
 
 It varies also not only with every adjustment, since 
 every movement of the collar changes the focus, but 
 also according teethe eye-sight of the observer, and the 
 power of the eye-piece. A near-sighted microscopist 
 must focus his objectives nearer the object, and thus 
 shorten the working-distance, while the presbyopic ob- 
 server usually lengthens it, especially when using lenses 
 of low power. With high-powers the same fact holds 
 good, but not to so great an extent. 
 
MICROSCOPICAL PRAXIS. 141 
 
 Magnifying Power. 
 
 The magnifying power of the microscope depends 
 upon the power of the objective and that of the eye- 
 piece, and upon the length of the body-tube. It is 
 ascertained by multiplying the power of the objective 
 by the power of the ocular, the length of the body-tube 
 being assumed to be of the standard length of ten 
 inches. If the objective alone magnifies fifty diame- 
 ters and the eye-piece five, the power of the combina- 
 tion will be two-hundred and fifty diameters. This is 
 absolutely correct however, only when what is called 
 optical tube-length, which is a very variable quality, 
 has been taken into consideration. For the unchanging 
 ten-inch body-tube it is not strictly accurate, yet it is 
 near enough for all practical purposes. 
 
 The power of the objective depends upon its focal 
 length, both increasing and diminishing together, while 
 that of the microscope, or combination of objective and 
 eye-piece, depends, in addition, upon the power of the 
 ocular and the length of the body-tube, the longer the 
 tube or the stronger the eye-piece, the greater the am- 
 plification. The power of the objective is also affected 
 by the adjustment collar, increasing as the adjustment 
 approaches the closed point and becoming greatest at 
 that point. 
 
 If the objective is correctly named according to its 
 focal length, its power will be the quotient of ten 
 inches (the arbitrary distance of distinct vision, and the 
 length of the body-tube), divided by the focal length; 
 M (magnifying power) =~, where / is the focal length. 
 Thus a one-inch objective should magnify ten diame- 
 ters without the eye-piece; a one-fifth, 10-^-0.2 or fifty 
 diameters; a one-tenth, one-hundred diameters. 
 
 But all objectives are not correctly named. Some 
 
14? MICROSCOPICAL PRAXIS. 
 
 opticians in their nomenclature under-rate their pro- 
 ductions, so that a lens marked J by one maker may 
 in reality be a -^, and may do as much as or more than 
 a one-fifth by another, to the honor and glory of the 
 former apparently lower power. Such nomenclature is 
 not only dishonest, but it is inconvenient and mislead- 
 ing when we wish to compare two objectives by 
 different makers, but nominally of the same focal 
 length. Opticians have been improving in this respect 
 of late years. Formerly the complaints were many 
 and bitter, and the opticians answered back, defending 
 themselves or trying to do so, and thus making things 
 lively. But the makers seem to have taken warning. 
 At least the complaints have become fewer. 
 
 To measure the power of the objective, it is neces- 
 sary to have a point from which to begin the measure- 
 ment, and this, it is said, should be the optical centre, 
 or "that point in a lens through which if a ray passes, it 
 enters and emerges in parallel lines;" but since nobody 
 knows where the optical centre may be, and since some 
 objectives have none, it is rather difficult to find. Yet 
 when it exists, it may be discovered by going through 
 with three pages of mathematical calculations to get 
 two or three formulae which, when combined and 
 solved, will give the point sought, and which, when ob- 
 tained will be practically worthless. 
 
 A method which is sometimes more convenient is to 
 measure from the posterior principal focus of the ob- 
 jective, and since this, in high-powers especially, is 
 close to the surface of the back lens, the back lens may 
 be taken as the starting point. 
 
 To obtain the distance of this posterior focus from 
 the upper end of the body, prepare a paper tube closed 
 at one end with a translucent diaphragm, and slip this 
 
MICROSCOPICAL PRAXIS. 143 
 
 down the body until a small spot of light is sharply 
 defined in the centre of the diaphragm. Where this 
 spot is formed is said to be the posterior focal point of 
 the objective. Mark the place on the outside of the 
 body, and measure the tube-length from it. With low 
 powers the length of the body-tube may be ten inches 
 from the front lens, since the posterior focus in such 
 lenses is higher up the body, for which the length of 
 the lens-mounting will make approximate compensa- 
 tion. 
 
 In the experiment with the paper diaphragm the tube 
 should be small enough to enter the mounting of the 
 objective, as its posterior focus is often close to the 
 back lens, frequently less than -^ inch above it, when 
 it is outside of the lenses; sometimes it is somewhere 
 inside of the objective, in which case this experiment 
 will fail, as the paper diaphragm ca'nnot then reach the 
 focal point. To be strictly accurate in these matters is 
 difficult, often impossible to any but the opticians, and 
 it is the optical tube-length that should be ascertained, 
 yet this is a varying quantity with every combination 
 of objective and eye-piece, since it is the distance 
 between the posterior principal focus of the objective 
 and the anterior principal focus of the ocular. The 
 former it is often impossible to find in any practical 
 way; the latter is usually at or near the diaphragm 
 within- the eye-piece tube. For all practical purposes, 
 however, it will be amply sufficient to measure the dis- 
 tance from the back lens of the objective to the dia- 
 phragm of the eye-piece used, both positions being ob- 
 tained without difficulty, as the diaphragm of the ocular 
 is always fixed at the proper point by the optician. 
 
 The position of the posterior focus of the objective 
 depends upon the construction given the lens by its 
 
144 ( MICROSCOPICAL PRAXIS. 
 
 manufacturer. Sometimes it is outside of the combina- 
 tion and can then be ascertained; often it is some- 
 where within the objective, when we have no conven- 
 ient means of getting at it. The late W. H. Bulloch, of 
 Chicago, paid some attention to the subject, examining 
 several objectives by different makers, his purpose 
 being to discover a practicable method which might be 
 employed by the microscopist that is not a manufactur- 
 ing optician, nor an expert in the application of micro- 
 scopical optics. His conclusion was that the use of 
 the posterior principal focus of the objective was not 
 possible, for the reasons already detailed. 
 
 His method of ascertaining this focal plane was to 
 place the objective to be examined in the sub-stage of 
 a microscope, with the front lens directed away from 
 the microscope-stage. He then used a low-power ob- 
 jective on the body-tube to find the position of an 
 image of a distant object formed by the lens in the sub- 
 stage. I find much difference, he says, among the 
 objectives of different makers; of those of about the 
 same magnifying power, some have the posterior focus 
 within the combination, others have it some distance 
 behind the back system; for example, in the Spencer 
 two-thirds inch of 36 the posterior focus was found to 
 be 0.18 inch within the combination, measuring from 
 the back lens; in another two-thirds of 30, 0.34 inch 
 behind the posterior combination. 
 
 These are short distances, but in work that is to be 
 followed by mathematically accurate results, they afe 
 important; yet for the amateur microscopist they may 
 be disregarded, and, as has been said, the back lens of 
 an objective higher in power than the two-inch may be 
 taken as the starting point for our measurements. 
 
 The following table is compiled from Mr. Bulloch's 
 
MICROSCOPICAL PRAXIS. 
 
 145 
 
 paper, and is here given for the convenience of those 
 possessing the objectives mentioned, and also to show 
 that the distances are so short that they be disregarded 
 by all except perhaps the professional, manufacturing 
 optician. 
 
 Maker of 
 
 Name of 
 
 Angle of Posterior focus Posterior 
 
 Objective. 
 
 Objective. 
 
 Aperture, from front sur- focus in- 
 
 
 
 face. side or out- 
 
 
 
 side the 
 
 
 
 combination 
 
 Spencer, 
 
 -^ horn. imm. 
 
 1.35 N. A. 0.145 inside 
 
 tt 
 
 TV " " 
 
 1.27 N. A. c.ii " 
 
 tt 
 
 A dry 
 
 not found at 
 imm. 170 open point: 
 atclosedo.25 
 
 " 
 
 i dry 
 
 115 0.15 outside 
 
 tt 
 
 
 /-o 0.18 inside of 
 
 
 
 3 6 back. 
 
 " 
 
 i inch 
 
 o 0.02 inside of 
 40 back. 
 
 " 
 
 2 " 
 
 0.67 " 
 
 Gundlach, 
 
 -j*g- imm. 
 
 105 0.15 inside 
 
 " 
 
 | horn. imm. 
 
 1.40 N. A. 0.53 " 
 
 " 
 
 -J- dry 
 
 135 o-43 
 
 Bausch & Lomb, -J- dry 
 
 140 0.41 " 
 
 4< 
 
 ^ imm. 
 
 1 80 not found 
 
 a 
 
 idry 
 
 no 0.49 outside 
 
 tt 
 
 T 4 o 
 
 0.15 inside 
 
 " 
 
 i inch 
 
 _f~o 0.04 inside of 
 3 6 back. 
 
 " 
 
 J- inch 
 
 98 0.02 outside 
 
 Grunow, 
 
 t . 
 
 0.85 
 
 " 
 
 ij 
 
 0.123 
 
 Zeiss, 
 
 HDD) dry 
 
 116 0.41 inside 
 
 *' 
 
 i ( c c ) 
 
 90 0.32 " 
 
 
 
 t ( A A ) 
 
 20 0.84 " 
 
 " 
 
 A* closed 
 
 8.53 outside 
 
 " 
 
 " at 5 
 
 7.8 
 
 " 
 
 " open 
 
 7.3 
 
 Queen & Co., 
 
 2 inch 
 
 10 1.57 
 
 
 
 ii 
 
146 MICROSCOPICAL PRAXIS. 
 
 The posterior principal focus of Zeiss's variable low- 
 power objective A*, and that of Queen & Co.'s 2 inch, 
 are so far outside of the combination of lenses forming 
 the objective that the reader may measure the distance 
 for his own satisfaction. For this purpose a bright 
 light is needed, either sunlight or the parallel rays from 
 the Acme lamp, from the Stratton Illuminator, or light 
 from a lamp-flame made parallel by the interposition of 
 a bull's-eye lens. The objective is then held with the 
 front lens toward the source of illumination, which 
 should be placed at the opposite side or end of the 
 room, and the light focussed on the wall. The focal 
 point will be represented by a minute, circular dot of 
 light of great intensity and purity, the little spark being 
 smaller than the head of the smallest pin. By moving 
 the objective toward or from the wall, the exact focus 
 may be readily obtained, as a slight movement will be 
 sufficient to increase the size of the spot, and greatly to 
 decrease its brilliancy. The distance from the front 
 surface of the front lens to the wall, will be the dis- 
 tance of the posterior principal focus. This distance 
 cannot be obtained in this way with any microscope ob- 
 jective higher in power than the two-inch. 
 
 As it is not always possible, and not often conven- 
 ient, to ascertain the optical tube-length by actual 
 measurement, Mr. A. Ashe has disc9vered a method of 
 learning this optical distance by a simple act of obser- 
 vation and an equally simple calculation. By this plan 
 the microscopist may know what optical tube-length- he 
 has in practical use without, as Mr. Ashe remarks, put- 
 ting his finger at certain points of the body-tube and 
 saying, Here is the posterior principal focus of the ob- 
 jective, and here the anterior principal focus of the eye- 
 piece, and then measuring the distance between them 
 
MICROSCOPICAL PRAXIS. 147 
 
 by a foot-rule. To know this optical distance, how- 
 ever, is always of great interest, often of much import- 
 ance. For instance, by Mr. Ashe's admirable method, 
 * I have learned that with a certain combination of ob- 
 jective and ocular the actual optical tube-length is 
 only a single inch, a fact that explains certain features 
 in connection with this special objective that were 
 otherwise more than obscure. 
 
 Mr. Ashe's method is, in his own words, as follows. 
 A careful estimate is made of the power of the micros- 
 cope with the draw-tube pushed home as far as it will 
 go; then having determined this, the eye-piece is with- 
 drawn three or four inches, the exact amount being 
 noted and the increased power of the instrument re- 
 measured. 
 
 We are now in possession of all the data necessary to" 
 calculate not the actual optical tube-length, but its 
 arithmetical equivalent a distinction to be observed, 
 though the difference is immaterial to the purpose in 
 view. 
 
 As it is a rule in optics that the relative sizes of im- 
 ages formed by a lens at different points in its axis are 
 in strict proportion to the distance of those points from 
 the focus of the lens, we may arrange the following 
 formula: 
 
 where A is the amplification of the instrument with the 
 draw-tube closed; B the distance to which the eye-piece 
 has been withdrawn; C the increase in power produced 
 by the effect of B. D is therefore the equivalent of the 
 distance separating the focus of the objective from the 
 anterior focal plane of the ocular. 
 
 To illustrate this simply, suppose an instrument mag- 
 
148 MICROSCOPICAL PRAXIS. 
 
 nifies 100. and that on withdrawing the eye-piece three 
 inches, the power is found to be increased to 130, the 
 equivalent of the tube-length will be, by the foregoing 
 rule, 5^p =IO , or ten inches. 
 
 A simplification of this method has recently been 
 recommended by its author. It is as follows: 
 
 Instead of measuring the power of the microscope 
 twice over, it is sufficient to place a micrometer, or 
 other divided scale, on the stage, and to count the num- 
 ber of lines that fill the field of view from side to side, 
 then to pull out the draw-tube some inches and repeat 
 the counting. 
 
 Of course the greater the increase in power the fewer 
 will be the number of lines seen. In other words, the 
 number of lines and the magnifying power are in inverse 
 proportion to each other. 
 
 Now. for the purpose in view, it does not matter one 
 iota what the actual powers of the instrument may be, 
 with its draw-tube in various positions, so long as we 
 know the proportion those powers bear to each other, 
 .and this proportion we shall find in the relative number 
 of lines which fill the field of view at the same points. 
 
 Hence (bearing in mind the inversion of the ratio) 
 we may look upon the number of lines counted as 
 though they were the actual powers of the microscope, 
 and proceed at once to apply the formula, thus obtain- 
 ing results which correspond precisely with those given 
 by the more lengthy process. 
 
 An example may be useful: 
 
 Magnifying power with the tube closed, 100 
 
 Magnifying power with the tube extended three 
 
 inches, - X 5 
 
 Increase, 5 
 
MICROSCOPICAL PRAXIS. 149 
 
 Therefore, I -^6 (inches), optical tube-length. 
 Again by the shorter way: 
 Number of lines that fill the field with the tube 
 
 closed, 30 
 
 Number of lines that fill the field with the tube 
 
 extended 3 inches, 20 
 
 Increase, 10 
 
 Therefore, inverting these, ~ = 6 (inches), optical 
 tube-length. 
 
 An objective well corrected for central light is not 
 necessarily well corrected for oblique light, and vice 
 versa. This is especially noticeable with the best, wide- 
 angled, dry objectives, although it holds true with the 
 best of any kind, although probably not to the same 
 extent as with the widest angled dry lenses. Certain 
 immersion-fluids also demand a different adjustment of 
 the objective with central and with oblique light, and 
 again vice versa, all these delicate little points in the 
 manipulation of his best tools being the task which the 
 earnest microscopist must teach himself by observation 
 and by experience. 
 
 A change of eye-pieces will likewise call for another 
 adjustment of the objective. Taking out the two-inch, 
 or A, ocular and dropping in the B, or higher-power 
 eye-piece, will make a change for the worse in the im- 
 age obtained with the best objective. And in this case, 
 to bring back the perfection so much prized by the ac- 
 complished manipulator, will call for a movement of 
 the adjustment-collar further toward the open point, or 
 zero, the reason being that with the shorter (higher- 
 power) oculars the optical tube-length is increased, the 
 effect being similar to that obtainable by the use of a 
 
150 MICROSCOPICAL PRAXIS. 
 
 cover-glass thinner than that for which a non-adjustable 
 objective has been corrected, and calling for the ex- 
 tension of the draw-tube, or for an adjustment of the 
 collar further toward the open point. A return to the 
 use of the lower-power eye-piece will again produce a 
 deterioration in the image, and will call for the shorten- 
 ing of the draw-tube, or the replacing of the adjust- 
 ment-collar at the point toward "closed" at which it 
 was before the higher-power ocular was employed. 
 
 This holds true with the ordinary Huyghenian oculajs, 
 but with the compensating eye-pieces as made by Zeiss, 
 the microscopist is relieved of this necessity since these 
 splendid optical tools are so constructed that the focal 
 plane is at the same place in each, the image being 
 formed in the same relative position with them all. 
 This, aside from the improvement which they produce 
 with even the ordinary high-power objectives, is an- 
 other argument for their use; but the reader should re- 
 member that, while the ordinary achromatic object- 
 tives are improved in performance by the use of Zeiss's 
 compensating eye-pieces, it is only the. high powers 
 that are so improved. The lower powers are affected 
 for the worse. It is therefore to the advantage of the 
 microscopist that wishes to work with high and 
 low powers, to have within his reach both kinds of eye- 
 pieces, provided he desires to get the best images 
 which all his objectives are capable of producing. 
 
 While the compensating oculars of Abbe and Ziess 
 were designed primarily to cancel the slight amount of 
 chromatic aberration remaining in the apochromatic 
 objectives, the permanent position of their focal plane 
 was a secondary consideration, but an exceedingly use- 
 ful one, as by their employment a change in the focus 
 of the optical combination is rendered unnecessary. 
 
MICROSCOPICAL PRAXIS. 151 
 
 When a higher-power compensating eye-piece is substi- 
 tuted for a lower power, no change is necessary in the 
 position of the objective's collar-adjustment. 
 
 Some time ago Mr. Edward Pennock, an accom- 
 plished theoretical optician and expert manipulator of 
 objectives, suggested the construction of a series of 
 oculars that should have one of the good qualities pos- 
 sessed by the compensating, although imitation was 
 not thought of, my impression being that these special 
 "parfocal" oculars were suggested and made before 
 the compensating eye-pieces were put in the market. 
 
 The Latin name, parfocal or equal focus, explains 
 the special quality of these productions, the one in 
 which they resemble the compensating. Such oculars 
 are made by Messrs. Queen & Co., of Philadelphia, and 
 by the Bausch & Lomb Optical Co., of Rochester, N. 
 Y. It would be an improvement if all opticians would 
 adopt the principle, as the microscopist would then gain 
 an important convenience, and as they do not require a 
 different collar-adjustment of the objective with a 
 change from one magnifying power to another, the ad- 
 vantage would be great indeed. 
 
 I have never had the pleasure of examining parfocal 
 eye-pieces with this subject of collar-adjustment in mind, 
 but I am assured by Mr. Edward Pennock that no re- 
 adjustment should be required, and by the Bausch & 
 Lomb Optical Company that with their eye-pieces 
 "when substituting a high power for a low power, it is 
 not necessary to change the collar-correction of the ob- 
 jective; a slight turn of the micrometer-screw (fine-ad- 
 justment screw) is all that is necessary." 
 
 Thus the microscopist that has the privilege of using 
 the parfocal oculars, and has carefully adjusted his ob- 
 jective under another power than that which he desires 
 
152 MICROSCOPICAL PRAXIS. 
 
 finally to use, may make the change with impunity, 
 feeling sure that if the correction has been carefully 
 made, he is losing nothing in the good performance of 
 the lens, although he may not have exercised his eye 
 over the performance of the objective with that special 
 power of ocular. This is an exceedingly great advan- 
 tage in favor of the parfocals, and one that should 
 bring them into more general use. 
 
