jStl'*'^ <3v|FT or COLLEGE OF PHARI^IACY t'^^r^;i -c v^ ip^ ■'■'' -i,' -|f*yv ;i^^'' .^ *.Mte' ^^!^<^ m*^^Mm ^'.*;:^^ l^l?ei i ^v^t ^. :j* l^r*^*^^ Digitized by the Internet Archive in 2007 with funding from Microsoft Corporation http://www.archive.org/details/artofprojexperiOOdolbrich The Art of Projecting. PHYSICS, ^^f Chemistry, and Natural History WITH THK PORTE LUMIERE AND MAGIC LANTERN. BY PROF. A. E. ^OLBEAR, M.E., Ph.D. (tufts college.). Neto Edition, laeijiseU, toitfj Munitions. ILLUSTRATED. BOSTON 1892 LEE AND SHEPARD PUBLISHERS 10 MILK STREET NEXT " THE OLD SOUTH MEETING HOUSE " K Copyright, 1877, By Lee and Shei-ard. Copyright, 1887, By Lee and Shepard. All rights reserved. Art of Pkoiecting. S J PARKHILL & CO., PKINTERS BOSTON PREFACE TO THE SECOND EDITION. Since the first publication of this book the author has received so many commendatory letters from many parts of the world that he is fully persuaded that the book met a real want ; and a sun- beam is now made useful in school work and in the study of phenomena in many places where no substitute is practicable. In preparing a new edition, some things have been added which it is hoped will make the book still more useful to such as consult it. Two things may be specially mentioned here : the electric lamps and lights for projection purposes, and the production and phe- nomena of vortex rings. Of the former there is pointed out what is at present practicable, and of the latter it may be said that the vortex-ring theory of the constitution of matter has so much philosophical as well as scientific importance, and the phenomena presented by vortex rings are so curious and unexpected, that the author has felt warranted in presenting what he believes to be the most complete series at present known, especially as he believes himself to be the discoverer of a considerable number of them. Several small treatises on the management and use of lan- terns have been published lately, and may be had on application to almost any of the larger dealers in physical instruments and lan- tern transparencies. It was not thought advisable to add anything on that subject. An excellent manual for experimental work with the lantern has been published by Lewis Wright, which every one interested in such matters should have, especially as many of the experiments described in it can be done as well or better by the use of a beam of sunlight, — the use of a beam of sunlight for projection being the peculiar province of this book. A. E. DOLBEAR College Hill, Mass., Sept. Qth, 1887. 42'lSii INDEX. Absorption spectra .... 114 Acoustic curves 61 Air thermometer ..... 144 Animalcule cage 33 Arc light, To project .... 161 Biaxial crystals 132 Bubbles 107 Bubbles, cohesion of . . . .168 Calorescence • 149 Camphor on water .... 47 Camera obscura ...... 80 Candle power 13 " flame, To project. .132,100 Capillarity 49 Caustics by reflection ... 92 " " refraction ... 104 Chameleon top 143 Chemical tank 34 " reactions .... 157 Chladni's experiment ... 62 Chromatic aberration ... 104 Chromatic aberration, lessen- ing of 169 Chromatrope . ...... . 142 Cloud formation 145 Cohesion 45 Cohesion figures 47 College lantern 41 Colors of thin films .... 107 Concave mirror, To project with 63, 91 Convection in water .... 156 " air 98 Condenser: its use .... 26 Convex mirrors 93 Crova's apparatus 77 Crystalline substances for po- larized light 133 Darkened room 5 Diagrams on mica 129 Diamagnetism 151 Diffraction 137 Disks for study of colors . 110, 143 Dispersion 105 Distortion 93 Divisibility of matter .... 44 Double refraction 126 Double salts. Prepared . . . 134 Drummond light 11 Eidotrope 42 Electric light 9 " To project ... 163 Electric spark, To project . . 166 Elements, Spectra of .... 165 Engravings, To transfer . . 32 Etcliing upon glass .... 31 Films, Vibration of .... 169 Floating magnets 167 Fluorescence 119 Focal length of lenses ... 21 Focusing 25 Fountain, Illuminated ... 96 Fraunhofer's lines Ill Galvanometer 147 Gases for lime light .... 11 Ghost . • 84 Glue, Marine 35 Gramme machine 9 Gravitation 50 Heat ........ 144, 155 Heliostat 1 Ice flowers 52 Illumination, Intensity of . • 81 Images formed by lenses . . 100 Incandescent electric lamp . . 163 " lamp fil- aments 164 Incandescent electric lamp, Currents for 164 Interference 71 '• spectra .... 118 Interlacing lines 70 Kaleidoscope 88 Kaleidophone 57 Lamps, Electric arc . . 159, 160 " Incandescent. ... 163 Landscape projection .... 170 Lanterns 14, 18, 19 Oxyhvdrogen, 12, 16, 18, 19 Electric 161 Lenses • .. . 19 " Magnifying power . . .33 " Mountings for .... 23 Light 80 '• Intensity of 13 vi INDEX. Light, Magnesium 10 " Lime 11 " Composition of, 109, 117, 136 «• Polarized 127 Lissajous' experiments ... 09 Mach's experiment .... 64 Magnetism 150 Magnetic phantom 150 Manometric flames .... 62 Marine ghie 35 MegascoiJe 38 Melde's experiment . . . • 58 Microscope, 8olar 100 " attachment ... 49 Minute substances 1.33 Mirage 95 Monochromatic light . . 108, V£l Newton's disk 143 " rings 109 Objective 25 Objects for projection ... 27 Organ pipe 65 Opeidoscope 59 Outline drawings 29 Overtones 71 Persistence of vision .... 139 Pepper's ghost 84 Plateau's (experiment) ... 56 Polarization of light .... 127 Porosity 45 Porte Lumiere, To make . . 2 " " its use ... 24 Porte Lumieres, various pat- terns 2, 3 Projection with single lens . 24 '* " condenser . . 27 " of large apparatus, 35 " Apparatus for ver- tical 40 Pyrometer 145 Rainbow 100 Reactions, Chemical .... 157 Reflections 82 Reflections, Multiple .... 83 Refraction 97 Resultants 72 Salicine crystals 134 Screens 6 Sciopticons 18 Silver crystals 53 Singing flames 64 Sinuous lines 69 Sizes of objects and of images, To compute 171 Soap bubbles, Persistent . . 108 •' " Tension of . . 107 Sodium line in solar spectrum, 170 Solar microscope 100 " spectrum Ill Spectacle glasses. To test . . 132 Spheroidal form 62 Spectra, Methods of project- ing 121, 153 Spectrum analysis 119 " of sodium .... 121 *' " " reversed . 122 Starch 134 Stroboscope 139 Sympathetic vibrations ... 75 Thermometer 144 Total reflection t)4 Tuning forks 57 Vibrations of strings .... 59 " " forks .... 57 Vision, Persistence of . . . 139 Vortex rings, To produce . . 172 " " their phenom- ena 174 Water, Decomposed .... 153 " Maximum density . . 146 Refraction of ... . 97 " Total reflection in . . 94 Waves in water 61 Whirling-table attachment . 77 Zoetrope 140 THE ART OF PROJECTING. A MAGNIFIED image of a picture, or of any phenom eiion, when thrown upon a screen by means of sunlight, and lenses, or with a magic lantern, is called a projec- tion. When sunlight is to be used for this purpose, it is necessary to have some fixture to give the proper direc- tion to the beam. The helinstat and the porte lutniere are the devices in common use. The latter was the earliest form, and was invented by Gravesand, a Dutch professor of natural philosophy, in the early part of the last century. It was afterwards reinvented by Captain Drummond, an Englishman, who called it the heliostat. The latter term is now only applied to an automatic arrangement, by which a mirror is moved by clock- work in such a way that a beam of sunlight reflected from it may be kept in one direction all day, if it be needed so long. Silberman and Foucault have each devised very satisfactory instruments, but they are too costly to be owned by any but the wealthy ; the catalogue price of the cheapest of these being five hundred francs. C. Gerhardt, of Bonn, however, makes a small one, carrying a good mirror three inches in diameter, for twenty dollars. THE PORTE LUMIERE HOW MADE. The poi'te lumiere is made o^ various patterns by different makers, but the differences consist chiefly in 2 THE ART OF PROJECTING. the devices for giving proper movements to the mirror. Their cost is from ten to twenty dollars according to their size, workmanship, and attachments. On the opposite page are engravings of several such as are in the market. It is recommended that one be purchased at the outset, if it can be afforded, but as many who would be glad to work with one may not be able to purchase it, directions will be given for making one that will enable any person who is familiar with the use of carpenters' tools to make one at a trifling cost that will answer for many purposes. The room in which the porte lumiere is to be used must, of course, be one into which the sun can shine. A room having windows only upon the North side, evidently cannot be used at all for such a purpose ; one having windows only upon the East or upon the West side could be used only in forenoon or afternoon ; while one with windows looking to the South can be used nearly all day. Choose then that window where the sun is available the longest, and opposite to which can be stretched the screen to receive the projections upon. Next, take a well-seasoned piece of pine board a foot or more in width, and an inch thick when dressed ; cut it to the length of the width of the window sash, so that it may fit into the window frame, and the sash be brought down upon it ; this will keep it tightly in place. With the compasses, scratch two concentric circles in the middle of the board, one with a radius of four inches, the other with a radius of four inches and a half. Saw out the inner circle completely, and cut the other but one half through the board, and then cut away, making a square rabbet, as shown at b b. Next, take a round piece of inch board of the same diametei Hawkkidge's. ^ Queen & Co.'s, port:6 lumieres. '"i'!ll!!|||||||!i'liiir'^"" ' ""'"'I*"*" Adams's. Strong's. PORT]il LUMIERES. PROJECTIONS FOR THE SCHOOL-ROOM. 3 as the outer circle (namely, nine inches), cut a rabbet upon one side of it so that it will nicely fit into the hole of the larger board, as indicated 2Xc c. Make the worked edges, and touching surfaces, quite smooth ; but the outer edge should be made a trifle smaller than the hole, in order to allow the disk to turn freely round in it ; then the hole may be cut in the disk to receive the lens, four or five inches in diameter, whichever it may chance to be. Procure a nice piece of thin looking-glass, twelve or fifEeen inches long and five inches wide. Fasten it to a back of wood made a little larger than itself, with broad-headed tacks, or bits of wire driven in and the top bent at right angles. This back will need to be an inch thick at the bottom, but may taper like a shingle to the top, where it need not be half an inch thick ; m is the mirror and // is the back in the figure adjoining. A common desk hinge h may be used to attach this mirror- mounting to the part c in the figure below. It must be so fastened that the mirror may swing through ninety degrees from a horizontal plane. The accom- panying figure will be sufficiently definite to enable any one to make the whole instru- ment. When the mirror is securely fastened to the part c, the whole can be inserted in the board /^ ^ and buttoned in, as is shown at b and b ; these buttons must 4 THE ART OF PROJECTING, not bind upon the part c, as this must have an easy rotation in its place, though they need to be tight in the board b ; three of them will be enough. Again, a string must be attached to the end of m, passed through a small hole in orte himiere in its place, and so adjust it that the beam of light / (Fig. 17) is reflected horizon- tally, and falls upon the screen s. It will appear as a bright spot, five or six inch- es in diameter. Darken the room, by draw- ing the curtains or closing the shutters, and the beam of :E'ig. It, light can then PROJECTIONS. 25 be seen from the window to the screen by the light re- flected from the dust particles, which are always in the air. Now fasten in the retort-stand a lens o four or five inches in diameter, and with a focus of a foot or more, and place it two or three feet from the opening, in the path of the beam, and perpendicular to it. It will at once be noticed that the light is converged by the lens, the rays crossing each other in front of it, at its focus, from which they diverge, and appear upon the screen as a large disk of light. If some object, as d^ (Fig. 17), be placed between the opening and the lens 0, a place may be found by trial, when the image of the object will be seen upon the screen. The outline should be well defined ; it will be inverted and much enlarged. Finding the right adjustment of the object and the lens, so that the image is in its proper place, and has a sharp outline, is called focusing In general it is best done by fixing the object in the path of the beam first, and then placing the lens rather close to it, and slowly moving the lens toward the screen, being careful to keep it perpendicular to the beam until the image is plainest. It will be well for a beginner to take a number of ob- jects : some opaque, like the finger, a pencil, or a key, and some transparent, as a grasshopper*s wing, or a piece of glass with a design drawn upon it, or a regu- lar lantern transparency. A lens thus used to project a picture is called an objective. These two pieces of apparatus, \.h& porte iumiere and the single lens, have a much wider application than one unfamiliar with them might suppose. Every picture made for the magic lantern, or the stereopticon, can be shown with these in the day-time, even better than with the others at night. Every school in the land may have one, for the carpenter can make the porte lumiere^ and t6 THE ART OF PROJECTING. the lens will cost but a trifle. The pictures themselves, though not half as costly as they were before photogra- phy was applied in making them, can be rented of any one who keeps them for sale, if one cannot afford to buy them outright. Most excellent transparencies, on all sorts of subjects, can be bought, from six to nine dollars a dozen, of any lantern-maker or dealer in pho- tographic materials. If the teacher wished to give a lesson on the elements of drawing, his copies could be prepared upon glass, by one of the methods given a little further on. These, when projected, would be so large that a large school could see them as plainly as if they had been drawn upon a huge blackboard, with chalk. The room could be light enough for any of the required work. Geometrical fig- ures, outline maps, botanical specimens, the kaleido- scope, chemical reactions in a large test tube ; natural history specimens, such as small fish, pollywogs, water beetles, butterflies, grasshoppers ; the splendid colors on huge soap-bubbles ; the vibrations of tuning-forks, and of cords ; the intensity of light, reflection, refrac- tion, magnifying power of lenses, and many more things, may be projected, in an admirable way, with only these two pieces. THE CONDENSER AND ITS USE. The rays of light reflected from the mirror a (Fig. 17) through the aperture, are parallel, and the diameter of the lens should be as great as the thickness of the beam, and it may have a greater diameter with advan- tage. If it has less, some of the light will pass its edge, and either be unused or, what would be worse, fall upon the screen and make a bright spot in the mid- dle of the picture. The smaller the object to be pro- POOJECTIONS. 27 jected, the smaller must be the lens used as the object- ive, and the shorter must be its focal length \ hence, if a beam of parallel rays is used, it may often be so small as to be nearly useless, for the divergence is so rapid beyond the focus of a short focus lens that the little light thus used would be too much scattered. It be- comes necessary, then, to make a large beam of light to pass through the small lens. This is accomplished by means of a sec'jnd lens, called the condenser^ because its office is to condense a large number of light rays for the double purpose of illuminating the object better and making them all to pass through the smaller lens. This condenser is usually four or five inches in diameter, though for special purposes it is sometimes made a foot or more in diameter. For the porte lu77iiere the con- denser may be the same lens that was used as the ob- jective, or any lens may be used that has a sufficient di- ameter and a focus of from one to two feet. It may be either double-convex or plano-convex. TO PROJECT WITH A CONDENSER. The object ^ (Fig. 18) is placed near the condenser c, and the objective is brought near to it and slowly moved toward the screen, as before, until the well de- fined image ap- pears upon it. It must be not- Fig. 18. ed here that the size and focal length of the objective must be such that all the light passes through it when it is at its proper distance from the object. If ^ be moved toward 28 THE ART OF PROJECTING. the object, it will be seen that some of the light does not pass through it. If the object d be moved toward the objective, then some parts of it will not be lighted, and consequently but a part of it would be projected. If the object d is quite small, like a fly, or a flea, or a small crystal, it will be necessary to bring it forward, toward the focus of the condenser, where it will be more strongly lighted, and allow the use of an objective of shorter focus, and consequently higher magnifying power. If the object be made of wood, or any kind of tissue, be careful about bringing it very near to the focus, as the great heat there may destroy it in a few seconds. This danger may be somewhat lessened by placing between the condenser and the object the chem- ical tank, containing a strong solution of alum. The common pocket botanical glass, having a focus of an inch or two, will answer for very much of this work, but Zentmayer*s inch-and-a-half gas microscope object- ive is superior to any other lens I have seen for such projections. This arrangement is essentially the solar microscope^ The object may be exceedingly minute if the objective has a very short focus, say half an inch, or less. It is possible to magnify an object a thousand diam- eters, or a million times, and still have it so well lighted that a large audience can see it plainly. A list of things that are suitable for projections with this ar- rangement is appended, mainly for the purpose of indi- cating the breadth of its field of usefulness : Hairs of various animals, which may be held between two strips of glass. Down from the wings of moths and butterflies ; these will adhere to a piece of glass with- out any pressure. Scales of fishes. Eyes, legs, and wings of flies, or the whole of any insect. Stings of- PROJECTIONS. 29 bees and wasps. Antennae of moths and mosquitoes. Fibres of cotton, woolen, silk, etc. Ferns, moss, lichens, leaves of trees. Thin sections of wood. Small flowers, stamens and pistils, pollen. Mites in cheese. Butter- flies, beetles, animalcules in stagnant water. Vinegar eels. Crystallization of camphor, sulphate of copper, and most solutions of crystallizable substances. Diatoms, and indeed most objects that are prepared for the microscope, appear to good advantage upon a screen. Any book upon the microscope will have many valuable hints upon obtaining and preparing ob- jects in a suitable way, and will be a very useful book to one interested in natural history but who cannot af- ford to buy a good microscope. OUTLINE DRAWINGS FOR PROJECTION. Every one who uses either a lantern or the porU lumiere for purposes of instruction, will need to make outline pictures to illustrate his subject, as it will be fre- quently impracticable to get a photograph of what is wanted. Moreover, a simple outline is often quite suf- ficient for the illustration, as, for instance, superposi- tion and inclination of strata in geology ; sections of machines ; writing, or musical notes ; outlines of leaves, roots, parts of a flower, insects, maps, etc. The surface of transparent glass is so smooth that it cannot be marked with either common ink or a lead pencil. If the glass be ground, so that a pencil will mark it, it becomes so opaque that but little light can go through it ; hence, a surface must be prepared which will be transparent and yet allow marking