 There are few exercises in the educating of the 
 microscopist's eye better adapted to teach him how to 
 obtain the best which his objective can give, than this 
 experimenting with the effect on the position of the ad- 
 justment-collar after a change of Huyghenian eye- 
 pieces over wide-angled objectives. An eye sensitive 
 to the usually slight deterioration in the image may be 
 considered a valuable possession by its owner, since 
 with it he will be able to distinguish smaller objects 
 and finer details, than will the microscopist that has 
 neglected to teach himself this accomplishment, and to 
 train his eye to be the valuable servant it should be, 
 and which it will be if its responsible owner will do his 
 part. 
 
 Balsam-mounted diatoms are of course the best ob- 
 jects over which to make the experiments, and the 
 secondary structure of those diatoms is perhaps the 
 most valuable object which may be used as the test. 
 Even with the best of wide-angled objectives it is no 
 light task to see distinctly a fracture passing through 
 this minute secondary structure, yet it can be done, 
 after the necessary preliminary eye-training, a training 
 which will amply repay, in future service, the outlay of 
 time, patience and labor demanded in its attainment. 
 If the microscopist is to enter into the serious investi- 
 gation of any natural object, either for his amusement 
 
MICROSCOPICAL PRAXIS. 153 
 
 only or for the instruction of his fellows, he should con- 
 sider the time well spent which he uses in the most 
 careful study of the action of his objectives with vari- 
 ous immersion-fluids, and under various eye-pieces, and 
 with as many kinds of secondary diatom-structure as 
 he may be able to obtain. The more acute the eye 
 becomes in this work, for it is work of the most deli- 
 cate character, the surer he may be that he is getting 
 the best out of his objectives when he comes to use 
 them over an object about whose structure he is 
 ignorant, and the surer too that he is getting a true 
 image. 
 
 To the diatoms the world owes a debt of gratitude 
 which it can never pay, for it is to the study of them 
 that is due in great part the immense advances made 
 in the construction of the modern objective, and to this 
 the world owes more than perhaps it knows. To sup- 
 ply the amateur microscopist with the^objectives that 
 he has demanded so that he may be able to see a few 
 more striae on a certain diatom, or to see them better, 
 the optician has exerted himself to apply all his optical 
 knowledge and to seek more, so that he may respond 
 to the demands coming primarily from the diatom but 
 through the amateur microscojjist. Without the ama- 
 teur microscopist and his efforts to have his objectives 
 so improved that he might the better resolve his little 
 diatom, the microscopical world would never have had 
 the immersion-objective of the present day, and the 
 great world of medical science would never have known 
 of the cholera spirillum nor of the Bacillus tuberculosis. 
 
 It is not too much to predict that within only a few 
 years the cholera will have become a disease unknown 
 except as it is remembered by the aged physician, or is 
 recorded in the books. Having learned the micro- 
 
154 MICROSCOPICAL PRAXIS. 
 
 scopic cause, the scientific man will before long be able 
 to overcome it, and to stamp out of existance the terri- 
 ble disease which it has provoked. If the microscope 
 had never done anything more than this it would de- 
 serve an immortal fame. But it has done more, and 
 will do still more in the future as the amateur micros- 
 copist shall continue, as he will continue, to stimulate 
 the opticians to renewed efforts to give the advanced 
 worker new and better tools with which to prosecute 
 his investigations. Without the amateur, the micro- 
 scope of to-day would be only a dream of the future; 
 without the diatom and the fussing of the amateur over 
 the striations of that diatom, the microscopical world 
 would still- be lingering near the edge of the dark ages, 
 and histological and hygienic science would yet be in 
 their swaddling clothes. 
 
 But the diatom and its good work have not yet been 
 exhausted. It is only within a short time that the mi- 
 croscopist has suspected that it has a secondary struc- 
 ture a thousand times more delicate than are the striae 
 over which he has labored so long and so hard; and it is 
 still more recently that he has suspected, from the little 
 that he has thus far been able to see, that there is within 
 the secondary a tertiary* structure, after which he is to 
 strive, and for which the optician should begin to pre- 
 pare himself. And all this reacts for the better, and 
 brings about the "higher criticism" in the best sense, 
 for while the scientific world is yet tempted to smile at 
 the enthusiasm of the amateur microscopist it never re- 
 fuses to take advantage of what that amateur offers. 
 
 The microscopist then that trains his eye, and studies 
 the action of his objective under various conditions, is 
 not wasting his time. The more he can see of the in- 
 exhaustable microscopic world and the better he can 
 
MICROSCOPICAL PRAXIS. 155 
 
 see it, the happier will he himself be, the better in- 
 formed will be his successor and the better prepared to 
 take up the work when his predecessor shall have fal- 
 len on sleep. 
 
 To ascertain the initial power of the objective, that 
 is, the power without the eye-piece, make the body-tube 
 about ten inches long from the back lens, focus the mi- 
 croscope on a stage-micrometer ruled to hundredths of 
 an inch for use with powers up to the one-fourth or the 
 one-fifth, to thousandths of an inch for higher powers, 
 and place the instrument in a horizontal position. Re- 
 move the ocular, and over the body-tube fasten a piece 
 of translucent paper, or a slip of finely-ground glass, 
 and if necessary re-focus until the micrometer-lines are 
 distinctly seen on the paper when strong light is re- 
 flected up the tube. It is usually necessary to use di- 
 rect sunlight, although strong lamp-light will some- 
 times answer the purpose. Mark the lines on the 
 paper, and measure the spaces with a rule divided to 
 tenths of an inch. If the micrometer one-hundredth 
 inch spaces become one-tenth inch on the paper, the am- 
 plification is represented by ten; if five-tenths, fifty, each 
 tenth representing an amplification of ten. If the one- 
 thousandth inch spaces on the micrometer are used 
 with high-powers, as they must be, and this space be- 
 comes on the paper one-tenth inch, then each tenth will 
 represent an amplification of one hundred. 
 
 Mr. E. M. Nelson has suggested another method 
 which may be more accurate, but which is scarcely as 
 convenient. He describes it as follows: 
 
 In practically measuring the power, it will be found a 
 more accurate plan to increase the distance [that is, 
 from the objective to a screen, or to the paper on the 
 end of the draw-tube], to, say, 60 inches, and to di- 
 
156 MICROSCOPICAL PRAXIS. 
 
 vide the result by six. These measurements are very 
 easily performed when one has a camera, but it is not 
 so easy to do them without. Therefore, another and 
 somewhat" loose way of getting at the initial power, 
 that is, the power of the objective alone, is as follows: 
 Measure the combined magnifying power of the object- 
 ive and, say, the two-inch, or A^ eye-piece, and divide 
 the result by 5. This method would do very well if 
 the multiplying power of the eye-piece was 5, and if the 
 length of the body remained constant. As it is not an 
 easy matter to find out the exact multiplying power of 
 an eye-piece, Mr. Nelson recommends any one desir- 
 ous of knowing this to measure, or to get measured, 
 the initial power of one of his objectives; then measure 
 the combined power of this lens and the eye-piece, pay- 
 ing great attention to the tube-length during the oper- 
 ation. This will give him once for all the multiplying 
 power of his eye-piece with that tube-length. He 
 will then be in a position to ascertain the initial power 
 of any other lens with that eye-piece and the same tube- 
 length. But as the optical tube-length may differ from 
 the actual tube-length, and does differ to a certain ex- 
 tent with objectives of ordinary construction, the pro- 
 cess is not so simple as it seems. In order to get fairly 
 accurate results with the higher powers, a certain per- 
 centage must be deducted. To give some examples: 
 Thus the one-inch objective at 60 inches from a 
 screen increases the image of o.oi inch to 0.66 inch, its 
 power therefore is 66, which at 10 inches becomes n, 
 or the initial power. The combined power of this lens 
 with the A eye-piece is 55, which gives 5 as the multiply- 
 ing power of the eye-piece. Now if the combined power 
 of this eye-piece with a^ inch objective is 75, we may 
 assume that the initial power of the YI> is 15. 
 
MICROSCOPICAL PRAXIS. 157 
 
 If however, we treat higher powers in the same way, 
 we shall get too high values. Thus the combined 
 power of a ^ and the eye-piece is 203; dividing by 5 we 
 get 40.6 as the initial power whereas 39.3 is the real 
 power. 
 
 Again, the combined power of a certain -fa and the 
 eye-piece is 600, which divided by 5 gives 120 as the 
 initial power, whereas it is in reality 113.2. The 
 empirical rule employed by Mr. Nelson is to deduct 2 
 per cent, for the ^ inch objective; 3 per cent, for the 
 % ; 4 per cent, for the -J-; 6 per cent, for the -J-, the fa, etc. 
 Thus, taking the y mentioned and deducting 3 per 
 cent, from the 203 we get 197, which divided by 5 gives 
 39.4, a result very near the truth. 
 
 A certain -J gives a combined power of 450; deduct 
 6 per cent, and we have 423; dividing by 5 gives 84.6, 
 the actual power being 85. For short bodies of 6^ in- 
 ches in length, or the Continental size, a different per- 
 centage must be employed. The following gives fair 
 results: 2 per cent for y 2 \ 4 per cent for ^; 6 per cent 
 for |-, 8 per cent for -J-; and 10 per cent for fa. 
 
 In reference again to magnifying power, Mr. E. M. 
 Nelson says in another and more recent paper, that the 
 limit of combined power for best definition maybe found 
 for any objective of any given aperture, by multiplying 
 its N. A', by 400, stating as an example, that the limit 
 of power for best definition with a z /z inch of 0.3 N. A. 
 is 1 20 diameters. 
 
 If, in this case, the ^ inch is truly named it will have 
 the initial power of 15 diameters. To acrtain the 
 highest powers that may be applied in the eye-piece to 
 be used, divide the 120 diameters by the 15 and the re- 
 sult, 8, will be the required amplifying power of the 
 occular needed to work the objective up to its limit of 
 
158 MICROSCOPICAL PRAXIS. 
 
 good definition, according to Mr. Nelson's theory, which 
 is doubtless correct. 
 
 To Measure the Focal Length of an Objective. 
 
 To ascertain this for an objective whose box or 
 mounting has not been marked by the maker, a rare 
 occurrence, or when it is suspected to be incorrectly 
 named or under-rated, a method is to multiply the tube- 
 length by the quotient obtained by dividing the dia- 
 meter of the object by that of the image. If F is focal 
 length, / the tube-length, D the diameter of the image, 
 and d that of the object, the formula will be, F (^)/; 
 and if the stage-micrometer is ruled to hundredths of an 
 inch, the body-tube ten inches long, and one micrometer- 
 space used as the object becomes five tenths inch on 
 the paper at the top of the tube, then J?=(?*) IO O r| 
 inch. 
 
MICROSCOPICAL PRAXIS. 159 
 
 High Magnifying Power. 
 
 The highest-power objectives ever made are a one- 
 seventy-fifth by the late R. B. Tolles, of Boston, and a 
 one-eightieth by Powell and Lealand, of London. The 
 immense amplification obtainable by such objectives is 
 chiefly a curiosity. It has been said that anything 
 more miserable than the one-seventy-fifth need not be 
 desired, a statement that I can readily believe. No 
 discoveries have ever been reported as having been 
 made with it, so far as I know, and few observations 
 have been described. In reference to the Powell and 
 Lealand one-eightieth I have heard nothing. 
 
 The use of such objectives calls for the highest skill 
 of the microscopist, and to see anything with them de- 
 mands more than ingenuity. No cover-glass can be had 
 thin enough to allow the -fa to act through it; mica 
 films are necessary. 
 
 A mistake often made by the amateur is to use too 
 high a magnifying power. Mere size in the image is 
 not a necessary nor even a useful feature. It is rather 
 a detriment. The microscopist should use the lowest 
 magnifying power compatable with distinctness of the 
 image, and with the requisite separation of minute and 
 contiguous parts. The image is best when it is clear 
 and vivid, rather than huge and foggy. It is separation 
 of closety contiguous parts, not the magnitude of those 
 parts, that the eye needs and appreciates. 
 
 It is probable that 2,000 diameters represent the 
 limit of useful amplification. Very much more has 
 been used but not with praiseworthy results. 
 
l6o MICROSCOPICAL PRAXIS. 
 
 Immersion-objectives. 
 
 Immersion-objectives are those that require a drop of 
 liquid between the front lens and the cover-glass when 
 in use. Among the advantages obtainable by their em- 
 ployment are increase of working-distance and increase 
 of numerical aperture, with, as a consequence, the ad- 
 mission of more light, and especially the partial, or in 
 some cases, the complete extinguishment of the cover- 
 glass, by which its aberrations are obviated and two re- 
 flecting surfaces, those of the cover and of the front 
 lens, are cancelled. The honor of discovering the prin- 
 ciple is usually conceded to the renowned Italian pro- 
 fessor, G. B. Amici, who exhibited water-immersion ob- 
 jectives at Paris in 1855. It is also said that he used 
 oil as well as water for the immersion-medium, and that 
 he therefore deserves the credit for originating oil-im- 
 mersion objectives. For the modern homogeneous-im- 
 mersion, however, we are indebted to Mr. J. W. Steven- 
 son, a well-known British microscopist. It was he who 
 suggested to Professor Abbe and to Dr. Zeiss that they 
 should turn their attention to the theoretical and the 
 practical application of the principle; it is to him, there- 
 fore, that we owe this great modern advance in practi- 
 cal microscopy. 
 
 The use of immersion-objectives is attended by a lit- 
 tle more inconvenience than that of dry objectives, and 
 the immersion fluid is likely to be carried over the edge 
 of the cover by the movements of the stage, and so is 
 liable to mingle with the mounting medium in those 
 preparations which are not permanently sealed. This 
 annoyance may be avoided to a great extent by using 
 square covers somewhat larger than the cement ring 
 enclosing the object, the movements of the stage then 
 bringing the ring indistinctly into view, and the pro- 
 
MICROSCOPICAL PRAXIS. l6l 
 
 jecting borders of the square protecting both the im- 
 mersion-fluid above it, and the object beneath. 
 
 The front lens of these 'objectives must also be care- 
 fully cleaned and dried after the immersion-fluid has 
 been as carefully applied. But the advantages obtain- 
 able more than counterbalance the inconveniences. 
 
 Tlie fluid is always applied between the lens and the 
 cover-glass. It is difficult to imagine any human being 
 endowed with such unmitigated stupidity, that he should 
 pour the immersion-liquid into the tube of the lens- 
 mounting, yet instances of the kind have been reported. 
 
 It is recommended by some that the water, glycerine, 
 oil or other fluid be applied in a small drop to the cover, 
 and the objective racked down until the front lens 
 comes in contact with it. The only advantage of this 
 method, and that advantage is very slight, is that by it 
 the probability of disarranging the object by the pres- 
 sure of the thick liquid compressed between the cover 
 and the lens, is lessened; and the reader may prefer this 
 method, especially when using glycerine-immersion or 
 homogeneous-immersion objectives, but a great dis- 
 advantage is that the moment the objective touches 
 the liquid, the microscopist loses the power to appreci- 
 ate the distance between the lens and the cover, and is 
 therefore likely to rack down too far and so to do some 
 damage, or not far enough and thus leave too much to 
 be done by the fine-adjustment screw. 
 
 When using glycerine or homogeneous-fluid, I am in 
 the habit of applying a drop to the front lens of the objec- 
 tive instead of to the cover-glass. This may be done by 
 means of the cork from the bottle of fluid, a drop 
 being allowed to form at one edge, whence it is care- 
 fully placed on the objective without touching the cork 
 to the lens-front; or a rod may be forced through the 
 
 12 
 
I 62 MICROSCOPICAL PRAXIS. 
 
 cork and the drop adhering to this applied to the lens. 
 One leg of a rubber hair-pin forced through the cork is 
 useful for the purpose, as none of the chemical liquids 
 used for immersion purposes will act on it, as some of 
 them will act on a metal wire. When the drop has been 
 applied to the front of the lens, the objective is attached 
 to the body-tube and racked down until the fluid 
 touches the cover; and as the microscopist looks across 
 the slide, between the cover and the lens, the objective 
 is still further lowered while the lessening distance and 
 the expansion of the fluid are watched, the expansion 
 being continued until the objective is supposed to be 
 approximately focussed, when the fine-adjustment fo- 
 cusses it upward or downward to the proper point. 
 These movements demand exceedingly great delibera- 
 tion and caution; deliberation so that the object may 
 not be disarranged by the sjow expansion of the thick 
 immersion-fluid, if it is not permanently -mounted, and 
 caution that the objective be not injured, for immersion 
 lenses are easily disordered. Their lenses are larger 
 than those of smaller angled dry objectives, their con- 
 struction is more delicate, and they must be treated 
 with more care. 
 
 When a water-immersion is to be used, that is, an ob- 
 jective with which water is the immersion liquid, I am 
 accustomed to focus it as a dry objective, as may easily 
 be done, although the definition will probably be abomi- 
 nable and the field -dimly lighted, yet enough may be 
 seen to show that the desired object is in view. With 
 a camel's-hair brush a drop of water is then added to 
 the cover near the edge of the objective, under which 
 it will run by capillary attraction. Here all danger of 
 forcing the object out of position or of injuring the ob- 
 jective or the cover, is with ordinary caution, reduced 
 to nothing. 
 
MICROSCOPICAL PRAXIS. 163 
 
 To clean the front of a water-immersion, after using 
 it, the careful employment of the Japanese filter-paper 
 is all that is needed. To remove the glycerine when 
 used by itself or in combination with a salt, as in the 
 homogeneous-immersion fluids, I am accustomed to 
 wipe away the greater portion with the Japanese paper, 
 and to remove the rest with a few touches. of the 
 tongue, finishing with the dry paper. 
 
 The cedar-oil used with homogeneous-immersion 
 lenses is thickened with dammar, so that a touch of the 
 moist tongue is likely to cause a deposit of some of 
 the gum on the lens, and to necessitate repeated appli- 
 cations of the paper moistened with alcohol to remove 
 it.* It is better with this liquid to employ the paper 
 alone, and to finish with another piece moistened with 
 alcohol, and to wipe the lens dry and perfectly clean 
 with still another piece. 
 
 It is always well to clean an immersion-objective as 
 soon as possible after using it. I have known instances 
 in which the front lens was so insecurely burnished into 
 the metal cell that the fluid has penetrated to the back 
 of the glass, and made necessary a journey to the manu- 
 facturer. When the objective is to be out of use for 
 only a short time, it should be placed on the table with 
 the glass surface upward, and when the evening's work 
 is finished, it should be carefully returned to its brass 
 box, after a scrupulously neat cleaning. 
 
 When about to measure the angular aperture of an 
 immersion-objective the front must of course be im- 
 mersed in the proper medium. This may be done by 
 applying a thin cover-glass to the front lens by means 
 of a drop of its special fluid. It is better however, to 
 estimate the numerical aperture by the method already 
 described. 
 
 * Since this was written I have received from the Bausch & Lomb Optical Com- 
 pany, a supply of cedar-oil not open to this objection. 
 
164 MICROSCOPICAL PRAXIS. 
 
 To Measure the Refractive Index of the Im- 
 mersion-fluid. 
 