upon it. This can be effected in many ways, and I give a num- ber which I know to be practicable : I. If a piece of glass be rubbed on one surface with 30 THE ART OF PROJECTING. a piece of hard soap, enough will adhere to it to make the glass semi-opaque. Now draw the design with a fine-pointed stick. It will clear the soap from the glass, and so permit the light to shine through the marks. This has the advantage of permitting the same glass to be used like a slate, for with a drop of water upon the finger the old design can be rubbed out, leaving the glass coated for another picture. The same thing can be done with a surface of beeswax, but the glass would need heating in order to re-spread the wax. 2. For more permanent pictures, a very good way is to flow the glass with photographers* transparent var- nish, and then scratch the design upon the varnish, not cutting through to the glass. The light is so much scattered from this scratched surface, that it appears as a dark line, and answers a very good purpose. The prepared plate can be laid over the design wanted if it is to be a copy, and is of proper size ; the transparency allows the picture to be plainly seen, and all its mark- ings can easily be followed. The varnish is quite hard when dry, and with a little care in handling these pic- tures, they need not become scratched. They can be entirely protected from that danger by covering them with another clean glass of the same size, and binding their edges with paper, as common lantern-pictures are bound. Photographers have also another kind of var- nish called ground-glass varnish, which, when spread upon glass, gives it an appearance similar to ground glass. This surface permits drawing with a pencil or with ink upon it, and then a coat of the transparent varnish will render the first coat transparent, leaving the lines in ink or pencil ; or the design may be drawn through the first coat of varnish, in which case, the light will shine through the lines and appear white upon the screen. PROJECTIONS. 31 3. If India ink be rubbed up in water until it is quite thick, it can be used for drawing designs upon ordinary glass. 4. A thin sheet of gelatin may be treated like the glass coated with the transparent varnish, and either have the design scratched upon it, or drawn with ink. It should be inclosed between two glasses for protection, 5. Thin sheets of transparent mica will receive lines drawn with india ink, or the figures may be scratched upon them with the needle or awl. 6. Designs may be nicely etched upon glass, by first coating the glass with a thin, even coat of beeswax, which can be well done by heating the glass over a lamp until the wax melts and flows over its upper sur- face. When it is cool, draw the design with a needle point or a small awl, cutting through the wax all the way. Take an old saucer, or some such dish which you are willing to spoil for other use, and put into it a table spoonfull of powdered fluor spar. Upon that pour a table spoonful of strong sulphuric acid, and stir them together with a stick. Fasten the glass, drawing upper- most, to a piece of board large enough to completely cover the dish. The fastening can be done by crowd- ing tacks into the wood, so that the heads shall lap the glass and keep it in its place. When thus fixed and laid over the mixture of spar and acid, gently heat the dish, being careful not to inhale the fumes that will escape. When the fumes begin to appear, put the whole either out of doors or in a good chimney draught, and let it remain eight or ten minutes, when the wax may be removed by heat and rubbing, and the drawing will be found etched into the glass. Beautiful pictures of crystals can be made in this way, by taking various crystallizable salts, such as am- J2 THE ART OF PROJECTINu. wonium chloride, cupric sulphate, etc., and making a rather dilute solution of them, and then adding a little dissolved gum arable. Flow the solution over the plate, and let it remain horizontal until it is dry. The crystals will be seen to have separated from the gum, which will fill up all the mtermediate space. Put over the etching dish as before. The crystals will quickly dissolve, and their outlines will be beautifully etched upon the glass, which may now be washed clean in water. 7. Engravings may be transferred to glass by first coating the glass with dammar varnish, or with Canada balsam, and letting it dry until it is very sticky, which will take half a day or more. The picture to be trans- ferred should be well soaked in soft water, and care- fully laid upon the prepared glass, and pressed upon it, so that no air bubbles or drops of water are seen un- derneath. This should dry a whole day before it is touched \ then with the wetted finger, begin to rub off the paper at the back. If this be skillfully done, almost the whole of the paper can be removed, leaving simply the ink upon the varnish. When the paper has been removed, another coat of varnish will serve to make the whole more transparent. 8. A piece of glass may be smoked in the ordinary way, and a design marked upon it. This makes a very good and plain picture. If the design is needed for keeping, heat some alcohol in a cup or small porcelain dish, and hold the smoked side of the glass in the alco- hol vapor for a minute or two, and afterward it may be varnished with photographers' varnish, carefully flowing it over the plate in the same way that plates are flowed Cor photographic purposes. ► PROJECTIONS. 33 TO DETERMINE THE MAGNIFYING POWER OF A LENS, OR A COMBINATION OF LENSES, IN PROJECTING. It will be evident, upon inspection, that the farther away the screen is from the lens, the larger will be the picture ; but for a given projection, the simplest way of determining the magnification is to choose some object of known dimensions for projection, and then to meas- ure its size upon the screen. Suppose it be a lead pencil having a diameter of one fourth of an inch. If its image is a foot in diameter, it is evident that it is magnified 4 X 12 =48 diameters. If it is three feet in diameter, then it has been magnified 4 X 12 X 3 = 144 diameters. It will be convenient to have a scale, either photographed or etched upon glass, for the purpose of directly showing the magnifying power of lenses. A ver7iier made upon glass by either of the described methods, will be convenient for study, and some meas- urements. THE ANIMALCULE CAGE. If one would exhibit the minute forms of life to be seen in water, an animalcule cage will be needed. This may be made in the following way: Take two quite clear pieces of white glass, about four inches long and one inch wide. Two other pieces of the same width, and one inch and a half long. Put these two short- jiig, 19, er pieces be- tween the longer ones, so as to separate them, and leave a space in the middle clear through. Fasten these together with japan varnish, being careful not to get any of the varnish into the opening. If any should 34 THE ART OF PROJECTING. get in, wipe it carefully out. When the varnish is dry, and the pieces are firmly fixed together, putty up the bottom of this hole so that it will hold water. When this is dry, it can be used to hold fluids of most kinds, but it is especially fitted for water containing ani- malcules, or vinegar with eels. It should be put back of the focus of the condenser, for the great heat there will boil the water in a little while, and the temperature of no more than 130° Fah. will quickly kill most all kinds of infusoria. Suitable water for examination can be found in old rain barrels, stagnant pools, water in which flowers have been standing for a day or two, an infusion of hay in water, and will be found very interest- ing. The larva of the mosquito is a lively and amus- ing thing when magnified to five or six feet in length. THE CHEMICAL TANK. For chemical experiments, and a variety of others, a tank of larger proportions will be necessary. The accompanying diagram (Fig. 20), shows the construc- tion. Provide two pieces of clear, white glass, of the same size, about five inches by six, for the sides. These may be kept apart by a strip of rubber, about one-half of an inch thick, bent and cut at the corners, the whole clamped together by three or four clamps, as shown. If rub- ber with flat sides is not easily pro- curable, a piece of rubber t u b - Fig, 20. ing will answer f PROJECTIONS, 35 nearly as well ; the tubing may be filled with sand to keep it firm. Such a tank will hold any kind of a so- lution, and may be quickly taken apart and cleaned. A tank which will answer for many experiments nearly as well can be made by cutting a semi-circular piece out of a board, of the proper size, and fastening the glass sides to it with cement. What is known as marine glue will be the best for this purpose, and as it is very convenient to have some of this glue for making and mending apparatus, because it will adhere to any surface, the method of preparing it is given : Dissolve, separately, equal parts of shellac and India rubber in naptha, and afterwards mix the solutions thoroughly, applying heat. It may be made thinner by adding more naptha. It may be preserved in a tin box. In order to use it, it must be heated, as well as the sur- faces which are to receive it. Marine glue may be dis- solved in ether, or a solution of potash. A METHOD FOR PROJECTING LARGE PIECES OF APP/-:^ATUS. Many pieces of apparatus used in illustration and demonstration are much too large to be projected in the ordinary way, as it is obvious that the size of the lens used as condenser will be the limit to the size of the object that can be shown with it. Thus, if sunlight is used, the diameter of the orifice c, ^/(Fig. ii), will be the measure of the largest picture that can be shown at once ; and if a lantern is employed, no picture larger than the condenser can be projected. Suppose that it is desirable to show to an audience a piece of apparatus much too large for ordinary pro- jection, and yet too small to be plainly seen, such, for instance, as the electroscope ; or the movement of a pith-ball under electrical excitement ; or the movement 36 THE ART OF PROJECTING. of a vibrating cord, or large tuning-fork ; or the ap- paratus for showing the linear expansion of metallic rods, etc. The following method will be found applica- ble to a great many such cases, where simply the outline of the instrument is needed. Place a short focus objective (and the shorter the better), so near the focus of the condenser that all the light falls upon it. After refraction the light will form a very divergent beam and the focus in front of o will Fig. 21. be a sharp point, practically a luminous point, and any object held between it and the screen j, will have a strong shadow cast upon the latter. The magnitude of this shadow will depend upon the distance from the focus. There will be no penumbra — the outline will be sharply defined. If one has a lantern, the condensing lens above will answer without the objective, as its focus for parallel rays will be sufficiently short. A globular glass flask, filled with water and placed in the path of the rays, will also be found to be satisfactory. When a lantern is used instead of sunlight, it will be necessary to use the microscope attachment, which is described further on. PROJECTIONS. 37 working in front of the lens, the same as with the porte lumiere. The following is a list of apparatus and of experi- ments which are suitable for such projection : Equil- ibrium of the same liquid in several communicating vessels; equilibrium of different liquids in communi- cating vessels ; cartesian diver ; the hydrometer j cap- illarity ; diffusion of gases ; Torricelli*s experiment j Mariotte's law ; the manometer ; SprengePs air pump ; fountain in vacuo ; the siphon ; the pyrometer ; the in- fluence of pressure upon the boiling point ; M. Des- pretz's experiment on the conductivity of solids ; con- vection j the thermo-pile ; umbra and penumbra ; action of magnets ; attraction and repulsion from electrical ex- citation. Natural history specimens, such as birds, rats, mice, squirrels, frogs, toads, live fishes, if in a tank with transparent sides ; leaves of trees, ferns, etc. ; well-de- fined crystals, such as quartz, feldspar, mica, pyrite \ diagrams on glass of machinery, as the steam ^ngine^ these diagrams can be drawn a foot square or more \ silhouettes, etc., etc., are all available with this method. There is an advantage in this plan, when it is at all applicable, that will commend itself to every one, namely, it is available at any point between the focus and the screen, hence it will only be necessary to place the object in the path of the rays to the screen at such a point as will be convenient and will make the shadow sufficiently large. The instructor can stand by the ob- ject, and with a pointer like a pencil call attention to any particular part. And again, the field is so large that several objects can be in it at a time, if need be, for comparison, such for instance as leaves of several species of oaks or maples, or a range of capillary tubes of various diameters. 38 THE ART OF PROJECTING. THE MEGASCOPE. Photographs that are taken especially for projection with the magic lantern are often called transparencies because all of the lighter parts of the pictures are made as transparent as possible, and they are shown by light that is transmitted through them. If one would ex- hibit a picture like a stereoscopic view or a common carte de visite, it is evident that recourse must be had to some other arrangement. The light must be reflected from the picture, but when only the ordinary amount which is reflected from a surface of nine square inches is distributed over seventy-five or a hundred square feet, it is evident that it will be but dimly visible. If a large amount of light is concentrated upon the picture it will, of course, reflect more, and its image will be corres- pondingly brighter. This can be effected in two ways : first, by using a large lens, or second, by using a large concave mirror. The following figures will serve to show how this may Fig. 22, be done. When sunlight is used, the larger the con- denser the better. One seven or eight inches in diam- eter, if possible, should concentrate the light upon a PROJECTIONS. 39 second plain mirror at r, which should have such an inclination as to reflect the converging rays upon the object to be shown at d, and strongly illuminate it ; the objective at o will be used in the same way as for any- other projection. This apparatus should be in a box made with sides a foot square and six or eight inches deep. At the back of it a hole should be left at d, in which the various objects for exhibition may be held. In place of the condenser and the plain mirror, a large concave reflector, such as is used behind lamps, may be placed at r, and the parallel rays from the porte lumiere allowed to fall upon it. It should be placed at such a distance from the object d^ that it will just illu- minate it j this will of course be determined by the focal length of the mirror. The room needs to be quite dark for the successful working of this apparatus, and especial care should be taken to prevent any of the light from th& porte lumiere from being scattered into the room ; paint the box black, inside and out, with lampblack mixed in japan varnish. If the lime light be used, as it generally is for such an exhibition, it is necessary to modify the lantern very much, — so much so as to require an entirely new in- strument. The following is the simplest plan of one : A square wooden box made eighteen or twenty inches on a side, and about fifteen inches deep, may have a little way made in it on one side for the fixtures holding the jet i and the lime / to slide upon. A hole r cut six inches square, may be made near the corner, and another one on the front side for the light to come through upon the lens o, which is the only lens needed for work. The size of this hole should be no greater than that of the lens o used for the projections, but this lens should be as large as possible. A lens six or eight 40 THE ART OF PROJECTING. inches in diameter, with a focus of from eighteen to twenty-four inches, will be found best for the purpose. This may be held in the re- tort-holder before mentioned, and set at such a distance in front of the hole that an ob- ject c, when strongly lighted, will be plainly projected upon the screen s. The whole of the back on the in side should be covered with white jjaper. Let a black cloth flap hang over the hole at r, so that no light will enter the room, save what is reflected from the il- luminated object. With these conditions a dark photograph of an in- dividual, upon a white background, will show quite well. Objects held in the hand, such as a watch with its movements, cameo pins, small flowers, surface of half an apple or orange. The latter, if squeezed when being shown, presents a very amusing appearance. Minerals, crystals, shells, bright-colored beetles, bugs, butterflies, etc., may all be exhibited, and appear, with the shades and shadows, like real objects. This con- stitutes the megascope. The accompanying cut ( Fig. 24 ) represents the scenic effect of the human hand, as projected by the megascope. Tig. 23. THE VERTICAL ATTACHMENT. It is often very desirable to project such phenomena as the ripples upon the surface of water, the move- PROJECTIONS. 41 Fig, 24. 42 THE ART OF PROJECTING. ments of a horizontal galvanometer needle, etc., such as cannot be exhibited with the common forms of ap- paratus for projections. At first the awkward method was adopted of turning the lantern up so that it rested upon its back. This endangered the condensing lenses of the lantern from the great heat immediately under them. Dr. J. P. Cook and Dr. Morton have great- ly improved upon Fig. 25. Fig. 26. this, and have added a most valuable attachment to the lantern. The cut (Fig. 25) represents this invention. It con- sists of a plane mirror inclined at an angle of 45°, and when so placed that the beam of light from the lantern falls upon it, it is reflected perpendicularly upwards upon a lens that converges the light when it passes PROJECTIONS. 43 through the objective above it, and falls upon a second mirror, which is so mounted as to allow reflection in any direction. The same device is made a part of the " College Lantern^'' manufactured by Hawkridge, of Hoboken, N. J. By an ingenious arrangement the change from the horizontal to the vertical can be made in less than half a minute. The microscope, the polariscope, the electric-light regulator, and several other fixtures, are fitted to this instrument, making it a most perfect and complete lantern. Such a vertical attachment as is shown in Fig. 25 is applicable to the porte lumiere, but one can be extem- porized, that will do good service, with such material as is accessible to every one. An iron filter-stand, such Fig, 27. as is in common use in every chemical laboratory, may be taken, and the condensing lens c laid upon the lower or largest ring, and the objective, 0, upon the upper or smaller one, as shown in Fig. 27. Below the lower ring. a plain mirror m may be placed, at such an inclination that the beam of parallel rays falling upon it from the 44 THE ART OF PROJECTING. porte lumiere will be reflected upward through the two lenses upon another smaller mirror, n^ which may be held in a retort-stand, and the beam directed to the proper place. PHYSICAL EXPERIMENTS. DIVISIBILITY OF MATTER. A good way to show the minute divisibility of matter is to dissolve, in water, a small quantity, say a gram, of cupric sulphate, and add enough ammonia-water to make a clear, blue solution. Put it into the chemical tank, having measured its capacity in cubic centimeters, or inches, fill it with water, and project the tank by the method described on page 183. A beautiful blue color will appear upon the screen. With a small syphon of bent-glass tube, draw out one-half of the solution and fill up with pure water. The amount of coloring mat- ter will be reduced one-half, but the solution will be strongly colored. Remove, in the same way, another half, and so on until the blue color is no longer visible — comparing the color with that of pure water, pro- jected, at the same time, in a test-tube. Keep account of the number of dilutions, and at last, when the blue color is on the vanishing point, calculate the weight of cupric sulphate in each cubic centimeter of water. In place of the copper solution, any of the analine dyes will do as well. The same thing can be illustrated with a soap-bubble, PHYSICAL EXPFKIMENTS. 