 The kinds of homogeneous-immersion media are 
 numerous, the object of all being to reach a refractive 
 index as nearly as possible that of the front lens of the 
 objective. The basis is generally pure glycerine in 
 which is dissolved, usually by the aid of heat, various 
 -chemical salts, among which are chloral hydrate, zinc 
 sulpho-carbolate, cadmium chloride, zinc iodide, dis- 
 tilled zinc-chloride, and perhaps others. The micros- 
 copist himself may prepare any of these fluids, but he 
 "would better buy them of the optician that makes his 
 objectives. He will thus not only save himself trouble, 
 but he will be sure that the index of the fluid will be 
 as nearly correct as may be. Upon the proper condi- 
 tion of the immersion-fluid depends the proper action 
 of the objective, the best performance of the best lenses 
 being defeated by a liquid that is not of the correct re- 
 fractive index. 
 
 In any event it is important that the medium should 
 have a refractive index as nearly like that of liquid 
 crown-glass as possible, and for the purpose of learning 
 its condition in this respect, several devices have been 
 suggested. With his cedar-oil, Zeiss sends out a bottle 
 with flattened sides, to the stopper of which is attached 
 a glass wedge which is to be immersed in the medium 
 and held against the light, when certain appearances in 
 a distant object will show when the correct refractive 
 index has been attained. Not long ago Prof. Hamilton 
 
MICROSCOPICAL PRAXIS. 
 
 165 
 
 Fig. it. H. L. Smith's test-slide 
 for refractive index. 
 
 L. Smith, the well-known American microscopist, 
 devised a little instru- 
 ment which is more con- 
 venient and more accur- 
 ate than Zeiss's. It is 
 shown in Fig. n, and is 
 described by its inventor 
 as follows. 
 
 It consists of a brass 
 adapter with the society screw above it to attach it to 
 the body-tube of the microscope, and one below it to 
 receive an objective, the one-inch being generally used 
 with it. Through openings in the sides two glass slips 
 are inserted, one having a polished cavity near the end. 
 These slips are of crown-glass with a refractive index 
 as nearly as possible that of the cover-glass. To test 
 the medium, a drop is put in the concavity, the slips 
 placed together, and inserted in the adapter above the 
 objective. The medium passes between the two by 
 capillary attraction, and the microscope is focussed on 
 an object, the microscopist looking through the slips 
 and the interposed medium. The focus will not differ 
 with or without the glass slips, and when the concavity 
 is pushed directly over the objective, if the medium be 
 optically homogeneous with the slips, the focus will 
 need no change and the definition will be unimpaired. 
 But if the medium has not the proper index, white the 
 focus may need no alteration, the outlines of the object 
 will be surrounded with colored fringes. 
 
 If the focus has been obtained by means of the rack 
 and pinion, the fine-adjustment always remaining the 
 same, one can readily ascertain, in the following way, 
 the refractive indices of the various media proposed for 
 use with immersion-objectives. Let a mark be placed 
 
l66 MICROSCOPICAL PRAXIS. 
 
 on the rack-bar or on the sliding draw-tube, as the case 
 may be, when the focus is obtained with the glass slips 
 in the position shown in the figure (Fig. n); this mark 
 will indicate, for example, a refractive index of 1.52, or 
 that of crown-glass. Filling the concave now with 
 cinnamon-oil, and focussing again (using the same ob- 
 jective and eye-piece), we get another position for a 
 mark indicating a refractive index of 1.6, the index of 
 cinnamon-oil. Using water we get still another, 1.33; 
 and glycerine 1.41; the extremes will be about half an 
 inch apart, as measured by the bar or by the tube, and 
 by interpolation, we can thus get pretty nearly the re- 
 fractive index of any fluid medium. Prof. Smith re- 
 ports that he has found the so-called homogeneous 
 media sold in the shops to differ very greatly, fully one- 
 fourth of an inch out of the way in many cases. 
 
 When one has a fine objective and with a certain 
 immersion-fluid has obtained certain positions of the 
 adjustment-collar for the best work on certain tests, 
 the exact refractive index of the medium can be ascer- 
 tained by this instrument, and afterwards always 
 secured. A non-adjustable immersion-objective, a one- 
 eighth by Spencer, performed most admirably, 
 both with oblique and with direct light, using the 
 medium furnished by the maker, but showed indiffer- 
 ently well with another medium which, on being tested 
 with this little apparatus, required an alteration of 
 focus necessary to obtain distinct vision, or rather the 
 most distinct vision, of fully one-fourth of an inch. On 
 diluting the second medium to bring it to the same in- 
 dex as that sent out by the maker, the performance 
 was entirely satisfactory. It will be understood that 
 there should be a diaphragm in the adapter of such a 
 size that it shall prevent the passing of any light except 
 
MICROSCOPICAL PRAXIS. 167 
 
 what actually passes through the fluid when the con- 
 cavity containing the immersion-fluid to be tested, is 
 put over the objective. 
 
 x- 
 
 To Focus an Objective. 
 
 This has already been incidentally referred to, but it 
 may well be repeated, as among his first lessons the am- 
 ateur or the novice should learn how to focus his ob- 
 jectives properly, everyone of which, it does not matter 
 whether it be a five-inch or a one-fiftieth inch, should 
 without exception be focussed in the same way. Some- 
 times there are two ways of doing the same thing. 
 Here there is but one. It is this: In no circumstances 
 should the objective be focussed by racking the body- 
 tube down while the microscopist is looking through 
 the instrument. Always look across the objective, be- 
 tween the front lens and the cover-glass, or the slide, 
 while the body-tube is carefully racked downward until 
 the front of the high-power objective is almost in con- 
 tact with the cover; then, while the eye is at the eye- 
 piece, rack the body upward until the focus is approxi- 
 mately obtained, finishing with the fine-adjustment. 
 Proceed in this way always, and at all times, and in all 
 circumstances, even though it should be necessary to 
 place your ear flat on the table-top whilst you look 
 across the objective-front toward the light, and it will be 
 
l68 MICROSCOPICAL PRAXIS. 
 
 to the amateur's advantage. It will be especially advan- 
 tageous to the novice, if he will from the very first, cul- 
 tivate the habit of racking the body-tube upward when- 
 ever the slide is to be taken from the stage. This good 
 habit will soon become automatic, and if both these 
 rules be observed, there will be no danger of breaking 
 or of scratching the soft glass forming the front lens of 
 the objective, nor of injuring the slide. The latter 
 might not be of any importance, whereas the breaking 
 of an objective is a disaster that cannot be remedied. 
 
 The Maltwood Finder and Similar Devices. 
 
 Reference has already been made to the difficulty of 
 finding a small object, or some special part of a larger 
 object, with a high-power unless a mechanical stage be 
 used. The field of the high-power objective is so 
 small that the chances of bringing the desired object 
 within its circumference are slight. Usually it is neces- 
 sary to substitute a low-power lens, bring the specimen 
 within its field, then to re-attach the higher-power ob- 
 jective, in or near whose field the object should be. 
 
 On mechanical stages there are commonly engraved 
 two sets of lines about one one-hundredth inch apart, 
 the scales- resembling those of a micrometer. These 
 are intended to facilitate the finding of the object the 
 second time, the objective being noted, and the posi- 
 tion of the stage recorded as read from the horizontal 
 and the vertical scales on its surface. When the stage 
 
MICROSCOPICAL PRAXIS. 169 
 
 is again placed in those positions, the same objective 
 used, and the slide laid on the object-carrier as it was 
 before, the object should then be in the field. 
 
 Several devices have been suggested for the conveni- 
 ence of those microscopists that do not possess a me- 
 chanical stage. These consist of lines photographed 
 ofruled on a glass slip, their number usually being 
 great and the spaces between them small, each of the 
 latter bearing one or more figures. The best known of 
 these finders is Maltwood's, a glass plate bearing 
 twenty-five hundred squares, so numbered that the 
 position of an object may be recorded by recording the 
 numbers within the space over which it may be when in 
 the field of a certain objective. 
 
 The object is brought into the centre of the field of 
 view, when the slide is removed and the Maltwood 
 finder substituted. The numbers on the square now 
 occupying the position previously occupied by the ob- 
 ject are noted, and whenever this special square is 
 again brought into the field of that objective, the stage 
 will be in a position to bring at once the desired object 
 into that field. 
 
 The Condenser. 
 
 The sub-stage condenser is the most important ac- 
 cessory that the microscopist can have, especially if he 
 intends to work with first-class, wide-angled objectives. 
 Its purpose is to supply a wide and solid cone of light, 
 
170 MICROSCOPICAL PRAXIS. 
 
 not primarily to illuminate the object, but to give the 
 very light of life to the objective itself. Without a 
 modern, wide-angled condenser the microscopist is 
 badly handicapped, although he may possess the best 
 objectives to be had for money. The condenser brings 
 out the good points of the lenses in a surprising way, 
 and it also reveals with as painful distinctness, the bad 
 qualities of the same lenses. 
 
 The microscopist should purchase the best condenser 
 he can find at the opticians', and attach it permanently 
 to his stand. It may be used to increase the illumina- 
 tion until the eye refuses to endure it, or the light, by 
 its means, may be reduced to the faintest glimmer. By 
 its means again, if it have the proper angular aperture, 
 the whole aperture of the objective may be filled by a 
 solid cone of rays; and by the use of the proper dia- 
 phragms, or by moving the entire condenser laterally, 
 illumination of the greatest obliquity may be obtained 
 for the resolving of tests, or for the study of obscure 
 structures of a certain character. 
 
 The defining and the resolving powers of the ob- 
 jective are improved by its use; indeed, the best, high- 
 est-power, homogeneous-immersion lenses will not do 
 themselves even partial justice without the use of wide- 
 angled condensers now fortunately becoming common. 
 The apparatus is a combination of two or more 
 lenses forming an instrument not unlike a greatly 
 enlarged objective. It is fitted to the sub-stage ring 
 so that it may be accurately centred, a condition ab- 
 solutely essential to its best performance, and so that 
 it may be moved upward or downward to bring the 
 light from the mirror to a focus on the object, or to re- 
 move it beyond the focus so as to reduce the intensity 
 of the illumination, although this method of using it is 
 
MICROSCOPICAL PRAXIS. 171 
 
 not to be unreservedly commended with wide-angled, 
 high-power objectives of the present day. Some me-' 
 thod, however, of changing its position vertically, either 
 by rack and pinion or by direct finger-movements, is 
 absolutely necessary. 
 
 It is always accompanied with diaphragms to reduce 
 the size of the illuminating cone, to obtain light of 
 great obliquity or black-ground illumination. In the 
 best condensers these diaphragms are applied below 
 the lenses, and this should always be their position. In 
 no circumstances should they be above the front lens 
 of the apparatus. 
 
 Authorities on the subject recommend that the con- 
 denser should never be used without being accurately 
 focussed. Dr. W. H. Dallinger says that it should al- 
 ways be racked upward until, when the microscopist 
 looks down the body-tube after the eye-piece has been 
 removed, the back lens of the objective is three-fourths 
 full of light. M. H. Peragallo, in the "Annales de Mi- 
 crographie," gives an elaborate discussion of the 
 theoretical principles involved in microscopical illum- 
 ination, and states that the back lens of the objective 
 should be only one-third filled with light. Dr. 
 Dallinger's teaching calls for more (three-fourths), and 
 the microscopist that attempts to use his objective thus 
 illuminated will have an experience that will teach him 
 that, in ordinary circumstances, the method is imprac- 
 ticable. According to Dr. Dallinger's contention, 
 which is here correct, if the condenser, after having 
 been focussed, be racked a little further upward while 
 the microscopist is still looking at the back lens of the 
 objective, there will soon appear on each side of the 
 illuminated disk a little dark spot showing that the 
 apparatus has been raised too high, and is within the 
 
IJ2 MICROSCOPICAL PRAXIS. 
 
 focus, as the late Dr. W. H. Carpenter wrongly ad- 
 vised the microscopist to use it. The proper position, 
 according to Dr. Dallinger, is said to be that reached 
 just before the dark spots appear; that is, the conden- 
 ser is then focussed. 
 
 This is undoubtedly the proper way in which to use the 
 apparatus, but it is an utter impossibility so to use it, 
 unless the microscopist have some way of reducing the 
 terrible intensity of light which will then pour into his 
 eye. This reduction is to be obtained, not by racking the 
 condenser downward, or beyond the focus, which would 
 be using it, as well as the objective, at a disadvantage 
 if the objective is a first-class lens, but by the employ- 
 ment of properly colored glass to be placed between 
 the lamp and the mirror, or between the mirror and the 
 condenser, the correct color being double cobalt-blue, 
 a kind of glass not to be had in this country, and until 
 recently scarce even in Europe. From this Dr. 
 Dallinger recommends that screens be made and 
 placed between the mirror and the source of light, so 
 to modify the illumination that the eye may endure it. 
 Or two disks of this special glass may be prepared and 
 inserted in the sub-stage below the condenser, with the 
 same purpose in view. 
 
 If used as M. Peragallo recommends, the conden- 
 ser must also be supplied with the blue glass to modify 
 the illumination, although the advanced microscopist 
 may be able to endure the unmodified light, since his 
 eye has, by practice, become less sensitive to such im- 
 pressions while it has also become better able to ap- 
 preciate minute points, delicate details and sharply de- 
 fined outlines. But while it may be possible or neces- 
 sary to use only one-third of the back lens, or even less, 
 doing so would not be wording the objective, nor the 
 
MICROSCOPICAL PRAXIS. 173 
 
 condenser, up to the best, if condenser and objective 
 are first-class. With the French microscopist's method 
 we should be losing in two particulars, yet while his 
 recommendation may be put into practice, Dr. Dallin- 
 ger's can not, unless the right kind of blue glass can be 
 obtained to reduce the terrible intensity of the illumina- 
 tion from the modern, wide-angled condenser. But the 
 only way in which, at present, the teaching of the expert 
 British authorities can be followed is to send to 
 Messrs. Powell and Lealand, the well-known opticians 
 of London, and import the blue glass, as they are the 
 only dealers who, so far as I know, supply the material 
 proper for the purpose. 
 
 It is indeed possible to use a wide-angled condenser 
 when almost in focus, if we employ a blue glass chim- 
 ney to the microscope-lamp, and put below the con- 
 denser three thicknesses of the blue glass as supplied 
 with the "Acme lamp," by Messrs. Queen & Co., and by 
 Messrs. James Stratton & Son with their "Stratton 
 Illuminator." By this means a one-tenth inch objec- 
 tive, or higher power, may be used with the condenser 
 accurately focussed; but with the one-fifth or with 
 lower powers the light must still be modified by racking 
 the condenser downward, or preferably by using dia- 
 phragms. The unprotected eye can not endure the 
 intensity of the light under these conditions, and if 
 more blue glass be inserted into the sub-stage, the illu- 
 mination becomes too much weakened, and the image 
 is deteriorated. 
 
 That the condenser will give us its best service when 
 carefully focussed under our first-class objectives, there 
 can be no doubt; and if the American microscopist can 
 modify the light without depriving it of any of its good 
 qualities, he should always use the accessory in that 
 
174 MICROSCOPICAL PRAXIS. 
 
 position. But I fear he will not be able to do so with 
 all objectives, unless the opticians come to his help, as 
 Messrs. Powell & Lealand seem to have come to the 
 help of the British microscopists, and give him the 
 right kind of blue glass. But in the present circum- 
 stances, the advantages of using the condenser when 
 accurately focussed are so great, that it seems best to 
 keep it in focus and to reduce the intensity of the 
 illumination by diaphragms. This will reduce the 
 angle of ,the illuminating cone, which is not always a 
 commendable thing to do, but at present is apparently 
 unavoidable. To get the greatest benefit to be ob- 
 tained from a condenser, the object should be exactly 
 at the apex of the illuminating cone issuing from the 
 front lens of the apparatus. 
 
 The microscopist may be certain that the condenser 
 is focussed even while looking through the instrument, 
 and without removing the eye-piece to gaze down the 
 body-tube. To do this, the condenser, after the ob- 
 jective has been focussed, is gently racked upward, 
 while the eye is at the ocular, and the ascent continued 
 until the dust-particles on its front lens are visible in 
 the field as blue or blue-bordered objects which are 
 rather indistinct yet plainly visible. These dust-parti- 
 cles are sometimes so conspicuous that they become 
 menacing annoyances in observations with high-powers, 
 and the front lens of the condenser must be dusted with 
 a camel's-hair brush, or racked slightly below the focus. 
 It will also sometimes happen that when the apparatus 
 is used with low powers, or when it is occasionally 
 racked far downward, that dust-particles and finger- 
 marks on the blue-glass in the sub-stage will become 
 apparent, and may possibly mislead the observer into 
 thinking that the instrument is in its proper position. 
 
MICROSCOPICAL PRAXIS. 175 
 
 The widest-angled condensers for use with the 
 widest-angled homogeneous-immersion "objectives, are 
 so made that they may be employed in immersion con- 
 tact with the lower surface of the slide. A drop of 
 glycerine or of homogeneous-immersion fluid is placed 
 on the upper surface of the condenser and brought into 
 contact with the lower surface of the slide. Such 
 immersion-condensers must necessarily be used in 
 focus; and with them the intensity must be in some 
 way decreased, since the normal eye can not endure it 
 when unmodified. 
 
 There are two forms of the accessory in the optical 
 market, one with no corrections for the spherical and 
 chromatic aberrations, the other carefully corrected 
 for both of these troublesome things. The most popu- 
 lar form is Abbe's, and belongs amongst the uncor- 
 rected, although in recent years its accomplished in- 
 ventor has issued a corrected condenser which he 
 recommends for use in microscopical photography. It 
 is also commendable for ordinary, every-day use, as the 
 uncorrected form adds an immense amount of spherical 
 aberration for the objective to contend against, so that 
 diaphragms should always be used with it to cut off 
 some of the uncorrected marginal rays. 
 
 In reference to the uncorrected accessory Prof. Abbe 
 says: "The condenser is not made achromatic for the 
 reason that, for the effect contemplated, it would be 
 altogether useless to seek to obtain a sharp image of 
 the cloud or other source of light, as it is in like 
 manner quite immaterial whether the image is formed 
 precisely on a level with the object, or somewhat above 
 or below it." Several British microscopists take issue 
 with Prof. Abbe on these points, and insist that the con- 
 denser should be corrected^ and as Prof. Abbe has re- 
 
176 MICROSCOPICAL PRAXIS. 
 
 cently made a corrected form of his own celebrated 
 apparatus, it would seem that he has changed his 
 opinion. 
 
 Messrs. Powell and Lealand, of London, make an oil- 
 immersion condenser of wide angle and a high price; 
 Watson and Son, also of London, make an excellent con- 
 denser which is corrected, its angle being i.o N. A. 
 against 1.40 of the Powell and Lealand form. In this 
 country, Messrs. Bausch and Lomb, of Rochester, N. Y., 
 make an achromatic form of the Abbe condenser, as 
 well as another which is not corrected. 
 
 Low-power objectives, those, for instance, below the 
 one-fourth or one-fifth inch, and those of small angular- 
 aperture, do not call for the use of the condenser. 
 Sufficient illumination, generally more than is needed, 
 may be had with these from the concave mirror alone. 
 It is also possible to use an objective of low-power as a 
 condenser, if the sub-stage ring is supplied, as it should 
 be, with an adapter carrying the society screw. The 
 objective, when used for this purpose, is screwed into 
 the adapter with the front lens facing upward, the light 
 being reflected through it from the plane mirror. If a 
 microscopical lamp be used, or a bull's-eye lens be in- 
 terposed between- the lamp and the mirror, the plane 
 mirror should also be used. 
 