45 blown thin, and projected in the diverging beam (Fig. 2i). The bubble will be sharply defined upon the screen, and its magnitude will depend upon the diverg- ence of the beam of light, and its distance from the screen. It may be made ten or fifteen feet in diameter, if the lens have a short focus. The colors will begin to appear around the pipe in bands, and computation of the thickness may be made, and of the probable num- ber of molecules in its thickness. For the considera- tion of this, see " The New Chemistry," by Professor Cooke, and Nature, Vol. I, p. 551; also Galloway's "First Steps in Chemistry," article 102. POROSITY. The gases dissolved in common water will be ex- pelled by gently heating some in a test-tube while the whole is projected. The bubbles will be seen to form and rise where nothing was before visible. The po- rosity of water can be shown by projecting a test-tube half filled with it, and its depth marked by a bit of thread tied about the tube at the level of the surface. A considerable quantity of salt or sugar can be added to the water without noticeably increasing its bulk. A piece of chalk dropped into a test-tube containing warm water will at once give out quite a quantity of included air. The ordinary experiment of showing the porosity of leather by forcing mercury through it by atmospheric pressure into a partial vacuum, can be exhibited by pro- jecting the upper part of the lube, while tiie exhaus- tion is going on. The mercury will be seen to fall upward on account of the inverting by the lens. A mixture of equal parts of strong sulphuric acid and water loses notably in volume when cool. Fill a 46 THE ART OF PR OJE C TING. test-tube with the fresh mixture, and tie a string about the tube at the hight of the mixture. It will be too hot for handling with the fingers at first, but it may be cooled in a few minutes enough to show the shrinkage, by stirring it in a dish of cold water. The surface will be seen to be considerably below the string which marked its original hight. This experiment may be used to exhibit compressibility of liquids. Most of the experiments which are suitable for pro- jection of the properties of matter are* chemical, and will be found described under that head. Diagrams, such as are given in most text-books on mechanics, can be made upon glass by one of the processes described on page 30, and will be found very convenient to a lec- turer upon that subject. COHESION. A drop of water or other fluid exhibits this, and may be projected with the lantern, or with the porte lumiere, and a single lens (Fig. 28). Sprinkle a little lamp- black or lycopodium-powder upon one side of a strip of glass, like a microscope slide- and place it in the proper place for projecting, keep- ing it horizontal that the dust may not slide off. Fxg, 28. '^o^ place a single drop of water upon the slide ; the powder will prevent it from spreading upon the glass, and it will gather itself up into a round globule with some of the dust over its sur- face, making an interesting object upon the screen. Again, a saturated solution of zinc-sulphate is put into a white glass square bottle, two inches square^ and PHYSICAL EXPERIMENTS. 47 three or four inches high. Let the bottle be about half filled with this solution. Into a few drops of bisul- phide of carbon drop a piece of iodine. It will at once stain the bisulphide a dark-brown color, which should then be carefully dropped^ upon the solution of zinc, where it will float. If now pure water be carefully- added, so as to rest upon the solution of zinc, the bi- sulphide will collect into an oblate spheroid, having the appearance of brown-colored glass. A square bottle will enable one to project it better, as a round bottle would make a cylindrical lens, and the projection would be indistinct, unless the vessel was quite large. Nearly fill the large tank (Fig. 20) with alcohol, and project the tank with the lantern, or with the single lens and porte lumiere. Now drop upon the alcohol, with a glass rod, or other convenient thing, any of the aniline dyes. As soon as the dye touches the alcohol it will go straight down for a short distance, then it will branch, and these will shortly branch again, and so on to the bottom of the tank, when there will be a large number of branches. Upon the screen the appearance will be as if a tree were growing ; if at short distances apart in the tank drops of different colors are placed, the branches will interlace and produce a fine effect. A tank of coal-oil, in which is dropped a little colored fusil oil, is said to produce an entirely different figure. But it is with the vertical attachment that the most novel and interesting phenomena, due to cohesion, may be shown. For this purpose it is necessary to have a horizontal tank, made by cementing a ring, an inch broad and four or five inches in diameter, upon a plate of clear glass. The ring may be made of glass, or wood, or zinc. This is to be placed upon the hori- zontal condenser, and half filled with pure water, the 48 THE ART OF PROJECTING. surface of which is to be projected. Let fall, from a height of two or three inches, a single drop of ether. It assumes a characteristic form, will move about, but will last only a few seconds, as it evaporates rapidly. Rinse out the tank, and fill again with pure water, and in like manner drop upon its surface any of the essen- tial oils, of creosote, lavender, turpentine, sperm, and colza oils. Each one will assume its peculiar form due to cohesion. Fig. 29 represents the pattern exhibited by a single drop of oil of coriander, and Fig. 30 the appearance J /'^. First, with the fork held as indicated, make it to vibrate. The string will divide up into segments, all of which can be plainly seen and counted. Second, turn the fork so that it vibrates Fig. 38. in a horizontal plain. The number of segments will be doubled. Third, hang another button upon the pin- hook, so that the weight will be doubled. Count the segments while the fork vibrates, both perpendicularly and horizontally. In this way some of the laws of vibrating strings can be demonstrated. Fasten a small piece of wire to one prong of the tun- ing-fork, and when the latter is vibrating draw it quickly across a piece of smoked glass. The undulating line will show well when projected. THE OPEIDOSCOPE. Take a tube, of any kind, that is five or six inches long and an inch or more in diameter, tie a thin rub- ber membrane or a piece of tissue-paper over one end, and on the middle of the membrane glue a piece of looking-glass that is not more than the eighth of an inch square. The light from the J>orfe lumiere falls ACOUSTICS. 6l upon a mirror a, and is received upon the bit of mirror upon the end of the tube. The open end of this tube is to be held at the mouth and various sounds produced, varying in pitch and intensity. The vibrations of the membrane will move the mirror, and the beam of light Fig, 39. reflected from it upon the screen will describe various beautiful and regular curves, depending upon the man- agement of the voice. It will be easy to find some pitch and intensity which will give a straight line : then, while the sound is being made, if the outer end be swung sidewise at right angles to the line, an undulat- ing line will appear, in every way like those produced by the vibrating tuning-fork described on another page. If there are prominent over-tones in the sound they will be made apparent by their interference, giving a trace just like the traces upon a smoked glass by Scott's Phonautograph. The forms are regular enough for a tone of a given pitch and intensity, to enable one to write his music with them for notes ; and if a tune like "Auld Lang Syne " be tooted in the instrument, the ef- fect is quite amusing". The size of these figures, at the distance of fifteen or twenty feet, may be six or eight feet or more. 62 THE ART OF PROJECTING. CHLADNl'S EXPERIMENT. A glass plate of any form, if fixed by a clamp, will give out a musical sound when a violin bow is drawn across its edge. If the surface of the glass be strewn with sand, the latter will be arranged in some symmet- rical form. The glass plate may be prepared as for the magnetic phantom, and the sand fixed after its acoustical arrangement, and afterwards projected as an ordinary transparency. It is generally best to exhibit this phenomenon during the process of arrangement, and this will require the fixtures for vertical projection. The glass to be sounded is to be made fast, and so placed that as much as is possible of it is over the con- denser of the vertical attachment ; then the sand sprin- kled upon it, and the focus adjusted for the upper surface. When the bow is drawn, the sand is seen to arrange itself according as the plate gives out one sound or an- other, which depends upon the part of the plate that is bowed, and where it is damped, also upon its form. It is well to have round, square, triangular, and hex- agonal pieces, eight or ten inches in diameter. To show water-waves upon a Chladni plate, Professor Morton has devised the following way: A plate of glass about a foot square is so held by its middle that one corner covers the condenser of the vertical lantern. To this corner is cemented a thin ring of soft rubber, of about five inches in diameter, and into this water is poured to the depth of one-tenth of an inch. Project the surface of the water and then draw the bow across the edge of the glass, as in the other cases, so as to produce a musical sound. The water within the rub- ber ring is thrown into a system of large waves, which ACOUSTICS. 63 form a shaded net-work of singular beauty. Drawing the bow so as to produce notes of different pitch, the waves will be large or small as the notes are low or high, and with a mixed note it is possible to get two or more systems superposed. If a common tuning-fork be struck and then have one of its prongs put in contact with the surface of the water in this tank, a beautiful radiation of ripples may be seen, resembling somewhat the arrangement of iron filings about the poles of a magnet. The motion of water in a shallow bell-glass can be projected by letting the parallel beam from the vertical lantern go through it, doing away with the condenser, as the vessel itself would act as a lens if water were in it. The bow may be drawn across its edge when it will give out a musical sound, the water will be thrown into ripples, and a large objective might be used to project the whole surface. The bell-glass may be filled with ether or alcohol, and then sounded. Some of the liquid assumes the sphe- roidal form, and these are driven over the surface to the nodal lines. (See Tyndall on Sound,) MANOMETRIC FLAMES. The flame of a candle, or lamp, or gas-jet, if a lumin- ous one, can be projected upon a screen by using a concave mirror (Fig. 40). It will be invert- ed and magni- fied. If while the flame is projected the mirror be tilted Fig, 40. so as to swing 64 THE ART OF PROJECTING. the beam horizontally, the flame will appear drawn out mto a band of light, due to persistence of vision. But if the flame be not a bright one, the image will be too dim to be useful, if the screen is ten or fifteen feet away. The intermittent character of the singing hy- drogen flame can be shown in this way, but it is much better to use common gas in place of hydrogen, as the flame is much brighter. The flame of commcn gas may be made still brighter by passing it through ben- zole or naptha, or tow saturated with ether. The room must be quite dark. (See Tyndall on Sound, p. 223.) In the American edition of Atkinson's Ganot's Physics is pictured Koenig's apparatus for observing manomet- ric flames. In place of the rotating reflector use the concave mirror, as above, and the same figures will ap- pear upon the screen. One can make a tolerable substitute for that apparatus, if gas be not obtainable, by fastening over the mouth of a small two-inch funnel, such as is used in chemical laboratories, a piece of tissue-paper or thin rubber. A piece of rubber tubing, two or three inches long, may be drawn over the stem of the funnel, and the other end drawn over the mouth of a common jeweler's blow- pipe. A sheet of pasteboard may now be rolled so large that the broad end of the funnel, which has the tissue-paper pasted to it, may fit snugly in it. The whole fixture may now be supported in any way, by means of retort stands. A gas- flame from a small round orifice, or a common candle may be used for the flame ; the end of the blow-pipe is to be inserted in the blaze, with the opening upward. If now, either a com- mon mirror be used to give angular motion to the re- flected beam, or the concave mirror to reflect the flame upon the screen, while a sound is made in the large ACOUSTICS. 65 tube, it will disturb the flame so much as to give a dis- tinctly serrate image either upon the screen or in the plain mirror. The annexed figure will give an idea of Fig. 41. the arrangement mentioned : a is the tube for produc- ing sounds, in b is the funnel with tissue-paper over its mouth, c rubber connection to the blow-pipe //, which opens upward into the flame from the candle e. THE ORGAN-PIPE. The vibrations of the air reed of a sounding organ- pipe may be shown, by having a small pipe made of iron gas-pipe and blown by illuminating gas, which may be lighted ; and when the pipe is sounding the reed will be seen to swing backward and forward in front of the emhonchure. That it really vibrates may be seen by re- flecting the light from a mirror upon a screen, and tilt- ing the mirror, as is done in showing the manometric flames. mach's experiment. The movement of the air within a sounding organ- pipe has been studied optically by Mach, a German physicist. His method was to stretch a membrane across the node of a pipe with glass sides, and in the open end he ran a fine platinum wire to the membrane, and thence out to be connected with a galvanic battery. 66 THE ART OF PROJECTING. A sponge dipped in strong sulphuric acid was drawn along upon the stretched wire, the acid gathering itself up into small drops at regular distances apart. When Fig. 42. a current of electricity of sufficient strength was sent through the wire it was heated red-hot, and the acid was vaporized in dense fumes that, on account of its great density, sunk down toward the bottom of the tube, making so many gaseous strings hanging from the wire. These, of course, were subject to the motions of the air in the tube, and when the other end of the tube was sounded by wind from a bellows, the free end partook of the vibrations. The motions were then observed through a revolving stroboscopic disk, described further on. Not only the swaying of these gaseous threads was observed, but some of the Lissajous's curves were seen. I think it highly probable that the motions of the air in such a sounding-tube can be shown to an audience, by having the tube with glass sides filled with dense smoke, and a strong beam of light converged in it, and having the stroboscopic disk so placed that the focus of the lens would be in the holes, and so permit a large amount of light to be used. Where the node was formed no movement would be visible ; but by giving the disk a suitable velocity, at any other place than the node, the vibration might be shown in its different phases. ACOUSTICS. 67 LISSAJOUS'S CURVES. The optical method of studying vibrations is attract- ive to old and young, lo students of science, and to musicians ; but the apparatus generally used is so costly that not many can afford to purchase it. The following directions will enable any one to have a pair of the tuning-forks made at the nearest blacksmith's shop, that will be found even more satisfactory for pro- jections than the more costly ones. Choose a piece of steel that is an inch broad, one- fourth of an inch thick, and about four feet and a half long. Have it made into two large tuning-forks, one of them to be about fifteen inches long, and the other twelve inches. Let the tines be two inches apart, and the flat sides should face each other on each fork. A stem may be now welded upon the bend j it 3(iould be about five-eighths of an inch in diameter, three or four inches long, and made of round steel. When one of these forks is struck in the manner of common tuning- forks, it will be seen to vibrate through quite a large arc, and will continue to vibrate perceptibly to the eye, for half a minute or more. If, while the fork is vibrating the stem be held upon a table or floor, or some other resonator, a deep sound will be heard, and the larger one will make about fifty vibrations per second, while the shorter one will probably make seventy or seventy- five vibrations per second. A stand will be needed for each of these, and may be made by mortising a post three inches square, and three or four inches high, into a board eighteen inches long and ten inches wide (Fig. 43). This post should have an inch-and-a-hal£ 68 THE ART OF PROJECTING, hole bored through it lengthwise, s o that a smooth stem may freely turn in it. This stem must have a large head upon it, thro' which is bored ^ ■^*^* ^^' hole to receive the stem of the fork. Set-screws should be provided, to fasten the stems in their proper places. These sup^ ports might be made of cast-iron, in which case they would not need to be nearly so large. Next make four slides of iron, an inch and a half or two inches long, and bent so as to slide upon the fork and be fixed with a set -screw where it is wanted. These are for loading the forks and making them vibrate slower, as they are nearer the ends. Lastly, each fork will need a small mirror fastened to its end. The small, round pocket mirrors, about an inch in diameter, I have found to answer well j but care should be taken, in selecting these glasses, to get plain mirrors. Most of these small ones are on poor glass, and will spread a beam of light over a large space. These mirrors may be fastened to the end of the fork with the cement known as marine glue, and will adhere strongly enough for all careful work ; but sometimes these are fitted with a screw in the back, and screwed into a tapped hole in the end of the fork. A still better way to fasten this small mirror, is to cement to its back a piece of rubber as long as the mg. 44. ACOUSTICS, 69 breadth of the fork, a quarter of an inch thick, and half an inch broad, this to be cemented to the end of the fork. The fork will not vibrate at all with this attach- ment at first j but if a thin wedge is cut out from each side of the rubber, until it moves very freely, the vibra- tions of the fork will not be much interfered with ; at the same time the amplitude of the vibrations will be much increased. When the mirror is fastened to each fork, it will be advisable to determine their pitch, which may be done by comparing them with a properly-tuned piano, organ, or another tuning-fork with known pitch. EXPERIMENTS WITH THE FORKS. / The Sinuous Line. Cut ofT most of the light from the lantern or J>orte lumiere with a diaphragm, so that the beam is not more than an inch in diameter and consists of parallel rays. Adjust the fork so that it JPig, 45, will vibrate perpendicularly, and place it so that the beam of light will fall upon the mirror at its end. This should be again reflected to the screen by a mir- ror m held in the hands, to swing the beam around the room. When the fork is made to vibrate by striking it with a small billet of wood, if the mirror m is held still. 