 Several diaphragms, which are used below the poste- 
 rior lens, for central, oblique and black-ground illumi- 
 nation, accompany all forms, or should do so. Those 
 for central light have a central opening, the size of the 
 cone of light and the obliquity of its lateral rays vary- 
 ing with the size of the diaphragm-opening employed. 
 For oblique illumination two lune-shaped diaphragms 
 are usually supplied. These are placed in the dia- 
 phragm-carrier, one at a time, of course, in any posi- 
 
MICROSCOPICAL PRAXIS. 177 
 
 tion that may be needed to produce the effects desired; 
 in some the diaphragm-carrier may be rotated. For 
 black-ground illumination those" with the central disk 
 supported by radiating arms are used, but to obtain the 
 effect with wide-angled objectives something more is 
 needed than the use of these special disks. A circular 
 diaphragm must also be placed at the back of the ob- 
 jective. 
 
 The diaphragm-carrier in the American forms of the 
 condenser is usually a sliding plate into whose aperture 
 the various diaphragms or stops are placed, when it is 
 pushed below the lenses until a spring catch indicates 
 that it is properly centred to the condenser; but this 
 has nothing to do with the centring of the condenser 
 to the objective. 
 
 For oblique illumination, the lunate disks are used, 
 the larger when the greater portion of the cone of light 
 is to be intercepted, the smaller when more of the rays 
 nearer the centre are desired. With either size the con- 
 denser will give light of greater obliquity than many ob- 
 jectives will receive. The object, however, may be 
 obliquely illuminated with rays from any direction, 
 either by withdrawing the carrier and inserting the 
 lune-shaped diaphragm in another position, or by ro- 
 tating it, so that the light shall sweep around a circu- 
 lar course. This requires delicate manipulation, an ob- 
 jective of the proper angular aperture to receive light 
 of that obliquity, and very accurate centring of all 
 the parts. It may be done, however, with fine effect in 
 the resolution of lined objects, diatoms for instance, 
 but if the microscopist owns the wide-angled apparatus, 
 and this form of oblique light is to be employed with a 
 dry objective, the front hemispherical lens of the con- 
 denser should be removed and the remainder of the 
 
178 MICROSCOPICAL PRAXIS. 
 
 combination focussed on the object without a dia- 
 phragm. Then insert the lunate disk, and, if all is well, 
 a glance down the body-tube, without the ocular, will 
 show a small double-convex spot of light near one 
 border of the back lens of the objective, with the diffrac- 
 tion spectra also, if they are specially looked for. 
 
 If the sub-stage has lateral movements, as it has on 
 some first-class stands, oblique illumination with the 
 circular diaphragm-openings may be obtained, but some- 
 what less effectually, by moving the entire condenser 
 from side to side. 
 
 In what is called black-ground illumination the ob- 
 ject appears to be self-luminous, gleaming with the 
 vivid radiance of molten silver, seeming to rest softly 
 on a back -ground of the blackest velvet. Living animals 
 appear like creatures fashioned from moonbeams; mi- 
 nute particles shimmer and flash like silver stars; a 
 little heap of colored sand-grains seems a little heap of 
 rubies and diamonds from Sinbad the Sailor's Valley of 
 Gems. And to obtain such exquisite pictures it is only 
 necessary to obstruct the central beam of light by a cir- 
 cular, opaque disk, allowing the object to be illuminated 
 by the light that comes to* it from the periphery. No 
 rays reach the objective directly. All must first enter 
 the object and there be properly refracted or inflected, 
 or after passing through the object, must be thrown 
 back on it by reflection from the cover-glass, so that 
 under the beating of those waves of light it shall appear 
 to glow with a soft intensity indescribable. This effect 
 may be obtained, sometimes better and more easily, by 
 sub-stage apparatus especially intended for the purpose, 
 rather than by any sub-stage condenser. 
 
 For black-ground illumination, the central disk-dia- 
 phragm must be used, and with the one-inch objective 
 
MICROSCOPICAL PRAXIS. 179 
 
 brilliant effects may be produced, with the proper 
 objects. To do this with the one-inch of 33 and the 
 uncorrected Abbe condenser of 1.40 N. A., it is neces- 
 sary to remove the front lens of the condenser, when 
 the effect will be exceedingly fine. Here again if 
 the bull's-eye lens is placed between the mirror and 
 the source of light, the plane mirror is to be used; if 
 the light is taken directly from the lamp-flame, the 
 concave mirror is the proper one to be employed. 
 Black-ground illumination may be obtained with pow- 
 ers of from five-hundred to six-hundred diameters, but 
 according to my experience it is not praiseworthy. 
 
 With the Abbe form of the condenser, Zeiss supplies 
 diaphragms to be applied to the back lens of his wide- 
 angled objectives when^lack-ground illumination is de- 
 sired with them, the proper disk-bearing plate being 
 placed in the diaphragm-carrier, and immersion contact 
 made with the lower surface of the slide. The diffi- 
 culty with any but Zeiss's objectives is to prepare the 
 diaphragms for the back lens. With his they are made 
 of metal, to fit properly when dropped into the mount- 
 ing, and the opening will ttyen be central. But if the 
 microscopist must cut them from paper, he need not 
 expect to obtain the best results. The directions are 
 to drop the diaphragms into the back of the wide-an- 
 gled objective, and then the microscopist is left, by all 
 opticians except Zeiss, to take care of himself. Yet it. 
 is of course impossible for any one optician to supply 
 these little parts, since no two objectives by different 
 makers, of even the same magnifying power, have the- 
 same sized mounting. The microscopist must depend 
 upon himself, and he will speedily observe that when, 
 the aperture is reduced by a diaphragm at the back 
 lens, the defining and resolving powers of the objective,- 
 
l8o MICROSCOPICAL PRAXIS. 
 
 suffer an injurious diminution. The experiment is 
 worth making although no other result than this be ob- 
 tained. 
 
 While black-ground illumination is beautiful, it has 
 little scientific value. I do not know that any. discovery, 
 or even any observation of importance, has ever been 
 made by its use. It will at times exhibit certain struc- 
 tural features in a conspicuous way, but only, I think, 
 after they have been previously observed, for in most 
 cases this peculiar lighting appears to make the struc- 
 ture obscure. It may render the contour lines more 
 distinct, and develop the whole object in a glamour of 
 brilliant beauty, but the microscopist, while he never 
 disdains beauty, never makes it the object of his pur- 
 suit. 
 
 Although oblique illumination may be obtained by 
 the lateral movement of the condenser, if the sub-stage 
 arrangements will allow it, such use of the apparatus is 
 not to be commended. It should, for ordinary work, 
 when oblique light is never wanted, be used with cen- 
 tral illumination, and the more perfectly centred it is 
 the better. To accomplish this, proceed as was de- 
 scribed in connection with the centring of the illumina- 
 ting beam with the objective and the mirror. But it 
 will sometimes happen that while the condenser ap- 
 pears to be centred when it is near the focus, that it 
 will be greatly out of centre when racked downward. 
 In such a common occurrence a slight change in the 
 position of the mirror, or of the lamp, or perhaps of 
 both, is all that will be needed to remedy matters. 
 
 It frequently happens that with even excellent objec- 
 tives, that after they have been carefully adjusted, and 
 the outlines of the image are as narrow and as black as 
 they can be made, that a narrow line of bright light is 
 
MICROSCOPICAL PRAXIS. l8l 
 
 conspicuously visible around the free edges of the ob- 
 ject. This external line of light has been said to be an 
 evidence of improper adjustment, and that a second- 
 rate objective is being used. The fact is, that such a 
 bright marginal line is evidence that the objective is 
 asking for more light to be given it in a wider cone of 
 illumination. The bright line can be almost cancelled 
 in the image formed by first-class objectives, and 
 greatly diminished in those of inferior quality, by sup- 
 plying a wider cone of rays. The experiment is an in- 
 structive one, and can be easily made by using the con- 
 denser in a way that needs no description. 
 
 Black-ground Illuminators. 
 
 This form of illumination as accomplished by the 
 sub stage condenser is not entirely satisfactory, 
 although the effect may be moderately well obtained by 
 that accessory with low-power objectives. It may be 
 better done with appliances specially designed for the 
 purpose, all of which deflect the illuminating rays so 
 that they pass beyond the objective without entering it 
 directly. 
 
l82 MICROSCOPICAL PRAXIS. 
 
 The Paraboloid. 
 
 This consists -of a solid parabola of glass whose sides 
 are so curved that the rays of light which impinge 
 upon them are internally reflected, so that they escape 
 from the front at such an angle that they pass beyond 
 the front of the objective. The anterior surface is 
 concave, the back one flat, and passing through the 
 longitudina.1 axis there is, in some forms, a movable 
 .stop formed of a flat disk at the summit of a stem. 
 The appliance is fitted into the sub-stage ring, the light 
 reflected from the plane mirror, and the instrument 
 focussed on the object. It gives excellent black- 
 ground illumination with magnifying powers up to the 
 one-fourth inch objective. When the central stop is 
 present it should be pushed up when the paraboloid is 
 used with a high-power lens and depressed for a low- 
 power. 
 
 It may be used as an immersion instrument, as sug- 
 gested by an anonymous writer, who then places the 
 microscope in a vertical position, and having greased 
 the stem of the stop to prevent the water from run- 
 ning down by its side, the hollow is filled and brought 
 into immersion contact' with the under surface of the 
 slide. ''With the highest-power objective generally 
 used with black-ground illumination, as a one-fourth of 
 from 75 to 110, the object seems no brighter than 
 usual, but the field is free from the foggy, diffuse light 
 otherwise present, and the object appears beautifully 
 distinct upon a jet-black ground. Even a one-fifth 
 or a one-eighth of 130 gives the same effect of a deep 
 black back-ground and shows the object with good 
 stereoscopic effect in Wenham's binocular. With objec- 
 tives of 170, the main effect is that of a dark back- 
 ground, though not so perfect as with the lower 
 angles." 
 
MICROSCOPICAL PRAXIS. 183 
 
 There is, however, an immersion paraboloid devised 
 by Mr. Wenham for use with powers higher than 
 the one-fourth. This is similar to the foregoing, but 
 the front surface is flat and the more readily to receive 
 the drop of immersion-fluid, and black-ground effects 
 are said to be obtained with objectives of comparatively 
 wide angular-aperture. 
 
 The Spot Lens. 
 
 This is a large hemispherical lens upon whose plane 
 surface is a central opaque stop to intercept all the rays 
 except those passing through the peripheral parts. It 
 is attached in various ways to the sub-stage, so that 
 it may be focussed on the object, which is illuminated 
 by the reflection from the cover-glass. It cannot be 
 satisfactorily used with objectives higher in power than 
 the one-half inch. With daylight or with parallel rays 
 obtained by a bull's-eye condenser, the plane mirror 
 should be used with this apparatus. 
 
184 MICROSCOPICAL PRAXIS. 
 
 The Woodward Prism. 
 
 For oblique illumination in the study of diatoms, 
 test objects and other preparations where delicate de- 
 tails are to be made out, the Woodward prism is a useful 
 and simple appliance consisting, as first suggested by 
 Dr. J. J. Woodward, of a right-angled prism attached by 
 immersion contact to the lower surface of the slide. 
 The original device was for use with certain first-class 
 objectives in connection with the study of their an- 
 gular aperture, but the opticians have so modified it 
 that it is is now a useful little thing under lenses of 
 very moderate aperture. 
 
 The upper face of the prism receives a drop of 
 glycerine and is then applied to the slide according to 
 the way in which it has been mounted, different 
 opticians adopting different methods. The simplest 
 is to have the little prism without metal mounting of 
 any kind, when it is to be attached bodily to the slide, 
 the drop of immersion-fluid holding it in place. 
 If it should slip out of position, as it probably 
 will when the stand is inclined, a strip of 
 paper pasted along the slide will hold it securely. The 
 prism is usually about half an inch long and half that 
 length in width, its small size making it somewhat 
 troublesome to manipulate, unless it is mounted for 
 attachment to the stage or to the sub-stage. 
 
 It should be accurately centred to the objective, and 
 the lower edge placed as nearly vertical as possible when 
 seen through a low-power. Then attach the higher 
 power to be used, swing the mirror to one side, using 
 the bull's-eye condenser or a microscope-lamp to obtain 
 parallel rays, and illuminate the field. The best effects 
 obtainable cannot be indicated here; the reader must 
 seek for them by experimenting with the mirror, the 
 prism and the light. 
 
MICROSCOPICAL PRAXIS. 185 
 
 Ex-Gov. J. D. Cox, speaking of this little accessory says: 
 It may be safely asserted that for whatever purpose 
 the Wenham reflex illuminator is useful, the Woodward 
 prism is superior; and we have no hesitation in pre- 
 dicting that it will eventually supersede the paraboloid 
 for black-ground illumination with low-powers. Indeed 
 there is hardly one of the elaborate and expensive sub- 
 stage illuminators whose work cannot be better done 
 by this amusingly simple accessory, which can be 
 so cheaply made as to form part of every outfit for a 
 'Students's Microscope,' being in fact less costly than 
 the very cheapest achromatic condenser furnished 
 with an educational instrument. 
 
 The late Dr. Allen Y. Moore was in the habit of 
 using the prism for the illumination of opaque objects 
 under high-power objectives, by attaching it with a 
 drop of glycerine between its broad side and the upper 
 surface of the slide. To accomplish the effect sought the 
 object must be mounted dry on the caver glass, a dry 
 objective should be used and the cement ring made of 
 Canada balsam or some other transparent material. 
 
 Fig 12. Illumination of opaque objects by the Woodward prism. 
 
 In the figure (Fig. 12), E is the objective, B the cover- 
 glass, C the cement ring, A the slide, D the prism, and 
 F and G two rays of light whose course is shown by 
 
l86 MICROSCOPICAL PRAXIS. 
 
 the dotted lines. The rays passing through the prism 
 are reflected to and fro by the surfaces of the glass, 
 brilliantly illuminating any transparent object that 
 may be properly mounted there. Dr. Moore states 
 that he has used this method with a magnifying power 
 of four-thousand diameters, having plenty of light and 
 good definition. 
 
 The Hemispherical Lens. 
 
 This is simply a solid hemisphere of glass attached 
 by -means of a drop of immersion-fluid on its plane sur- 
 face to the lower aspect of the slide. It is used to 
 concentrate on the object oblique light from the mirror 
 when the latter is swung to one side. It may be pre- 
 vented from slipping when the microscope is inclined, 
 in the way suggested for the same purpose with the 
 Woodward prism. A better method, however, is to use 
 only a small amount of water, glycerine or whatever 
 the immersion fluid may be. 
 
 Its. optical action is such that the object is practi- 
 cally at the centre of a hemisphere, where the light is 
 also brought to a focus, the lens being in effect ex- 
 tended by the immersion-fluid to include the slide and 
 the object. It is used, as are so many of these illumi- 
 nating accessories, in the resolving of diatoms or of 
 other finely lined tests, and to assist the objective in 
 the work. For other purposes it is of little or no im- 
 portance. 
 
MICROSCOPICAL PRAXIS. 187 
 
 Supra-stage Illuminators. 
 
 There are many pieces of apparatus for illumination 
 from above the object, but, as with those for sub-stage 
 use, the majority will be here omitted. Some have 
 been forgotten and are not worth recalling. Others are 
 so seldom used that they are rapidly on their way to 
 oblivion, and are rarely seen anywhere except in the 
 opticians' lists. Among these is the Lieberkuhn. 
 
 The Lieberkuhn. 
 
 This is a cup-shaped metal reflector, its inside sur- 
 face polished so as to direct the light upon an opaque 
 object after it has been attached to the objective, over 
 which it is slipped by means of a collar. It was de- 
 vised in 1738 by Johann Lieberkuhn, a German anato- 
 mist and microscopist for whom it is named. At one 
 time it was extensively employed, but it has so many 
 objectionable features that it has now fallen into disuse, 
 although its effects are often admirable. 
 
 It must have a special focus and therefore a special 
 curvature for every objective for which it is intended, 
 and these are usually low-powers. 
 
 Each objective must therefore have its own Lieber- 
 kuhn. In using it, the light must be intercepted by a 
 central opaque disk, or a dark-well, so that no rays 
 shall reach the object directly, but pass around it to 
 
l88 MICROSCOPICAL PRAXIS. 
 
 the Lieberkuhn thence to be reflected downward. Con- 
 sequently the object must not be too large. Intense 
 surface-illumination may be obtained equally well, 
 much better in^ some respects, by other means, the 
 Lieberkuhn therefore being a device that the microsco- 
 pist can get along without. It is interesting chiefly on 
 account of its age and history. 
 
 The Parabolic Speculum. 
 
 This too has gone out of fashion, but it might well be 
 revived for use with low-powers up to the one-half inch, 
 and over large opaque objects. It is a silvered and 
 polished surface of such curvature that parallel rays 
 are reflected from it to a focus on the object, after the 
 apparatus has been attached to the objective, and the 
 mirror swung above the stage. A large object may be 
 examined with all the shadow effects preserved and in- 
 tensified, and these effects are in some cases exceed- 
 ingly fine. 
 
 Mr. E. H. Griffith has suggested the use of a silver- 
 plated spoon as a cheap and effective substitute for the 
 opticians' finely finished accessory. He winds a clean 
 copper-wire of one-twenty-fourth inch in diameter three 
 times around the base of the objective, bending both 
 ends so that they may reach somewhat beyond the front 
 of the lens. He than cuts a section of about half an 
 
MICROSCOPICAL PRAXIS. 189 
 
 inch from the bowl of a new tea-spoon, and solders the 
 ends of the wire to the convex surface. .The wire loop 
 serves to hold the apparatus in position, easily sliding 
 on and off the objective, and being readily bent as the 
 adjustment of the spoon-speculum may require. This 
 is to be used with parallel rays, as is the more expen- 
 sive but scarcely more effective parabolic speculum 
 supplied by the opticians. 
 
 The Vertical Illuminator. 
 
 With the vertical illuminator the objective acts not 
 only as an objective but as an achromatic condenser, 
 focussing on the object the light sent through it from 
 the rear, opaque objects being seen* therefore, by sur- 
 face illumination, and transparent substances by reflec- 
 tion from their inferior surface, the light being forced 
 to act so that the transparent object appears self- 
 luminous. 
 
 The apparatus, which was invented by Prof. Hamilton 
 L. Smith, of Geneva, N. Y., consists of a hollow, brass 
 cylin'der fitted at one end for attachment to the body- 
 tube, and at the other to carry the objective. At one 
 side a projecting milled-head bears a pin which carries, 
 within the hollow of the cylinder, a disk of thin cover- 
 glass which is to act as the reflector, to throw upon the 
 back lens of the objective. the light received through a 
 lateral opening about one-fourth inch in diameter and 
 
igo MICROSCOPICAL PRAXIS. 
 
 opposite the glass disk. At the side of this aperture, 
 a diaphragm is frequently attached so that by it the 
 amount and the direction of the entering light may be 
 altered. As originally devised the reflector was of 
 metal, but this has been replaced by the equally effec- 
 tive thin-glass. 
 