70 THE ART OF PROJECTING. a band of light will appear upon the screen, three to five feet long, depending upon the amplitude of vibra- tion and the distance to the screen. If now the mirror tn be turned so as to swing the beam at right-angles to the band of light, a long sinuous line of light will be wwwww JPig. 46. spread upon the wall. It may be seen to be forty or fifty feet long if the mirror be moved fast enough. At the time the fork is struck attention may be called to the sound. If two beams of light, about half an inch apart, and one above the other, be made to fall upon the first mirror while it is vibratinsr, and the mirror m ITig. 47. (Fig. 45) be moved as before, two undulating lines will appear, one above the other (Fig. 47), with phases ex- actly corresponding. Let the two beams of light be r\ r\ r\ r\ r\ r mxmjj J^Hg. 48. brought side-by-side and they will appear to have op- posite phases (Fig. 48), and will show as beautiful in- terlacing lines. A double image prism put in the path ACOUSTICS. 71 of the beam just in front of the fork, serves well to give this double line of light. //. Overtone, If the fork be struck about midway of its length, a much higher sound will be heard along with the fundamental. Let the mirror be turning when the fork is struck, and the large sinuous line seen be- fore will now be seen covered with ripples due to the overtone. ///. Interference. In the place of the mirror at m, place the second fork so that the beam of light from the first will fall upon it, and be reflected to the middle of the screen, having both forks to vibrate perpendicu- larly. Now load the shorter fork with slides until it is nearly in unison with the long fork. When they are both made to vibrate, the line of light upon the screen will be seen to lengthen and shorten with regularity ; at the same time beats will be heard corresponding with the lengthening of the line. These beats may be made to vary in frequency by moving the slides. If the beats are as many as five or six a second, or more, and the second fork be swung upon its vertical axis, the inter- Pig. 49. ference may be noted (Fig. 49) ; the swellings corres- ponding to the periods of coincidence, and the con- traction to the periods of interference. If the forks are now brought to unison and struck, the resultant figure will depend upon their relative phases. If they have like phases, so that each one reaches its limit at the same instant, the line of light upon the screen will be much elongated, the amplitude 73 THE ART OF PROJECTING. being equal to the sum of the two amplitudes. If their phases are opposite, so that one reaches its upper limit at the same instant that the other reaches its lower limit, then the spot of light will not be drawn out into a line at all, but will remain quiescent. These various relative vibrations can only be obtained by trial, but usually in four or five strokes one will develop such a phase as he requires. IV. Resultants. Keeping the two forks in unison, turn the second fork so that it vibrates horizontally. Adjust the light so that it falls upon the second mirror as before, and thence to the middle of the screen. Now, if both forks be struck, the resulting figure may be a straight line, an ellipse, or a circle depending upon the phase of the first fork when the second one begins to vibrate. Fig. 50 represents these unison forms. By moving one of the slides so that the fork is not quite in tune with the other, the figure will move through each of these phases alternately. When the Wig. 51. circle is obtained upon the screen, swing the second fork through a small arc, and the circle will be drawn out into a luminous scroll, (Fig. 51). If the forks are ACOUSTICS. 73 not quite in unison, the same experiment will give the scroll of irregular amplitude, (Fig, 52). Fig. 52. Remove the slides from the short fork and fix them upon the long one near the end, and, if necessary, at- tach two pairs, and adjust them so that the ratio of vi- brations is as 2 to I ; that is, their pitch is an octave apart. The resulting figures are shown in Fig. 53 j and Fig. 53. when the forks are tuned exactly, the figure first de- veloped will remain, with no other alteration than a decrease in size, and may be a parabola, an 8, called a lemniscata, or an intermediate form. While this figure 8 is upojj the screen let the second F%y. 34. fork be rotated through a small arc, as before with the unison, and the scroll shown in Fig. 54 will appear. By trial the slides may be so adjusted upon one of the forks that all the varying ratios in the octave may be obtained. The simpler the ratio the simpler the 74 THE ART OF PROJECTING. Fig. 55, figure, and such ratios as 2 to 3 {do to sol^^ and 3 to 4 iij^urf, cinu &UU11 1 alius as z lu 3 \uu lu jt/^y, aiiu 3 lu 4 {do iofa)^ may be known by their representative figures, Fig, 56, 55 and 56. The ratio i to 3 {do to sol^ in next octave,) will present such forms as those in Fig. 57. Fig, 57, In any case, the figure will remain constant when the ratio is exact, and the ratio may be known by counting the number of loops upon the top and one side. Thus, in the fully developed figure, with the ratio 2 to 3, there may be counted two loops upon the top and three loops upon the side, which indicate that the fork that vibrates horizontally makes three vibrations, while the other one makes two. The overtones may be developed and exhibited upon each of these compound forms by striking upon the fork rather lightly, about midway of its length, while it is giving any particular figure. Thus, if the forks are in unison and a circle has been obtained, the overtone ACOUSTICS, yr developed will cover the circle with ripples which ap- pear to move around it. For the exhibition of the Lissajous curves with such forks as have been described, it is not necessary to use a lens, but the whole light from the porte lumiere may be allowed to enter the room, and the first fork placed with its mirror in the middle of the beam. If, however, it be desirable to admit less light into the room, a dia- phragm may be used that admits a beam only an inch in diameter or less. A lens may be used which will concentrate the light upon a much. smaller, space, mak-. ing a much brighter spot, but will very much reduce the size of the figures. When a lens is used, it must be so placed as to project the mirror upon the second fork. Its focal length should be two feet or more. All of these phenomena can be shown by means of a lantern, — even an oil lantern will answer. It will be found best to use a beam of parallel rays, which may be used in such a lantern as is represented in Fig. 26 by simply removing the front lens of the condenser. With other lanterns it will be necessary to remove the objective, and push forward the light until the beam emerges with parallel rays : then, with a diaphragm cut off all the light except a beam of the size of the mir- ror upon the forks. The conditions are then the same as with sunlight, and a lens may or may not be used. SYMPATHETIC VIBRATIONS. Let the two forks be brought to unison and at right angles, so as to give, when struck, one of the forms of Fig, 50. If now, but one of the forks be struck, the straight line due to its vibration will slowly swell into an ellipse, which will be due to the absorption by the second fork of the vibrations of the- first. This^ioay. 76 THE ART OF PROJECTING. be demonstrated by changing the pitch of one of the forks, when no change of form of the projected beam will be observed. One of the conditions for the suc- cess of this experiment is that both forks should rest upon the same table, in order that the vibrations may be conveyed through the solid wood from one fork to the other. The intensity of the sound-wave in the air is not sufficient to communicate a motion that will be perceptible. A voice sounding the same fundamental note as one of the forks, will set it vibrating, as will be evident by the spot of light upon the screen being drawn out into a line. With one of these forks Melde's experiment may be shown in the most satisfactory manner. Choose a soft white cord eight or ten feet long (a silk cord is best, though a cotton twine will work very well), tie one end to the fork at a and let the other end hang over a hook driven in the wall at b. Weights varying from a pound to Fig. 5S. half an ounce or less may be hung upon this free end of the string, with which its tension may be varied. The fork may be struck with a billet of wood, as in the former experiments, when the string will be made to vibrate, either as a whole, or in equal segments, the number of which will be inversely proportional to the stretching weight. The amplitude of these vibrations of the string will be considerable, and if the string vi- brates as a whole it may be eight or ten inches, or even ACOUSTICS. 