 With the vertical illuminator, opaque objects may be 
 studied with high-power immersion-objectives. Yet 
 even with them its use is limited, since the object must 
 be mounted dry and adherent to the cover. It can be 
 utilized for the examination of the surface of blood- 
 corpuscles, diatoms, insect scales, ruled lines, or similar 
 objects capable of being dried without injury, or 
 naturally existing in a dry condition. It is reported 
 that good results have also been obtained in the ex- 
 amination of mucus, pus and liquids containing bac- 
 teria, etc.; also in the minute structure of muscle and 
 of nerve-fibres. 
 
 To use it, the objective is attached at the lower end 
 and focussed. The light from the mirror is then re- 
 moved, and the illuminator rotated until the lateral 
 aperture faces the lamp, whose flame should be on a 
 level with the opening, so that its rays may enter and 
 fall upon the glass disk within. By manipulating this 
 disk by means of the external milled-head, the light 
 may be thrown obliquely or centrally on the back lens 
 of the objective, through which it passes to be con- 
 densed on the object at the focus, where it will appear 
 as a narrow transverse band. The objective should be 
 carefully adjusted and focussed, and the band of light 
 made as narrow and brilliant as possible. The best 
 condition of the light can be obtained only by experi- 
 menting with the lamp, the. glass disk and the dia- 
 phragm zt the lateral opening. 
 
MICROSCOPICAL PRAXIS. 19! 
 
 Mr. Adolph Schultze, writing in the "Journal of the 
 Quekett Microscopical Club," with reference to the use 
 of this accessory, says: After having roughly focussed 
 the lens on the slide, adjust the lamp in a vertical direc- 
 tion so that a line perpendicular to the optical axis of 
 the microscope, drawn through the centre of the aper- 
 ture of the vertical illuminator, passes through the low- 
 est point of the flame, .or just over the top of the wick. 
 Adjust now the reflecting surface of the vertical illumi- 
 nator on its horizontal axis, so that a distinct image of 
 the flame appears in the field of vision. This image 
 will of course appear brightest, and the definition best, 
 when the narrowest side of the flame is turned toward 
 the instrument, which can be easily ascertained by turn- 
 ing the oil-vessel of the lamp a little round its axis, 
 whilst looking into the microscope. The field is now 
 quite dark, and nothing is seen but a streak of light 
 about a quarter of an inch in breadth, which passes 
 through the middle of it antero-posteriorly. If all this 
 has been carefully attended to, and if a diatom, adher- 
 ing closely to the cover, is moved into this image of the 
 flame, its markings will appear most beautifully and 
 distinctly resolved, provided they are lying across the 
 path of the light. Various little advantages may be 
 gained by a more careful regulation of the relative 
 heights of the light to that of the aperture of the verti- 
 cal illuminator, or by shutting off one-half of the aper- 
 ture of the latter, or by allowing the light to fall on the 
 lower half of the aperture, etc., all of which have for 
 their object to let the light fall on only one-half of the 
 reflecting surface, leaving the other for tt;e passage of 
 the rays from the object to the eye-piece. The image 
 of the flame does not always intersect the whole of the 
 field, and in this case it falls more in the fore part. 
 
192 
 
 MICROSCOPICAL PRAXIS. 
 
 By means of this apparatus the lines on the nineteenth 
 band of Nobert's test-plate have been separated, and 
 Amphipleura pellucida has been resolved into dots by 
 the proper objectives. 
 
 Fig. ig. The vertical illuminator. 
 
 It is exceedingly difficult to use, but its effects on 
 the proper objects are remarkable. It is shown in Fig. 
 13, A being the instrument, B the milled-head pin to 
 hold the thin cover-glass, and C a sectional view as at- 
 tached to the microscope and objective, D being the 
 reflector and the parallel lines the entering and re- 
 flected rays. 
 
MICROSCOPICAL PRAXIS. 193 
 
 The Amplifier. 
 
 The amplifier is a double concave, or an achromatic 
 concavo-convex lens placed in the tube of the micro- 
 scope between the eye-piece and the objective, to in- 
 crease the size of the image formed by the latter, and 
 so apparently to increase the magnifying power. The 
 accessory has long been used in the telescope, and for 
 many years to a certain extent in the microscope. Yet, 
 although known, it did not come into even limited em- 
 ployment until the late R. B. Tolles produced his achro- 
 matic amplifier, and even since that time it has not been 
 very extensively used although this achromatic form is 
 immensely superior to the common double-concave lens 
 sometimes employed, and which is accompanied with 
 considerable loss of light and impairment of definition. 
 
 Mr. Tolles's lens is so mounted that it may be 
 screwed into the draw-tube. It is said that by its 
 means resolution has been had of the nineteenth band 
 of the Nobert test-plate, which could not be had with- 
 out it by the same objective, and that it does not neces- 
 sitate any readjustment of the objective, and only the 
 slightest alteration of the focus. 
 
 Dr. G. Devron of New Orleans, in an enthusiastic de- 
 scription of this special form, says: Every microsco- 
 pist should possess a Tolles's amplifier, or a similar in- 
 strument; its cost is but little more than that of an or- 
 dinary eye-piece, and as it may be used with every eye- 
 piece, its possession is equal to having twice as many 
 such glasses; and the possessor of a good, modern ob- 
 jective of moderate power can accomplish with it al- 
 most anything that would require an objective of the 
 same grade and of double magnifying power; thus a 
 one-twelfth with an amplifier will do the same work 
 that a one-twenty-fourth would do without the ampli- 
 
 14 
 
194 MICROSCOPICAL PRAXIS. 
 
 fier. Object glasses of high-power are very expensive 
 when well made, and require great manipulative skill 
 for their use, while a medium power of the same quality 
 is not half as expensive, is easily worked and is more 
 frequently needed. Other microscopists do not agree 
 with Dr. Devron, condemning the apparatus for de- 
 stroying some of the objective's definition. 
 
 Personally I have never used Mr. Tolles's or any 
 other amplifier. With it the late Dr. J. J. Woodward 
 of Washington, and Mr. G. W. Rafter of Rochester, N. 
 Y., have obtained excellent results in photo-microg- 
 raphy. 
 
 The Erector. 
 
 The inversion of the image renders dissection or in- 
 deed any manipulations with the compound microscope, 
 difficult to most persons, as they are unable to cultivate 
 readily the habit of moving the hand towards the left 
 when it is desired to have the image move toward the 
 right, or vice versa. The opticians therefore offer a 
 prism of peculiar form, so mounted that it may be 
 placed between the ocular and the objective, and by its 
 action erect the image, thus constituting what is called 
 
MICROSCOPICAL PRAXIS. 195 
 
 the erector. Mr. Joseph Zentmayer makes one which 
 is said to interfere very little with the definition of the 
 objective, and Messrs. Bausch and Lomb add a body- 
 tube to their dissecting and mounting microscope in 
 which is an erecting prism. 
 
 An anonymous writer describes a home-made erector 
 formed from one of the right-angled prisms which are 
 used for ornament around certain chandeliers. If this 
 be held horizontally over the eye-piece, with the widest 
 face directed from the observer, and the image viewed 
 through it, the object will seem to be in its normal po- 
 sition; and if the prism be held in exactly the right po- 
 sition, the definition will be slightly if at all impaired. 
 
 I have experimented with this simple contrivance 
 with objectives up to the half-inch, and nothing above 
 this is ever used in ordinary dissections, and I find that 
 it does all that could be expected of it, and with but 
 little deterioration in the definition. It would not be a 
 difficult task for anyone possessing even a little me- 
 chanical ability, to cut off a portion of such a prism 
 and to mount it above the eye-piece, after the cap had 
 been removed, if necessary. 
 
 The image may also be erected by the use of an ob- 
 jective placed over the ocular, the screw-end down- 
 ward, or in the draw-tube above the objective. I have 
 obtained excellent results without much impairment of 
 the definition, by using Messrs. J. W. Queen & Co v 's 
 two-inch objective in the draw-tube with the front lens 
 directed upward, with Spencer's one-inch of 33 on the 
 body-tube. The working distance of the one-inch is 
 thus greatly increased, the magnification reduced, and 
 the field contracted in size, but various manipulations 
 about the stage may be accomplished with the great- 
 est comfort under the combination. 
 
196 MICROSCOPICAL PRAXIS. 
 
 The Polariscope. 
 
 This apparatus, so important in the study of crystals 
 and of the structure of rocks, divulging secrets that 
 could be learned by no other means, consists of two 
 prisms of Iceland spar, one placed below the object 
 and called the polariser, the other, the analyser, above 
 it, usually with a plate of selenite on the stage beneath 
 the object. The use of ordinary light may show noth- 
 ing of the organization of certain bodies, which is at 
 once revealed when the polariscope begins to question 
 it. In the study of crystallography and of petrology it 
 is indispensible. And for the revelation of that per- 
 fection and richness of color so appreciated by the nor- 
 mal eye, it is the only means that we have for its exhi- 
 bition in the microscope. 
 
 Polarisation of light was discovered by accident in 
 1808. Etienne Louis Malus, a French optician, while 
 experimenting with light, happened to view through a 
 doubly refracting prism of Iceland spar, the light of 
 the setting sun as it was reflected from a glass door 
 which was standing open. The phenomenon then seen 
 for the _first time he named polarisation, because, on 
 further study, he supposed that certain of its properties 
 bore "some analogy to the opposite properties of the 
 different poles of a magnet," so that this peculiar kind 
 of light was said to be polarised. The supposition, 
 however, was an entirely gratuitous one. 
 
 The effect is produced by "light reflected from or 
 transmitted through glass at an angle of incidence of 
 54 35' or light propagated by only one plane of vi- 
 brations." It is supposed that a ray of common light 
 consists of vibrations in all possible planes, but for 
 practical purposes they are considered to be in only two 
 directions at right-angles. to each other, polarised light 
 
MICROSCOPICAL PRAXIS. 197 
 
 being that kind in which the vibrations are reflected or 
 transmitted in one plane only, all the other kind of vi- 
 brations having by some means been intercepted. 
 Prof. A. P. Gage in his "Elements of Physics," illus- 
 trates this by Fig. 14, saying that the action of the 
 
 Figure 14. "Explanation of polarised light. 
 
 tourmaline (the polarising substance used in the experi- 
 ment) may be compared to that of a grating A' in the 
 figure, formed of parallel vertical rods, which will al- 
 low all vertical planes, as C C' to pass, but stops the 
 planes D D' that are at right-angles to these rods. Any 
 plane that has succeeded in passing one grating will 
 readily pass a second similarly placed. But if the 
 second grating j9, is turned so that its rods are at 
 right-angles to the first, the plane that has succeeded 
 in getting through the first grating will be stopped by 
 the second. 
 
 Iceland spar is the substance used for the microscop- 
 ical polariscope, being a mineral not rarely found m 
 many countries. It is a colorless, entirely transparent 
 carbonate of lime, and capable of being separated, by 
 the proper treatment, into a six-sided solid of a peculiar 
 inclined form, called a rhomb of Iceland spar. From 
 this are made the Nicol prisms, being so named for a 
 Mr. Nicol, an optician of Edinburgh, who first pre- 
 pared them by dividing these rhombs into two equal 
 parts by a plane passing at a certain angle diagonally 
 through them. 
 
198 MICROSCOPICAL PRA*XIS. 
 
 In the undivided rhomb a ray of light falling perpen- 
 dicularly on the surface is divided into two separate 
 parts, one taking the direction of the original ray, and 
 so named the "ordinary ray," the other being refracted 
 to form what is called the "extraordinary ray," so that 
 any small object looked at through a slice of Iceland 
 spar, will appear to be doubled. Mr. Nicol, by his treat- 
 ment of the rhomb, succeeded in producing single-image 
 prisms for use with the polariscope, in which there are 
 two, one forming the polariser, the other the analyser, 
 the extraordinary ray alone being allowed to pass, 
 the ordinary suffering total reflection. 
 
 The polarising prism is so mounted that it may be at- 
 tached to the sub-stage or inserted into the stage-open- 
 ing. It must be below the object, and either it or 
 the analyser must be capable of rotation at the will of 
 the microscopist, but which shall be movable is of no 
 importance. For convenience, the polariser may be 
 used with the sub-stage condenser and below it. 
 
 The analyser must be above the object. Whether it 
 is placed in the body-tube or above the eye-piece has 
 some effect upon the intensity of the illumination and 
 the size of the field, but none upon the polarising action. 
 If the analyser is in the body-tube the light is dimin- 
 ished; if above the eye-piece the field is greatly reduced 
 in size, while the amount of light suffers little or no 
 diminution. 
 
 When the prisms are applied to the microscope, they 
 should be in such a position, that is, with their polaris- 
 ing planes parallel with each other,- tha't thefield shall 
 be brightly illuminated. If it should be darTorVonly 
 faintly lighted,-the prisms are at or. near -right-angles to 
 each other, and are 'then ^saitf to be grossed and th^ 
 light may^be turned 'on by rotating one or the other 
 
MICROSCOPICAL PRAXIS. 199 
 
 Inmost work with the polariscope it is usually best to 
 use a low-power objective only, which is attached to the 
 body in the usual way, the field illuminated and the ob- 
 ject focussed. The polariser or the analyser is rotated 
 and the effect studied. There is, however, nothing in 
 the polariscope to prevent its use with high power-ob- 
 jectives over suitable objects. 
 
 When the prisms are crossed at right angles to each 
 other, the field will be absolutely black if the apparatus 
 is perfect, and with the field thus darkened, any trans- 
 parent object capable of being polarised, as all sub- 
 stances are not, will, when placed under the objective, 
 be lighted and may be exquisitely colored. Yet some 
 objects, although they may show the effect of polarised 
 light when the prisms are rotated, may not be colored. 
 In such cases a plate of selenite is placed beneath it, 
 when it will exhibit at every quarter turn of the prism, 
 complementary colors which will vary according to the 
 thickness of the selenite. 
 
 The instrument deserves to come into more general 
 use. It is employed with ease, demanding no compli- 
 cated or delicate manipulations, and its effects are al- 
 ways pleasing and such as can be obtained only by it. 
 That it is not oftener employed by microscopists is 
 probably due to its cost, and probably to a feeling that 
 its practical usefulness is not sufficient to warrant the 
 outlay; but this is a mistake. It merits careful study. 
 
200 MICROSCOPICAL PRAXIS. 
 
 Drawing the Object. 
 
 The microscopist that cannot draw is unfortunate. 
 There are many occasions when to be able to sketch the 
 appearances seen at the moment, is a great convenience, 
 whereas to be compelled to attach the camera lucida to 
 the eye-piece, and to prepare the paper and the light, 
 not only consume time, but may result in the loss of 
 the object if it be a living one. All forms of camera 
 lucidas are intended for use in drawing the outlines 
 only of the object. The shading and the fine artistic 
 finish must be added afterwards. All the devices are 
 attached to the ocular above the eye-lens after the cap 
 has been removed, and considerable practice is required 
 with all the different kinds before they can be used 
 with much success. 
 
 The light must be correctly modified or the pencil 
 point will be invisible. This is one of the necessities, 
 and the invisibility of the pencil point one of the diffi- 
 culties in the use of all these drawing contrivances. 
 The light on the paper should not, as a rule, be strongeA 
 than that on the object; it should usually be weaker./ 
 If it must be increased, as it must with certain forms, 
 the lamp can be arranged so that it may be nearer to 
 the paper, and a bull's-eye condensing lens be inter- 
 posed between them, while the illumination of the ob- 
 ject may be modified by diaphragms or by increasing 
 the distance between it and the mirror, if the concave 
 face is used. The pencil should be rather soft, with a\ 
 long, sharp point, whose visibility some microscopists \ 
 are in the habit of attempting to increase by the addi- \ 
 tion of a little tin-foil wrapped about it, contending that 
 the bright foil is more easily seen than the graphite J 
 pencil point. Others blacken the end of the pencil \ 
 with india ink, making for this the same claim as for ^ 
 
MICROSCOPICAL PRAXIS. 2OI 
 
 the tin-foil. The proper illumination seems to be more 
 important than any of these aids, and its careful regu- 
 lation better than any addition to the^fencil point. 
 
 Mr. T. Suffolk describes, in "Science Gossip," his 
 method of drawing without a camera lucida of any form. 
 He places on the diaphragm of the eye-piece a convex 
 lens of shallow curvature, upon the surface of which is 
 a grating ruled in squares, the lines being about one- 
 twentieth of an inch apart. The drawing, which is 
 made on paper also ruled in squares, may then be en- 
 larged or reduced after the well-known method em- 
 ployed by draughtsmen. "The process also possesses 
 the additional advantage of requiring no change in the 
 position of the microscope, as is the case with the cam- 
 era lucida, and can be used for a long time without any 
 of the strain on the eye inseparable from the use of in- 
 struments where the image and pencil point are viewed 
 through the divided pupil of the eye." 
 
 The Thin-glass Reflector. 
 
 The simplest form of all the appliances intended for 
 the microscopist that cannot draw free-hand, is a thin- 
 glass cover attached in any convenient way to the 
 ocular, so that the image shall be received upon its 
 
202 . MICROSCOPICAL PRAXIS. 
 
 upper surface at the proper angle, which may be de- 
 termined by experiment, but which is usually 45 de- 
 grees if the body-tube becomes horizontal when in- 
 clined, as it will not become on all stands. The micro- 
 scope is depressed into this horizontal position, the 
 paper arranged on the table beneath the eye-piece, and 
 while the pencil is seen through the thin cover-glass, 
 the object appears to be projected on the paper, where 
 the pencil draws the outlines. 
 
 Mr. T. B. Jennings has described the following sim- 
 ple method of mounting the reflector. In a flat cork he 
 cuts a central opening large enough to pass over the 
 mounting of the eye-lens, and just below this aperature 
 he makes an incision to receive the cover-glass so that 
 it may stand at an angle of 45. 
 
 Beale's Neutral Tint Reflector. 
 
 A form similar to the thin-glass disk but differing in 
 
 being tinted a neutral color, is Beale's reflector. It is 
 
 ?%-* employed quite extensively, more so than the colorless 
 
 ''^Vcover-glass, but in the same way. It is" made by op- 
 
 >"--..- -ticians generally, being intended to slide over the 
 
 '^-^ Amounting of the eye-lens, after the cap has been re- 
 
 - moved. " 
 
MICROSCOPICAL PRAXIS. 203 
 
 A Simple Mirror-reflector. 
 
 An anonymous writer recommends a reflector to be 
 made as follows. Take a small portion of the silvering, 
 about one-sixteenth of an inch in diameter, from the 
 back of a mirror, where there should be a thick coating 
 of paint on the amalgam to support it, or it will not 
 break off. This small amalgam reflector is to be 
 mounted with cement on a piece of watch-spring at the 
 proper angle. The spring is bent round and fixed to a 
 brass tube fitting over the eye-piece, so that the re- 
 flector may stand about one-fourth of an inch from the 
 eye-lens and central with it. On looking into it, the 
 object on the stage is seen and appears to be projected 
 on the paper below. 
 
 The objection to all these simple forms, especially to 
 the neutral tint reflector and its imitators, is that they 
 not only invert the image, the top appearing at the 
 lowest part, but seem also to turn it over, the left-hand 
 side being transferred to the right-hand. To finish a 
 drawing thus inverted and reversed is difficult. The 
 microscopist will be sure to lose his way when, after 
 glancing through the microscope for the details, he 
 attempts to add them to his sketch. With the neutral 
 tint the field is also very small. To see the whole of 
 the field at one time is impossible. 
 
2O4 MICROSCOPICAL PRAXIS. 
 
 Wollaston's Camera Lucida. 
 
 One of the earliest forms of drawing-prisms, and one 
 of the best, is the four-sided accessory known as 
 Wollaston's camera lucida. It is so mounted that it 
 slides over the eye-lens, after the cap has been re- 
 moved, the image being apparently projected on the 
 paper upon the table, and seen in its proper position, 
 since it has been twice reflected by the prism, the 
 pencil and the paper being seen direct, that is, without 
 reflection. The acute edge of the prism bisects the 
 pupil of the eye, one-half receiving the rays from the 
 microscope, the other half those from the paper and 
 the pencil. For this reason it is at first somewhat 
 difficult to use, but a little practice soon conquers 
 it. Like all similar kinds of apparatus it is used with 
 the left-eye closed. 
 
 The microscope must be in a horizontal position, and 
 the light on the drawing-surface should be much 
 stronger than that on the object. When the illumina- 
 tion has been properly adjusted the prism can be used 
 with great facility, and most satisfactory drawings 
 made with it. If the field projected on the paper below 
 the prism is not evenly illuminated, or if it is colored 
 at the margins, the camera lucida should be altered by 
 rotating it slightly, or by changing its position in re- 
 lation to the eye-piece. When the object is focussed, 
 and the field as seen' on the paper is properly illum- 
 inated, the eye is kept steadily over the acute edge of 
 the prism while the pencil sketches the outlines of the 
 image. The secret of success is to keep the eye mo- 
 tionless. If it turns ever so slightly the pencil point 
 will mysteriously disappear, in which event it should be 
 held immovable until the eye again finds it. 
 
 If the microscopist is accustomed to the use of spec- 
 
MICROSCOPICAL PRAXIS. 205 
 
 tacles he will find them an advantage when drawing 
 with any kind of camera. One of the troubles experi- 
 enced by some that use spectacles is that the pencil 
 cannot be seen at the same time with the object when 
 the spectacles are removed. Those whose eyes are 
 perfect occasionally have the same trouble. This may 
 be imperfectly remedied by using a very low-power con- 
 vex lens placed immediately below the prism, between 
 it and the paper. By its use the rays from the drawing 
 surface are given the same degree of divergence as those 
 from the camera. Some opticians attach this lens 
 movably to the apparatus, so that it may be utilized or 
 not as the microscopist may desire. 
 
 The size of the drawing will depend upon the dis- 
 tance between the paper and the prism, increasing with 
 that distance, so that almost any enlargement may be 
 obtained. Immense diagrams may be made, according 
 to Dr. L. S. Beale's method, by placing the paper on the 
 floor, or what may be more satisfactory for ordinary pur- 
 poses, by raising the microscope on a box or other sup- 
 port above the table on which the drawing is to be 
 made. 
 
 
 The Abbe Camera Lucida. 
 
 Perhaps the most prominent camera now offered by 
 the opticians is that of Professor Abbe. It consists of 
 two prisms cemented together so as to form a cube, 
 
206 MICROSCOPICAL PRAXIS. 
 
 one of the contacting hypothenuse surfaces being 
 silvered, or gilded, with the exception of a small spot in 
 the centre, and slightly removed from the prism so that 
 there is a thin film of air between them. A plane mir- 
 ror at the end of a lateral arm reflects the rays from the 
 pencil and the paper to the silvered prism, whence they 
 are reflected to the eye. 
 
 This camera may be used with the microscope either 
 vertical or inclined, the best effects being obtained 
 when the body-tube is somewhat inclined, as the distor- 
 tion of the image is then less than when the micro- 
 scope is vertical. When the prism is at one side of the 
 instrument and the paper at one side of the microscope- 
 foot, the distortion is so great that a special drawing- 
 desk must be used and inclined at an oblique elevation. 
 
 It is to Prof. S. H. Gage, of Cornell University, that 
 we owe a more complete knowledge of the distortion 
 produced by this camera and of the best means of over- 
 coming it. When using this form the aperture in the 
 silvered surface of the prism must be exactly in the op- 
 tic axis of the microscope and at just the right distance 
 above the eye-piece. If the field as seen through the 
 camera is not evenly illuminated, or if the entire field 
 can not be seen at once, the prism has not been 
 properly placed, and must be re-arranged by altering 
 the little screws in the collar of the instrument. The 
 mirror of the Abbe cameras which I have seen is per- 
 manently fastened at an angle of about 45, so that the 
 microscopist is relieved from all manipulations of this 
 part, except to slide it to and fro on the mirror-bar so 
 as to select the portion of the drawing paper or of the 
 table that is to be reflected. As Prof Gage says in 
 reference to the use of all forms of camera lucida: 
 "In order that the picture drawn . . . may not be dis- 
 
MICROSCOPICAL PRAXIS. 
 
 207 
 
 torted, it is necessary that the axial ray from the image 
 on the drawing-surface shall be at right angles to the 
 drawing-surface." And that the end of the drawing 
 board shall be in a plane parallel with the stage of the 
 microscope; the mirror must also have its edges parallel 
 with the edges of the drawing-board. 
 
 If the mirror of the camera be movable on its axis, 
 when the microscope is inclined the mirror must be 
 manipulated so as to prevent the distortion otherwise 
 inevitable. The general rule in these cases, again quot- 
 ing from Prof. Gage, is to raise the drawing-board 
 twice as many degrees toward the microscope as the 
 mirror is depressed below 45. With the mirror at 45 
 the drawing-surface should be horizontal; with the mir- 
 ror at 40 the drawing-surface should be elevated 10, 
 and with the mirror at 35 it should be elevated 20 
 toward the microscope. Even in these conditions there 
 will be distortion from front to back unless the drawing- 
 board is again elevated in the proper position. To 
 accomplish all these points Mrs. Gage has devised 
 a drawing-board shown in Fig. 15. It is to be used with 
 
 Figure 15. Mrs. S. H. Gage's drawing-desk for the Abbe camera lucida. 
 
 the mirror at 35 and the microscope inclined at 30. 
 For other positions of the camera-mirror and of 
 the microscope, a drawing-board may be devised with 
 the proper surfaces by Prof. Gage's rule already given. 
 
208 MICROSCOPICAL PRAXIS. 
 
 Messrs. Bausch & Lomb make a camera lucida to be 
 used on the microscope when inclined, with which they 
 claim that the pencil-point is well seen, and the distor- 
 tion less than with even the Wollaston prism. I have 
 not seen it. 
 
 The Grunow Camera Lucida. 
 
 In this device, which is made by Mr. J. Grunow, of 
 New York, the plane mirror of the Abbe camera is re- 
 placed by a rectangular prism to reflect the rays from 
 the drawing-surface. It consists of three rectangular 
 prisms, two forming a cube, one surface being silvered 
 or coated with gold, and having a perforation in this de- 
 posit essentially as in the Abbe form, the third prism 
 being intended to reflect the rays from the paper. The 
 apparatus may be used with the body-tube inclined or 
 vertical, giving, it is said, a very distinct image of the 
 pencil and the object. "A portion of the surface of the 
 work-table of the size of about twelve or fifteen inches 
 is projected into the field of view, so as to be distinctly 
 and clearly seen with the object on the stage." The 
 camera must, however, be especially fitted to the eye- 
 piece with which it is to be used. I have not seen it. 
 
MICROSCOPICAL PRAXIS. 209 
 
 Live-boxes, Growing-cells, and other Acces- 
 sories. 
 
 On the upper left-hand corner of many microscope 
 stages will be noticed a small hole intended to receive 
 the stem of the stage forceps, a piece of apparatus 
 sometimes furnished with stand. It was more frequently 
 added by the older opticians than it is by those of the 
 present day, and it was oftener used by the older 
 microscopists than it is by those of the present time. 
 
 It consists of a delicate forceps opened by a slight 
 pressure on a projecting pin, closing by its own elastic- 
 ity, and with a long handle having universal motions 
 by means of a ball-and-socket joint, or by some other 
 way. It is used to hold small insects or opaque objects 
 under the objective, generally a low-power, so that they 
 may be examined in all their parts and in all positions. 
 
 The Mechanical Finger. 
 
 The microscopist that does much mounting soon 
 feels a desire to produce some preparation to show his 
 skill in the manipulation of small objects, as well as the 
 adaptability of small objects to the formation of beauti- 
 ful slides. Butterfly scales are arranged into bouquets 
 of the most delicate and gorgeous hues; diatoms are 
 laid down in intricate geometrical patterns and figures 
 that are a joy to the eye, or they are selected from a 
 mass of dirt, and lifted into the light and the purity of 
 another slide, where the" expert microscopist studies 
 them with increased pleasure when he recalls the source 
 
210 MICROSCOPICAL PRAXIS. 
 
 whence he took them. All such delicate work is done 
 by the use of a mechanical finger, which is essentially a 
 fine bristle, or a filament of spun glass, attached in 
 some way below the objective. The filament picks up 
 the little object while the microscopist's eye is at the 
 ocular, holds it suspended in air until he moves a slide 
 beneath it, when it gently lowers the selected speci- 
 men into the spot prepared for it, and returns under the 
 microscopist's intelligent guidance for another similar 
 load. In this way, after repeated visits to the source 
 of supply, the mechanical finger produces exquisite 
 arrangements of diatoms, of insect scales or of other 
 minute objects. 
 
 To use even the simplest mechanical finger demands 
 much preliminary practice, yet those that have mas- 
 tered the apparatus seem to have little difficulty in pro- 
 ducing the wonderful results just referred to. 
 
 Fig. 16. E. A. Apgar's mechanical finger. 
 
 An uncomplicated form which has been very success- 
 fully used by its inventor is that shown in figure 16, 
 
MICROSCOPICAL PRAXIS. 211 
 
 having been devised by Mr. Ellis A. Apgar, who at my 
 request has thus described it. 
 
 It is constructed out of the stage forceps, the forceps 
 part of that appliance being removed, and in the end of 
 the arm a minute hole is drilled one-eighth of an inch 
 deep. Into this is inserted a bit of fine spun-glass or a 
 cat's whisker, and kept in .place by wax. A wooden 
 collar is fitted to the front of the objective, and in the 
 wood is bored a hole of the proper size to receive the 
 stem of the forceps, and from which it may be removed 
 at pleasure. Thus for a few cents an almost valueless 
 implement may be converted into one that is often use- 
 ful, and in the hands of an expert, one that is capable 
 of performing marvels in the preparation of arranged 
 diatoms. 
 
 The tube through which the stem passes, together 
 with the socket-joint give the operator complete con., 
 trol of the point of the hair or of the glass filament, en- 
 abling him to place it in exact focus or not, as he may 
 desire, and by the use of the rack-and-pinion move- 
 ment of the body-tube with one hand, and the shifting 
 of the stage with the other, the minutest diatoms may 
 be selected and placed wherever desired; delicate ob- 
 jects may be spread out and arranged, and fine parti- 
 cles of dust may be removed before the cover-glass im- 
 prisons them. 
 
 The slide intended to receive the diatoms or the 
 scales in patterns, is usually slightly coated with a thin 
 and weak solution of gum arabic, in which is a little 
 acetic acid, and allowed to dry. The object being de- 
 posited in the proper place and position, the slide is 
 gently breathed upon, the moisture of ihe breath soft- 
 ening the gum sufficiently to allow the object to adhere 
 the thin film of dried mucilage being invisible when the 
 Canada balsam is added. 
 
212 MICROSCOPICAL PRAXIS. 
 
 The Animalcule Cage or Live-box. 
 
 It often happens that the microscopist who is work- 
 ing with pond-life needs some means of preserving the 
 life of the aquatic creatures longer than is possible with 
 the ordinary cement cell and the cover-glass. There 
 are numerous life-slides, or growing-cells, well adapted 
 to this purpose, but the animalcule cage of the optician 
 has the additional convenience of being usable as a 
 compressorium as well as a life-slide. It is somewhat 
 inconvenient however, because it is heavy, difficult to 
 clean, and as ordinarily made, almost unusable with 
 high-power objectives. With comparatively large ob-~ 
 jects and low powers it is commendable. 
 
 Mr. E. J. Whitney has suggested a yery cheap and 
 effectual substitute for this rather expensive piece of 
 apparatus, which he makes by obtaining at the hard- 
 ware store, a full set of about a dozen ferrules of grad- 
 uated sizes, fitting snugly one inside the other. Take 
 any two which fit well together and cement the smaller 
 one, large end down, to the centre of an ordinary glass 
 slip. To the top of the ferrule cement one of the thick- 
 est cover-glasses that will fit. Now take another cover- 
 glass that fits inside the larger ferrule, and cement it to 
 the inside at the top. The box is now complete, and 
 all that remains to be done is to slip the larger ferrule 
 over the smaller. 
 
MICROSCOPICAL PRAXIS. 213. 
 
 Life-slides or Growing-cells. 
 
 Some of these numerous devices are complicated and 
 expensive, and are to be obtained only from the optic- 
 ian. Others are simple, easily made by the microsco- 
 pist himself, and are as praiseworthy for their working 
 qualities as those offered by the dealers. The purpose 
 of every one is to supply enough air to keep the im- 
 prisoned plants and animals in good condition, so that 
 their life-processes may be performed as in freedom. 
 
 This part of the problem is not so difficult, but to 
 supply the animals with the proper kind and amount of 
 food, and to imitate pretty closely the environment 
 which they must have or suffer, are not so easy of ac- 
 complishment. No life-slide has successfully solved 
 these parts of the question. All forms succeed for a 
 time, some longer than others, but all fail within a very 
 limited period. Those animals whose life-history is be- 
 gun and finished within a few hours, or a day or two at 
 most, can be accommodated, but those which develop 
 slowly and live comparatively long, give the microsco- 
 pist a difficult and troublesome subject to consider. 
 
 With microscopic, plants the question is no more easily 
 settled. They die as readily as the animals, even after 
 they have received the most careful attention. They 
 refuse to continue their life-history. Their chlor- 
 ophyll grains fade, and slowly group themselves to- 
 gether near the centre of the cell; the protoplastic gran- 
 ules increase in number and swarm and quiver in their 
 ceaseless pedetic dance, and the fungi come to envelop 
 the whole in a fluffy winding sheet, which, however in- 
 teresting at the proper time and place, is not accept- 
 able in one's growing-slide. 
 
 There is never any difficulty in cultivating microscopic 
 fungi of a certain kind. They cultivate themselves, and 
 
214 MICROSCOPICAL PRAXIS. 
 
 force themselves on the notice of the microscopist that 
 has them always with him. They will even grow and 
 flourish and be happy in the horribly astringent solution 
 within the alum-cell of the oxy-hydrogen microscope. 
 
 The simpler the life-slide the better the results, and 
 the hastily-made and home-made productions are often 
 more satisfactory than the elegant and elaborate con- 
 trivances offered by the dealers. 
 
 For my own purposes I take for covers for such cells 
 large thin-glass squares only. There are several ad- 
 vantages to be had in their use in this connection over 
 that of thin circles, one being the facility with which the 
 water supply can be renewed. By carefully adding, 
 with a camel's-hair brush, a drop of fresh water at the 
 corner of a large square cover projecting beyond the 
 cement cell, the fluid will usually flow under so gradu- 
 ally that the object, even a minute Infusorian, will not 
 be moved from the field, the inward rush of the current 
 being tempered by the cement ring. This supply can 
 be easily added by one hand holding the wet brush 
 while the eye is intent at the ocular, the secret of suc- 
 cess here being in not having too large a brush and in 
 not filling it too full of water. At the beginning of 
 daily evening work the brush is wetted and thrown on 
 the table to become thoroughly moistened, when a sin- 
 gle dip into the tumbler of water, with a slight shake 
 to prevent dripping, takes up enough, although some 
 pressure of the brush against the slide may be needed 
 to squeeze out a small drop. It is better to make sev- 
 eral journeys to the tumbler than to lose the object. A 
 dipping tube adds too much at once, and cannot be so 
 readily controlled as a brush. 
 
 In studying the morphology of minute animal organ- 
 isms, I use only a shallow, shellac cell with about one- 
 
MICROSCOPICAL PRAXIS. 215 
 
 fourth of the ring scraped away from both the upper 
 and the lower margins, thus leaving two curved sup- 
 ports for the square cover, one on each side. The dia- 
 
 Fig. 17. A simple life-slide. 
 
 gram shows the arrangement, the shaded parts repre- 
 senting the remnants of the cell. This gives the en- 
 closed drop, with its animal life, plenty of air, and 
 facilitates the application of the wet brush 
 at the point where the square cover projects beyond the 
 lateral cell-wall. In this simple affair I have fre- 
 quently kept Infusoria and other small creatures alive 
 and well from early in the evening until after midnight, 
 and when compelled to leave them have washed 
 them into the aquarium in as good condition and as 
 lively as when first imprisoned. Here the secret of 
 success consists, I think, in leaving enough of the ce- 
 ment ring to support the cover properly and to lessen 
 the force of the inflowing water-supply, and also in hav- 
 ing the cell shallow or deep according as the animals 
 are microscopically small or large. Much depends on 
 the depth of the cell in all cases. A comparatively large 
 Infusorian, a Rotifer or a Chatonotus can be injuriously 
 hampered in its movements and in the proper perform- 
 ance of its functions, by a cell of insufficient depth, and 
 a good objective can as the reader knows, be greatly 
 hampered in its functions by a cell of too great depth. 
 If it is desired to convert this or any other slide of 
 the kind into a growing-cell, it is done by the well- 
 known method of placing the slip across a small 
 
2l6 MICROSCOPICAL PRAXIS. 
 
 saucerful of water with a doubled and twisted thread of 
 sewing-cotton in close apposition with the edge of the 
 cover, both ends of the thread hanging freely in the 
 water. The liquid will flow up and supply that lost 
 by evaporation, provided the water is always in con- 
 tact with the lower surface of the slide. This "dodge" 
 is successful for a few days, but it always ends badly, 
 as the salts in the water will crystallize at the cover- 
 margins and cut off the oxygen supply. 
 
 It often happens that the conditions and the environ- 
 ment are such that an immense number of minute 
 Infusoria, all belonging to the same species, are sud- 
 denly developed in the aquarium, infusion or macera- 
 tion, and it becomes interesting to isolate a few to study 
 their life-history. Such an advent of Monads, Heter- 
 omita and other similarly minute creatures is not rare. 
 For the study of such truly microscopic objects I have 
 devised a simple life-slide, describing and figuring it in 
 "SCIENCE GOSSIP" for January, 1894. To make it, ce- 
 ment with Canada balsam in the centre of a slip a thin 
 glass disk one-fourth inch or less in diameter; use a 
 one-sixteenth inch cover-glass if possible. In the 
 letter case, then take a glass, or zinc or other kind of 
 ring with a quarter-inch aperture, break a small piece 
 from one side, and fasten this broken circular band 
 about the central disc. From another ring with a 
 three-eighths inch or larger aperture, break a piece as 
 before and cement this broken band around the inner 
 ring so that its broken part shall be opposite the un- 
 broken curve of the latter, and with a thin square cover, 
 the cell is complete, the depth depending upon the dif- 
 ference in the thickness of the outer rings and the cen- 
 tral circle. 
 
 To use, place on the central disc a small drop of the 
 
MICROSCOPICAL PRAXIS. 2 17 
 
 water containing the organisms to be kept alive, and 
 over it arrange the square cover, taking care to prevent 
 the water from overflowing into the inner annular space. 
 With the camel's-hair brush carefully, and in small 
 quantities, add fresh water at the top or the side of the 
 square, never at the bottom or near the opening in the 
 outer ring. It will be found that the water will flow be- 
 tween the square and the upper surface of the exterior 
 ring, will enter through the break in the latter, partly 
 filling the outer annular space, and by capillary attrac- 
 tion will occupy a part of the vacancy between the 
 
 Fig. 18. A simple life-slide. 
 
 cover and the interior ring, (as shown by the diagonal 
 lines in the diagram, Fig. 18), but unless too much wa- 
 ter is used or it is supplied in too great quantities at 
 once, it will not pass through the opening in the inner 
 ring, thus leaving an abundance of air to supply the 
 animals under observation. The imprisoned air at 
 once becomes saturated with moisture, as is evidenced 
 by the fogginess of the cover; the central drop can- 
 not evaporate and the external water will not come in 
 contact with it. if care be taken in filling the slide and 
 in supplying that lost by evaporation. To admit en- 
 tirely fresh air the water can be drawn off by bibulous 
 paper, or allowed to evaporate, without in any way dis- 
 turbing the central drop. The reader must remember 
 however, that the cell is intended for only the smallest 
 of microscopic animals, with which it is -entirely suc- 
 cessful. 
 
2l8 MICROSCOPICAL PRAXIS. 
 
 Logan's Life-slide. 
 
 Mr. Logan's device consists of a strip of wood with a 
 central perforation, in which is fitted a glass cell 
 formed of a thick platform surrounded by a deep 
 groove. The object is placed in a drop of water on 
 the central platform, and a cover-glass cemented over 
 the whole with wax. A ring of sheet-wax is applied 
 to the projecting ring around the groove, and the 
 cover is fastened down by running a warm wire around 
 the edge to melt the wax, the thickness of the cell, 
 that is, the distance of the cover-glass above the cen- 
 tral platform, of course depending upon the thickness 
 of the wax used. The air confined in the groove by the 
 cover will be enough to supply the microscopic 
 creatures for a long time. 
 
 Mr. Logan's slide is objectionable because it is so 
 heavy, the central disc being so exceedingly thick that 
 no sub-stage condenser can be focussed through it, and 
 the annular depression so deep that the glass sides 
 affect the light in an undesirable way. For compara- 
 tively large aquatic objects to be examined with a low 
 power it is useful, but I find my own modification of it 
 better for more delicate work. 
 
 A small square, cut from glass of any desired thick- 
 ness, is cemented with Canada balsam to a slip, and 
 surrounded by a thick, glass or zinc ring so as leave a 
 wide space between these parts. On the ring place a 
 ring of wax and, after the object has been arranged on 
 the central square, cover the whole with a thin cover 
 and cement it fast by running a warm wire around the 
 edge to melt the wax. A small drop of water may be 
 placed in the annular space if desired. 
 
 A diagram of this slide is shown in Fig. 19. The 
 reader of course understands that the thickness of the 
 
MICROSCOPICAL PRAXIS. 219 
 
 slip and square, and the depth of the cell, must be de- 
 termined by each worker according to his needs. For 
 myself I have them as thin and shallow as possible. 
 
 Fig. 19. A simple life-slide. 
 
 The secret of success here is to be sure that the joint 
 between the ring and the slip is air-tight, and to secure 
 the cover firmly, using an abundance of wax. 
 
 In this simple contrivance Hydra viridis has lived un- 
 complainingly for an entire month, and for four weeks 
 a Chironomus larva has existed, with no food except 
 what may have accidentally been in the water, and no 
 air except that within the annular space; not only has 
 the same larva lived in that confined place, but it there 
 ceased to be a larva, and became the perfect insect, of 
 course soon dying for the want of air, and on account 
 of its inability to expand and to dry its wings. ' In the 
 same form of life-slide, with an abundance of algae and 
 of infusorial creatures, the rotifer Furcularia has thrived 
 and built her mucilaginous home and deposited her 
 eggs, being contented there for a whole month and ap- 
 parently willing to stay another. 
 
 For showing living Infusoria, Rotifers, Chcetonoti, 
 aquatic worms and other animals at microscopical ex- 
 hibitions, nothing could be more satisfactory than Mr. 
 Logan's slide. With it the writer has kept a quantity 
 of Turbellarian worms well and active until the small 
 hours of the morning. In this instance the slide was 
 prepared for an exhibition, so hurriedly and so late that 
 the cement was not dry before the opening hour 
 
22O MICROSCOPICAL PRAXIS. 
 
 arrived, but an external application of the thin and 
 rapidly-drying Brown's rubber-cement made all tight, 
 and apparently not unpleasant for the worms. 
 
 Hitchcock's Moist Chamber. 
 
 Mr. Romyn Hitchcock has used and recommended 
 two forms of simple life-cells. One is prepared by in- 
 verting a small salt-cellar in a dish of water to serve as 
 a support to the specimens, a tumbler being inverted 
 over the whole. The objects are placed on glass slips, 
 made by cutting an ordinary slide across the middle, 
 and covered with thin glass. 
 
 The second plan the inventor thinks the more desir- 
 able. --In this a piece of glass four inches square is 
 placed on a support so that it is about on a level with 
 the top of a dish to hold water, an ice-cream saucer 
 being used by the inventor. A piece of blotting-paper 
 is then placed on the glass, and the edge allowed to dip 
 in the water. Objects to be examined are placed on 
 large cover-glasses, and either protected with a smaller 
 cover, or left exposed. These cover-glasses are laid on 
 the blotting-paper with watch glasses above them. A 
 single large watch-glass may be used, or a number of 
 small ones, one for each specimen. Objects can be 
 kept fresh and moist in this way, with far less trouble, 
 Mr. Hitchcock says, than by any other method that he 
 has tried. 
 
MICROSCOPICAL PRAXIS. 221 
 
 Beale's Growing-cell. 
 
 In Fig. 20 is shown a contrivance almost as uncom- 
 
 Fig. 20. Beale's growing-cell. 
 
 plicated as the preceding, taken from Beale's "How to 
 Work with the Microscope," and used by that microsco- 
 pist with much success. A small piece of glass tube is 
 fixed to an ordinary slip to serve as a reservoir to 
 supply the water. It is covered with a piece of thin 
 glass, a small opening left at one side being sufficiently 
 large to allow a fine thread of silk or cotton to conduct 
 the water from the reservoir to the specimen placed in 
 the centre of the slide. I have used this contrivance 
 with considerable satisfaction. 
 
 H. L. Smith's Growing-cell. 
 
 This is a very efficient cell made upon the old princi- 
 ple of the bird-fountain. It has been described by the 
 author as follows: 
 
 The whole slide is a trifle more than one-eighth of 
 an inch in thickness, and consists of two rectangular 
 glass plates, three by two inches, and about one- 
 twenty-fifth of an inch thick, separated by strips of 
 the same thickness cemented to the interior opposed 
 
222 MICROSCOPICAL PRAXIS. 
 
 faces, as shown in Fig. 21. This closed cell, ulti- 
 
 Figure 21. H. L. Smith's growing-cell. 
 
 mately destined to be filled with water, is not of such 
 thickness as to prevent the use of the achromatic con- 
 denser, a very important requisite. The upper plate 
 has a small hole, A, drilled through it, and one corner 
 removed, as at B. A small strip of glass cemented 
 below prevents the thin glass cover over the object 
 from slipping. Another strip is cemented on the lower 
 side of the cell at D, but not extending as far as the 
 removed part at B. The object of this is to prevent 
 the water in the cell from being removed by capillary 
 attraction, in case the slide in the neighborhood of B 
 should be a little wetted. This strip is not, however, 
 absolutely necessary. 
 
 To use the slide, fill the space between the two 
 plates with clean water, introduced at B, by means of a 
 pipette, and also place a drop on A, to remove the air. 
 The object being put on the top of the slide and wetted, 
 is now to be covered with a large square of thin glass, 
 C, at the same time covering the hole A. The slide can 
 now be placed upright, or in any position, and no water 
 can escape, but as it evaporates from under the 
 cover, more is supplied through the hole, A, and from 
 time to time an air bubble enters at *B; thus a constant 
 
MICROSCOPICAL PRAXIS. 223 
 
 circulation is maintained. A cell of the size named will 
 need replenishing only about once in three days, and 
 this is readily effected without disturbing the object. 
 I have been enabled, Prof. Smith continues, to make 
 observations by means of this slide, which it would 
 have been very difficult, if not impossible, to have made 
 without it. 
 
 The cell is not well-adapted to the study of minute 
 animals, if the examination is prolonged for any length 
 of time, unless the aperture in the cell be exceedingly 
 small, otherwise the active creatures will surely discover 
 it and pass through it into the reservoir. They will 
 often increase greatly there, but they are then always 
 beyond the reach of any but the lowest-power object- 
 ives. For algae, desmids or other still-life the slide is 
 commendable. 
 
 Sternberg's Culture-cell. 
 
 A simple form of culture cell is described by Dr. Geo. 
 M. Sternberg, as a member of the Havana Commission 
 for the Investigation of Yellow Fever. It is made by 
 drilling an opening about one-fourth inch in diameter 
 through the centre of a slip, around which a very thin 
 circle of cement one-half inch in diameter, is turned on 
 one side of the slip and a thin cover attached to it by 
 gentle pressure. When the cement is thoroughly dry, 
 the cell is ready to receive the drop of blood or other 
 
224 MICROSCOPICAL PRAXIS. 
 
 fluid to be observed. This is placed in the bottom of 
 the cell, Fig. 22, which shows the slide in section, and 
 
 c 
 
 Figure 22. Sternberg's culture-cell. 
 
 flows by capillary attraction into the space between the 
 thin cover and the slide until it extends to the circle of 
 cement. Thus there is a thin stratum of fluid between 
 the points b and c which may readily be examined by 
 inverting the slide, and bringing the objective to any 
 point between the central air-chamber and the cement 
 circle. Finally, the cell is closed by fastening a large 
 thin-glass circle to the opposite surface. 
 
 Strassburger's Moist Chamber. 
 
 The inventor describes this to consist of an ordinary 
 slide, upon which is placed a ring of pasteboard moist- 
 ened with water. The object which is to be observed 
 and kept alive, is placed in a drop of water on a cover- 
 glass and inverted over the pasteboard chamber, the 
 cover being made to adhere to the cell by pressure. 
 The evaporation of the water is greatly retarded, if not 
 entirely prevented, so that in this simple manner Prof. 
 Strassburger has kept Spirogyra in conjugation alive for 
 several days. By moistening the pasteboard from time 
 to time the cover will remain attached indefinitely. 
 
MICROSCOPICAL PRAXIS. 225 
 
 Deby's Cell for Bacteria. 
 
 This is a three by one inch slip, having a glass ring 
 with ground edges cemented to it to form a cell. A 
 small hole is bored through the slip inside and near the 
 edge of the cell. The objects are placed-with a very 
 minute drop of water on a thin cover, which is inverted, 
 and attached to the top of the cell by a little lard. The 
 slip is then laid upon another of the same size, but not 
 perforated, and a couple of India-rubber bands are 
 passed over the ends. One end of this arrangement is 
 then placed in a little water, which, by capillary action, 
 will occupy the space between the two slips, and by 
 evaporation will rise into the cell and prevent 
 the drying up of the minute drop on the cover. 
 By this contrivance a drop of water no larger than a 
 pin's head can be kept of nearly the same size for 
 weeks together, and the development of bacteria, or 
 other minute organisms, retained constantly under ob- 
 servation. 
 
 M. Deby has also described what he considers to be 
 a simplification of the foregoing. Take a slip and 
 make in its centre a circular opening ^ inch in diam- 
 eter; lay it upon a slide not perforated, and bind the 
 two together by rubber rings. Grease the upper slip 
 for a short distance around the opening, arrange the 
 object on a thin cover, adding to it another cover % 
 inch in diameter, which will adhere by capillary attrac- 
 tion, and invert the whole over the central aperture. 
 When not under observation place in a shallow vessel 
 of water, as in the original. 
 
 16 
 
226 
 
 MICROSCOPICAL PRAXIS. 
 
 Pagan's Growing-slide. 
 
 The Rev. A. Pagan has devised the following for 
 the study of rotifers, algae, and other microscopical or- 
 ganisms needing a frequent change of water. He states 
 that the results obtained with it were remarkable. It 
 may be kept constantly on the microscope stage, if de- 
 sired. 
 
 Fig. 23, Pagan's life-slide. 
 
 It consists, as shown in Fig. 23, of a slip A slightly 
 longer than the stage, so that it shall project a little at 
 both ends. On it is placed a piece of blotting-paper 
 which leaves only the margins of the slip free, and a 
 hole is cut in the centre of the paper, B C, and at one 
 end is the triangular prolongation, B ', which is bent 
 downward close to the slide. Water is then drawn from 
 the vessel, Z>, by means of the capillary tube, E, and 
 drops on the blotting paper. Tne tube should be only 
 wide enough to allow one drop to fall every twenty sec- 
 onds, the water being drained off by the triangular pro- 
 longation. An inverted flask, F, may be filled with wa- 
 ter, and so placed that its mouth shall just touch the 
 surface of the fluid in the tumbler, and keep the level of 
 the water constant, thus ensuring the regular escape of 
 drops from the capillary tube; or to simplify the appar- 
 atus, this vessel may well be omitted. 
 
MICROSCOPICAL PRAXIS. 227 
 
 Slack's Tubular Live-box. 
 
 Mr. J. H. Slack makes a tubular cell for the examina- 
 tion of the mouth-organs of insects. He takes a small 
 homeopathic phial about half an inch long, and a quarter 
 of an inch wide at the mouth. This is inserted into a 
 hole cut in a wooden slip, the rim of the bottle prevent- 
 ing it from falling through. Another wooden slip has a 
 hole through it of larger diameter, and on the top is 
 cemented by shellac a thin glass cover. This slip is 
 laid on the other, the glass cover forming a lid which 
 closes the bottle, the whole being held in position by a 
 rubber band. A little cotton wool is put into the bot- 
 tom of the bottle to suit it to the length of the fly, which 
 must be inserted mouth upward, and kept moderately 
 near the cover-glass, upon which a drop of syrup is 
 placed. Flies will readily feed in this position, and they 
 are sufficiently limited in their power of lateral motion 
 to be easily kept in the field of the one-inch or of the 
 one and one-half inch objective. 
 
 This might be modified by cutting off the bottom of 
 the bottle so as to make a tube of it. The fly could 
 then be introduced, after the cover-glass had been ar- 
 ranged, and in the proper position with its mouth up- 
 permost. It could thus be gently forced to ascend to 
 the desired place by forcing the cotton wool in behind 
 it. 
 
 There are many other forms of life-slides or of grow- 
 ing-cells, but these will be enough to give the reader an 
 idea of what is needed, and of the simplest way to ac- 
 complish the desired results. The less complicated the 
 apparatus, the better it will be. 
 
 THE END. 
 
228 MICROSCOPICAL PRAXIS. 
 
 Index. 
 
 A. 
 
 Page 
 
 Abbe apertometer, - 120 
 
 Abbe camera, - 205 
 
 Aberration, chromatic, - 104 
 
 " spherical, 107 
 
 Acme lamp, 31, 35, 36, 38, 146 
 
 Acme stand, Queen & Co.'s, - 22 
 
 Adjusting the objective, - 93 
 
 " by color, - 98, 99 
 
 " by tube-length, 101 
 
 " over Pleurosigma, - 99 
 
 " " Podura, - - 96, 98 
 
 Adjustment, change of by change of eye-piece, 149 
 
 " coarse, - 9, 18 
 
 " collar and value of micrometer-spaces, 64 
 
 fine, - 9, 20 
 
 " " position of, 21 
 
 " " " of Queen's Acme, - 22 
 
 Air bubbles, 76 
 
 " to make, 77 
 
 " to recognize, 7 8 
 
 Air, refractive index of, - 113 
 
 Amphipleura pellucida, - i3 2 
 
 Amplifier, the, *93 
 
 Analyser, the, - *9& 
 
 Angle of field, 122 
 
MICROSCOPICAL PRAXIS. 229 
 
 Page 
 
 Animalcule cage, 212 
 
 Apertometer, - 120 
 
 Aperture, angular, - 112 
 
 " " by image-forming rays, 121 
 
 " " " " " " to measure, 1 23 
 
 " of certain objectives, - 126 
 
 " to measure, - 117 
 
 numerical, 112 
 
 " " and resolving power, - 131 
 
 " " to measure, 127 
 
 " table of, - 114 
 
 " to use, -. 113 
 
 Apgar's, E. A., mechanical, finger, - 209 
 
 Ashe, A., cited. 146, 147 
 
 B. 
 
 Bacillus tuberculosis, 153 
 
 Bacteria, Deby's cell for, - 225 
 
 Balsam, Canada, refractive index, - 113 
 
 " " diatoms in, - TOO 
 
 Bausch's, Edward, adjustment-method, - 99 
 
 Bausch & Lomb's camera lucida, - 208 
 
 Bausch, Edward, cited, - 99, 104 
 
 Beale's adjustment-method, - 95 
 
 Beale's growing-cell, 221 
 
 Beale, L. S., cited, - * 95 
 
 Beale's reflector, 202, 203 
 
 Beetle, Gyrinus, key to species, 6 
 
 Beck, R. and J., cited, - - 17, 96 
 
 Black-ground illumination, 117, 178 
 
 " " illuminators, 181 
 
 Blackham, J. G., cited, - 123 
 
 Blow-fly, proboscis of, - 128 
 
 Body of stand, 8 
 
 Body-tube, - - - n 
 
230 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 Body-tube, cloth-lined, 12 
 
 " " standard length, 35 
 
 Brownian movement, 76, 79 
 
 Bulloch's apparatus to measure power of eye-pieces, 54 
 
 Bulloch, W. H , cited, 23, 54 
 
 Bull's-eye, position of, - 35 
 
 " " to measure foci of, 35 
 
 % C. 
 
 Camera lucida, - 61, 200 
 
 Abbe's, 205 
 
 " " drawing-desk for, 207 
 
 Bausch & Lomb's 208 
 
 " " Grunow's, - 208 
 
 measuring by, - 61 
 measuring thickness of cover-glass by, 85 
 
 Wollaston's, 204 
 
 Cedar oil, refractive index, 113 
 
 " " to free front lens from, 163 
 
 Cell for monochromatic light, - 42 
 
 Central illumination by air bubbles, 78 
 
 Centre, importance of the, 44 
 
 Centring illumination, Pennock's method, 44 
 
 Cholera spirillum, - 153 
 
 Chromatic aberration, - 104 
 
 tests for, -104, 108 
 
 Closed point, - - 92, 94 
 
 Collar adjustment, - 92 
 
 Color, adjusting by, 98 
 
 Color corrections, table of, 105 
 
 Coma, - 94 
 
 Condenser, Abbe's, - 175 
 
 " achromatic, 175 
 
 11 Bausch & Lomb's, - 17? 
 
 " central illumination with, - 180 
 
MICROSCOPICAL PRAXIS. 231 
 
 Page 
 
 Condenser, diaphragm carrier of, - . 177 
 
 diaphragms of, i 7I> I7 6 
 
 immersion, - ^ 
 
 sub-stage, T 6 9 
 
 to focus, - _ J74 
 
 to modify illumination with, - 172 
 
 objective used as, - ^5 
 
 Powell & Lealand's, I7 6 
 
 " Watson & Son's, - - . I7 6 
 
 Concave and convex surfaces, aspect of, - - - 80 
 
 Correction, over and under, - - - 98, 104 
 
 Cover-glass, correcting objective for, 91 
 
 " forceps, 89 
 
 " on mounted objects, to estimate 
 
 thickness, - . 86, 87 
 
 " optical effect of, ' .. , . gi 
 
 " thickness, collar graduations and, - 103 
 
 Cox, J. D., cited, - - 185 
 
 Culture-cell, Sternberg's, ,. . 223 
 
 Czapski's method of measuring covers on 
 
 mounted objects, - ... 86 
 Czapski, S., cited, . ' -82, 86 
 
 D. 
 
 Dallmger, W. H. cited, - - 42, 171 
 
 Daylight, mirrors and. - - - 40 
 
 use of, . . 3 8 
 
 when commendable for microscopical 
 
 work, . 4 o 
 
 Definition, . . . . I2 8 
 
 limit of power for best, - 157 
 
 tests for, - i 2 8 
 
 Deby's cell for bacteria, - - - 225 
 
 Devron, G., cited, - 193 
 
 Desk, drawing, for Abbe camera lucida, - 207 
 
 Diameters defined, - - 4 
 
232 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 Diaphragm, - 46 
 
 " action of, 48 
 
 defined, 10 
 
 " displacement of in body-tube, 73 
 
 of condenser, 171, 176 
 
 " position of, - 46, 47, 48 
 
 Diatom, balsam mounted, 100, 131 
 
 " dry mounted, 100, 131 
 
 " secondary structure, - 152 
 
 " striae and Nobert's lines, table of, 136 
 
 " tertiary structure, 154 
 
 " test plates, - 132 
 
 tv usefulness of, 152 
 
 " variation of striae, 131 
 
 Douglas's device for handling thin covers, 89 
 
 Drawing the object, 200 
 
 Drawings, enlarged, to make, - 205 
 
 Draw-tube, 8, 15, 17 
 
 " " for each inch extended, increase of 
 
 power, - 1 8 
 
 " " increase of power by, - 17 
 
 " " low-power objectives in, 16 
 
 " " to extend, - 16 
 
 Dust, effect of, 75 
 
 u to dispel from eye-piece, - 56 
 
 E. 
 
 Enlargement of drawing, to ascertain, 67 
 
 Erector, the, - 194 
 
 Swell's micrometers, 58 
 
 Eye, effect of artificial light on, 39 
 
 Eye-lens of ocular, to measure power of, - 51, 52 
 
 Eye-piece, - 8, 49 
 
 " " anterior focus of, 143 
 
 " " compensating, 150 
 
MICROSCOPICAL PRAXIS. 233 
 
 Page 
 
 Eye-piece, diaphragm, use of for micrometer, - 50 
 
 " " for micrometer, best, - 65 
 
 " " function of, 49 
 
 " " Huyghenian, - 5 
 
 " " micrometer, 62 
 " " " to learn value of spaces of 63 
 
 " " negative, 49 
 
 " " parfocal, - 15 l 
 
 " " particles on, 7 6 
 
 " " positive, - 5 
 
 " " power of, - 55 
 " " " " affects value of micrometer 
 
 spaces, 64 
 
 " " rating of, - - 53 
 
 " " specks on, - 7^ 
 
 Eyes, keep both open, - 73 
 
 Eye-shades, - - 74 
 
 Eye-training, - 13* T 5 2 
 
 F. '.* 
 
 Fasoldt's test-plate, - - 43, 5 8 
 
 Field, actual, - 5 1 
 
 " " of certain objectives, 5 2 
 
 " u to measure, 5 1 
 
 angle of, I22 
 
 " apparent, - 5 1 
 
 "" " to measure, - 5 1 
 
 cause of size of, - 5 
 
 centre of for measuring objects, 109 
 
 flatness of, - I0 9 
 
 sweeping the, 2 7 
 
 " to. illuminate, 7 T 
 
 of view, 9 
 
 Filter paper, Japanese, 
 
 Finders, - l68 
 
234 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 Fine adjustment, - - 9, 2 o 
 
 used as micrometer gauge, 84 
 
 Finger mechanical, - - 209 
 
 Focus, - 9 
 
 " objective, 167 
 
 ' of concave mirror, - 33 
 
 " of objective, to measure, 158 
 
 " posterior principal, - 142 
 
 table of, 145 
 
 " Queen's 2 in. objective, 146 
 
 " Zeiss's A* objective, - 146 
 
 " within and without the, - 94 
 
 Foot of stand, . 8 
 
 Forceps, mounting, Chase's, 89 
 
 stage, - 209 
 
 Frey, H., cited, 7 8 
 
 Frustulia Saxonica, - 132 
 
 G. 
 
 Gage, A. P., cited, I97 
 Gage, S. H., cited, - 56, 77, 88, 102, 206 
 Glass, blue, - - 43 
 " " for condenser, - 173 
 " cover, - 8 1 
 " crown, refractive index of, - 113 
 " thin, circles and squares how cut, - 82 
 " " device for handling, 89 
 " " Hanaman's method of storing, 83 
 " how made, - 82 
 " thickness used by the various opticians, 88 
 " " to clean, - 83 
 " to measure thickness by camera lucidia, 85 
 " " micrometer-gauge 84 
 " " on mounted ob- 
 jects, 86, 87 
 
MICROSCOPICAL PRAXIS. 235 
 
 Pag 
 
 Glycerine, refractive index, 113 
 Graduations of adjustment collar and cover- 
 thickness, 103 
 Griffith-club stand, 25 
 Griffith, E. H., cited, - 25, 188 
 Grooves, optical appearances of, 81 
 Growing-cells, 209, 213 
 " ' Beale's 221 
 " " Pagan's 226 
 " " simple, - f 214 
 " " Smith's 222 
 Grunow's camera lucida, - 208 
 Gymnastics, microscopical, 43 
 Gyrinus, key to species, - 6 
 
 H. 
 
 Hall, J. B., cited, 74 
 
 Hanaman, C. E., cited, 83 
 
 Heads, milled, - n 
 
 Hitchcock, R., cited, 220 
 
 I. 
 
 Iceland spar, - 197 
 
 Illumination affected by position of mirror, - 33 
 
 " black-ground, i77> J 7 8 
 
 central by air bubbles, - 78 
 
 " with condenser, 180 
 
 " diabolical, - 7 8 
 
 " general, of work table, - - . * 4 
 
 " oblique, 7 8 
 
 " '" with condenser, 177 
 
 " proper, of microscope, - 31 
 
 " to make central, - 44 
 
 " white cloud, 4 
 
 Illuminator, Stratton's, 3 r > 35 3 8 
 
 " vertical, l8 9 
 
236 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 Illuminators, black-ground, - 181 
 
 supra-stage, 187 
 
 Image, effect of daylight on, - 39 
 
 Image -forming rays, - . I2 i 
 
 Immersion-fluid, to apply, - !6i 
 
 " to measure index of, 164 
 
 " to remove, - . 163 
 
 Immersion-media, homogeneous, - - 164 
 
 Immersion paraboloid, - - ^2 
 
 Index, refractive, of various substances, - - 113 
 
 Indicator for ocular, ... c6 
 
 Instrument, to care for the, - - 67 
 
 L. 
 
 Lamp, Acme, 3Ij 35> 3 s 
 
 " best position for, - 34 
 
 " flame, edge of, ... 3 8 
 
 " position of, . 34> 7! 
 
 " Str *tton, . 31,35,38 
 
 with circular or flat wick, 41 
 
 Length of body-tube, standard, - 4 
 
 Lens, convex, as camera lucida, - 41 
 
 " hemispherical, i 33) 186 
 
 " s Pt, 183 
 
 Lepisma, scales of, - . I3 ^ 
 
 Lieberkuhn, the, - . 187 
 
 Life-slides, - . . 213 
 
 " Logan's, 2l8 
 
 " simple, 219 
 
 Light, artificial, advantage of, - 39 
 
 " for drawing, to modify, - 200 
 
 11 from oil or gas, 41 
 
 " lamp, to make rays parallel, 33, 35 
 
 " monochromatic, *- - 41, 42 
 
 cell for, - 42 
 
MICROSCOPICAL PRAXIS. 237 
 
 Page 
 
 Light, monochromatic, copper sulphate solution for, 41 
 " " potassium bichromate 
 
 solution for, .- 43 
 " oblique, 23, 29, 78, 133 
 
 " polarised, 196 
 
 " ' reflected, 45 
 
 " transmitted, - 45 
 
 " when more is needed by objective, - 180 
 
 Lighting, proper, for microscope, - 31 
 
 Lines, ruled, Fasoldt's, - 58 
 " " Nobert's, 43, 58 
 
 " " visibility of, - 131 
 
 " to inch, greatest number seen, 58 
 
 " " " million, 58 
 
 Live-box, 209 
 
 " " Slack's tubular, 227 
 
 Lubricating working-parts, 76 
 
 M. 
 
 Magnifying power, - 141 
 
 " high, - . 159 
 
 " initial, - 155 
 
 " " limit of for best definition, - 157 
 
 " " limit of useful, - 159 
 
 Maltwood finder, - 168 
 
 Measuring by camera lucida, - 61 
 
 " objects, centre of field for, no 
 
 Mechanical finger, - 209 
 
 Mica covers, - 81 
 
 Micrometer, - 58 
 
 " Ewell's, 58 
 
 " eye-piece, 62 
 
 " " " proper ruling of, - 65 
 
 " " " to learn value of spaces, - 63 
 
 Fasoldt's, 5 8 
 
2 3 8 MICROSCOPICAL PRAXIS. 
 
 Micrometer guage, ... g 4 
 
 " Bausch & Lomb's, 85 
 
 " fine-adjustment used as, 84 
 
 Rogers's, . 5 g 
 
 spaces, estimating parts of, - 60 
 
 recording value of. - - 65 
 
 stage, - 59 
 
 standard for, - . 62 
 
 Micron, value of, 62 
 
 Microscope, to illuminate, 7o 
 
 to remove from case, 68 
 
 to return to case, . 7^ 
 
 Microscopical gymnastics, 
 
 Microscopist, amateur, influence of, ^3 
 
 Microscopists, American Society of, - 62 
 
 Mirror, I0 
 Mirror-bar, 
 
 " best position of, ,. 
 
 " concave, action of, ^ 2 
 
 axis of, 34 
 
 distance from lamp, proper, - 36 
 
 " object, 37 
 
 size of, - - 31 
 
 to learn diameter of its hollow sphere, 33 
 
 to measure focus of, 33 
 
 " plane, action of, - 32 
 
 " reflector, a simple, 203 
 
 " size of concave, - 3! 
 
 Mirror, to manipulate the, 7 T 
 
 " use of the, - 3I 
 
 Moist chamber, Hitchcock's, - 220 
 
 Strasburger's, - 224 
 
 Moller's test-plate, diatoms on, - 134 
 
 Moore, A. Y., cited, . 9 8, 127, 185 
 
MICROSCOPICAL PRAXIS. 239 
 
 N. 
 
 Naegeli & Schwendener, cited, 105, in 
 Nelson, E. M., cited, 128, 155, 157 
 
 Nicol prisms, - 197 
 
 Nobert's lines, - 43, 58, 135 
 
 " " and diatom striae, table of, 136 
 
 Nose-piece, safety, - 14 
 
 O. 
 
 Object, how image should appear, - 93 
 
 " opaque, under Woodward prism, - 785 
 
 " to place small in field, - 81 
 
 Objectives, actual field of certain, - 52 
 
 " adjustable and non-adjustable, 91 
 
 " adjusting by cover imperfections, - 95 
 " adjustment, change of with change 
 
 of eye-piece, - 149 
 
 " aperture of certain, 126 
 
 " as an erector, 195 
 
 " corrected for certain tube-length, 4 
 
 " correcting for cover, - 91 
 
 " defined, 8 
 
 " defining power of, - 128 
 
 " . for cover, correcting, - 91 
 
 " homogeneous, Stevenson and, 113, 160 
 
 " how rated, - 138 
 
 " immersion, - 160 
 
 " " to clean, - 161 
 
 " in draw-tube, low-power, 16 
 
 " low-power, to illuminate, - 72,73 
 
 " needs more light, when, - 180 
 
 " optical centre, 142 
 " position of posterior principal focus 
 
 of certain, - 145 
 
240 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 Objectives, posterior principal focus, 142, 143 
 
 " " " " to obtain, 144 
 
 " Powell & Lealand's -^, 159 
 
 " specks on, seen in field, - 76 
 
 the, 90 
 
 to adjust by coma, 94 
 
 " " " " cover imperfections, - 95 
 
 " to attach to tube, - 69 
 
 Tolles's ^, 159 
 
 " to measure focus, - 158 
 
 " to obtain posterior focus, 144 
 
 " used as condenser, 176 
 
 u Queen & Co.'s 2 inch, - 146 
 
 " Zeiss's A*, - - 52, 72, 146 
 
 Oblique light, 23, 29, 78, 133 
 
 Ocular, - - 8, 49 
 
 Oil bubbles, - - 76, 77 
 
 " to make and recognize, - 78 
 
 Opaque objects under Woodward prism, - 185 
 
 Open point, - - 92, 94 
 
 Optical centre of objective, - 142 
 
 Over and under correction, - - 98, 104 
 
 P. 
 
 Pagan's growing-slide, - 226 
 
 Paper, Japanese filter, 56 
 
 Parabolic speculum, 188 
 
 Paraboloid, 182 
 
 Pedesis, - 79 
 
 Penetration, - 137 
 
 Pennock, Edward, cited, - - 44, 72, 101, 104, 151 
 
 Pennock's method of centring the illumination, 44 
 
 Peragallo, H., cited, 171 
 
MICROSCOPICAL PRAXIS. 241 
 
 Page 
 
 Pillars of stand, 8 
 
 Pinion of stand, 9 
 
 Plaster-of-Paris plate, 40 
 
 Pleurosigma angulatum, - 99, 101, 128, 132 
 
 " formosa, 128 
 
 Pocket-lens, chromatic and spherical abberrations, 106 
 
 " combination, i 
 
 " convenient power for, - 2 
 
 " diaphragm of, - i 
 
 " simple, i 
 
 " test for, 5 
 
 to measure focus, 2 
 
 " " magnifying power, - 3 
 
 Podura, - 95 
 
 " scale, appearance of, - - 96, 97 
 
 Polariscope, - 196 
 
 Polariser, 198 
 
 Posterior focus of objective, - 142, 143 
 
 " " " " to ascertain, - 144 
 
 Power, increase of by draw-tube, -.- 17 
 
 " initial magnifying, - 155 
 
 " magnifying, - 141 
 
 high, - 159 
 
 " " limit of for best definition, - 157 
 
 " " " " useful, - 159 
 
 u " table of increase for each inch 
 
 of draw-tube, - 18 
 
 " resolving, - 129 
 
 " " to increase, - 136 
 
 Prism, Nicol, - 196 
 
 " right-angled as erector, 195 
 
 " Woodward, - 184 
 
 " " opaque objects under, 185 
 
 Proboscis of blow-fly, 128 
 
242 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 R. 
 
 Rack of stand, - 9 
 
 Ray, ordinary and extraordinary, - 198 
 
 Reflector, a simple, - 203 
 
 " neutral tint, - 202 
 
 " " " disadvantage of, - 203 
 
 " thin-glass, 201 
 
 Refractive index of immersion fluids, to measure, 164 
 
 " " Smith's test for, - 165 
 
 Resolving power, - 129 
 
 " " to increase, - 136 
 
 Roger's micrometers, 58 
 
 S. 
 
 Scale and vernier, - 15 
 
 Scales, Lepisma, 135 
 
 " use of Podura, 96 
 
 Schultz, A., cited, - 191 
 
 Screw, Butterfield, - 13 
 
 " fine-adjustment, value of graduations, - 22 
 
 " society, - 12 
 Selenite, - 196, 199 
 
 Slack's tubular live-box, - 227 
 
 Slide, 10 
 
 Slip, 10 
 Smith, H. L., cited, - 165, 189 
 
 Smith's growing-cell, 222 
 
 Smith's test for refractive index, - 165 
 
 Soap and water, Brownian motion of, 80 
 
 Spherical aberration, 107 
 
 Spaces of micrometer, number, 60 
 
 Speculum, parabolic, JfS8 
 
 Spot lens, 183 
 Spring clips, - - 10, 24 
 
 " of Griffith-Club stand, - 25 
 
MICROSCOPICAL PRAXIS. 243 
 
 Page 
 
 Stage, - 9, 25 
 
 " mechanical, - 25 
 
 " micrometer, - 59 
 
 " " to use, - 60 
 
 " safety, - 27 
 
 " ' 4< Stewart's, - 28 
 
 " special, 25 
 
 Stand and its parts, 7 
 
 " Griffith-Club, 24 
 
 " to prepare for use, - 69 
 
 " without coarse adjustment, - 19 
 
 Star, artificial, - 106, 107 
 
 Sternberg's culture-cell, - 223 
 
 Stevenson, J. W., and homogeneous objectives, 1 13, 160 
 
 Stop defined, - 10 
 
 Strasburger's moist chamber, - 224 
 
 Stratton illuminator, 31, 35, 38, 146 
 
 Styrax, refractive index of, 113 
 
 Sub-stage, n 
 
 Suffolk, T., cited, - 201 
 
 Sunlight, direct, when used, - 40 
 
 Surirella gemma, - 132 
 
 T. 
 
 Table, aperture, 114 
 
 " " of certain objectives, 126 
 
 " numerical aperture, to use, - 113 
 
 " the work, 68 
 
 " tube-length, - 103 
 
 Test-plate, Abbe, - 107 
 
 MOller's and Thum's, - 134 
 
 " of diatoms, - 132 
 
 Thickness of cover glass, - 84 
 
 Thin glass, 81 
 
 Times and diameters defined, - 4 
 
244 MICROSCOPICAL PRAXIS. 
 
 Page 
 
 Triplet, achromatic, - i 
 
 " " useful power for, 2 
 
 Tube-length, adjusting objective by, 101 
 
 " of various opticians, - 102 
 
 " optical, 143 
 
 " " to measure, 146,148 
 
 u standard, - 4 
 
 Tubular live-box, Slack's, 227 
 
 V. 
 
 Vernier, scale and, - 5 
 
 Vertical Illuminator, 189 
 
 Vision, arbitrary distance for distinct, 3 
 
 W. 
 
 Water, refractive index, - 113 
 Wead's method of measuring covers on mounted 
 
 objects, 87 
 
 Wenham's method of adjusting objective, 94 
 
 Wenham, W. H., cited, - 94 
 
 Whirligig beetles, key to species, - 6 
 
 Whitney, E. J., cited, 212 
 
 Wollaston camera lucida, 204 
 
 Working distance, - - i, 138 
 
 " " to measure, - 139 
 
 Woodward, J. J., cited, - - 43, 65 
 
 Woodward prism, - 184 
 
 Z. 
 
 Zeiss's A* objective, field of, - 52 
 
 " " " posterior focus, 146 
 
 " " " to illuminate, - 72 
 
 Zentmayer, J., cited, 23 
 
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