77 a foot j and when the number of segments is as many as sixteen or twenty, they can all be seen and counted by a large number of persons at a time. If the string a, by is twice as long, and may reach back to «, the free end may be held in the left hand while the fork is struck with the right. It will then be very easy to vary the tension of the cord while it is vibrating, and the segments can be made to change through its whole series of one, two, three, four, etc. The various forms and motions of the cord may be shown to still better advantage, by making a strong beam of light from the porte lumiere or lantern to fall upon it in the direction of its length. Crova's apparatus consists of disks of glass about fifteen inches in diameter, which may be made to turn upon a suitable rotator. These disks are at first painted black, and then curves of various forms are traced through the paint to the glass. The upper part of the disk is projected in the ordinary way, and then if it be rotated, the lines which are drawn upon it will appear to move or to be quiescent, according as they are con- centric, eccentric, or some other form. If a diaphragm with a slit in it, long enough to reach across all the lines which are drawn upon the disk, be placed behind it, a series of dots will appear upon the screen, which will change their positions as the disk turns round. With properly drawn curves the various wave-motions in air in organ-pipes, reflection of sound-waves, nodes, interference, and so forth, as well as the transverse vi- brations in light-waves, may be well shown. AN ATTACHMENT TO THE WHIRLING TABLE FOR PRO- JECTING LISSAJOU'S CURVES. Two posts p and p' are made fast to the frame upon the opposite sides of the inertia plate a. A small THE ART OF PROSPECTING. wooden pulley s^ about an inch in diameter, is made to turn upon an axis that is made fast in the post/, and with such adjustment that the pulley rests upon the Fig, 59. plate a and turns by friction on that plate. It is best to have a thin India rubber ring upon the friction pulley to insure it from slipping. Above the pulley the mirror m is so mounted as to swing in azimuth, and is made to do this by a wire fastened to it at its hinge, and bent into a loop 2 at its lower end, which is opposite the face of the pulley s. Another twist in the wire at o will be needed, for a pin which is fast in the post / ; this will make a lever of the wire /, with the fulcrum at o, and if it is properly fastened to the hinge Fig. 60. Qf ^jjg mirror will cause it to vibrate in a horizontal plane when the plate a revolves. ACOUSTICS. 79 A somewhat similar arrangement :s made for the other side, save that the friction pulley sf has its bear- ing made fast in a separate piece c, which is so fastened to the end of a long screw d that the whole fixture can be moved to or from the centre of the plate a. The piece c is furnished with two guides, which keep it steady in any place where it is put. The mirror m' is made to tilt in a perpendicular plane by an arrange- ment quite similar to the former one, save that the wire connection has its lower end bent into a horizontal loop, through which a pin in the face of the pulley / is thrust. This is practically an eccentric, and, being directly fastened to the hinge of the mirror /«', gives to it an angular motion proportional to the distance of the pulley face-pin from the centre. The mirrors should be not less than two inches square. If then the pin is an eight of an inch from the centre of the friction pul- leys, they will have ample angular motion j much larger than can ever be got from forks. Experiments. — It is evident that if the two friction pulleys have equal diameters, and they are at equal dis- tances from the centre of the plate a, they will vibrate in unison in their respective planes. Now let a beam of light r, from the porte lumiere, fall upon the mirror tn at such an angle as to be reflected first upon the mirror «?', thence to the screen. If the plate a is now revolved, the beam of light will describe a circle, an ellipse or a straight line, either of which can be made at will by simply adjusting the crank of one of the mirrors to the required angle. Thus, suppose the mirror m' is tipped back its farthest by bringing the pulley pin at the top, as indicated in the drawing, at the same time that the mirror m is at its maximum an 8o THE ART OF PROJECTING. gular deviation. The beam of light will describe a circle. If it moves slowly, the path and direction of the moving beam can be nicely observed. These two ad- vantages are not to be had with forks ; for, first, it is accidental if one gets a circle or any other desired re- sultant figures from forks in unison, for the obvious reason that the phases cannot be regulated, and second, the vibrations of the fork are so rapid that the analysis of the motion can only be made in a mechanico-mathe- matical way. By moving the fixtures on the left side toward the centre of the plate ^, the pulley / will not revolve so fast. If moved half-way it will make one revolution while the other makes two, and the vibrations stand in the ratio i : 2 represented by forks in octave. Such ratio is shown upon the screen by a form very much like the figure 8, and known as the lemniscate. Between these two places, every musical ratio in the octave can be got, and the resultant motions projected in their proper curves. More than that, while the mir- rors are both vibrating, any of the ratios desired can be moved to at once by merely turning the thumb screw d, which is wholly impossible with any forks which require stoppage and adjustment of lugs for each different curve. Again, if the fixture c is moved still farther toward the centre than half-way, the curves projected will be those belonging to the second octave, until the pulley reaches three-fourths of the way, when the ratio will be 1 :4, and the resultant figure will be like a much-flat- tened double eight. If one would show the phenomenon of beats, it will be necessary to have the mirror m and its attachment LIGHT. 81 so adjusted as to have it vibrate in a perpendicular plane like m' . This can be done by fixing its hinge at right angles, and the rest the same as for mirror m' , The reflected beam from the second mirror may be received upon a large mijror held in the hands, and thence reflected upon the wall or screen. LIGHT RECTALINEAR MOVEMENT. That light moves in straight lines can be shown by admitting the light from the porte lumiere through a small hole. It goes straight across the room, and its course can be tracked through the room by the dust particles, or a little smoke, which it will light up. Also, by having the room otherwise quite dark, permit the light to come in the round orifice, half an inch in diam- eter, as it is reflected from the landscape outside, and not reflected from the mirror. The room is thus a large camera obscura, and an inverted image of the landscape will be seen upon the walls, or upon a small screen held a foot or two from the orifice. This image will be par- ticularly strong if the ground be covered with snow, as much more light is reflected from that than from grass or foliage. If persons are passing their forms wiU be seen, and appear as if walking head downward. Parallel rays A will be reflected from the mirror of 82 THE ART OF PROJECTING. t\\Q portelumiere, whiles converging b and diverging c rays will be obtained by interposing a convex lens of any size in the path of the parallel rays. Fig. 61. Transparent substances, like glass, some crystals, gases, and water permit the rays a to go through them and appear upon the screen. Translucent substances, like paper, ground glass, milk, allow but a few scattered rays to go through them, and a diffused light appears on the screen. Opaque substances, such as metals, thick pieces of wood, stones, etc., stop all the light, reflecting some and absorbing the rest. INTENSITY OF ILLUMINATION. When the lens is interposed in the path of the beam the light appears as a circular disk upon the screen, and as the rays cross each other at the focus f^ that point may be considered as the source of light. Cut a sheet of paper or a board j", one foot square, and hold it any distance from the focus, say two feet. Its shadow upon the screen will be bounded by ^, ^, which may be measured in square feet. Now move the paper to /, twice as far from the focus, and again measure the shadow b^ or^e lumiere. If instead of using disks of glass, disks are made of wire gauze, perforated tin, or paper or lace, very curious interference figures are produced, and this form is called the eidotrope. LIGHT. 143 TRe accompanying figure represents a chromatrope with an arrange- .„. . ment for quickly ^^___ j replacing one disk by another of different pat- tern. Rotation is given by fric- tion pulleys. With this form there is a disk with the so-called seven primary colors to illus- trate Newton's theory of colors, i 5 3 4 one to illustrate Brewster's theory, two to illustrate Young's theory, and a chameleon top, designed by President Morton, of Stevens Institute, Hoboken. The effects with all the forms of chromatropes are due to persistence of vision. Interesting subjective effects may be observed by projecting in the ordinary way bits of colored glass an inch or two square, so as to have upon the screen a large patch of color with a boundary of white light. The eyes must be fixed attentively upon the colored patch for about half a minute, when the colored piece must be quickly removed, the eyes to be kept meanwhile upon the screen. To prevent the eyes from unconsciously wandering while lacking, it will be found advisable to pin a large black button or a piece of black paper to the screen in the middle of the disk. This is to be kept in the centre of vision. The effects observed will of course depend upon the color upon the screen, and 144 THE ART OF PROJECTING. the sensitiveness of the eyes for various colors. Gen- erally, after looking steadily at a given color, and the disk is made suddenly white, the outline of the colored part will be seen in a color complementary to the one looked at first. Thus, if a square red glass should be projected the residual image would be a square green one. If a blue one was projected its complementary image would be orange, and so on. A great variety of su:h effects are obtainable with various colored pieces of glass, or of films of gelatine^ by projecting them singly, in juxtaposition, or superposed. Let disks of white cardboard a foot or two in diame- ter have partial sectors painted black, with India ink, so that the white and black parts alternate four or five times in the circumference. This is to be rotated while a powerful beam of light falls upon it. The persist- ence of some of the elements of white light being greater than of others, the disk will appear of various colors ; purple, green, and yellow being generally well developed. HEAT. AIR THERMOMETER. A bulb blown upon one end of a small glass tube, five or six inches long, answers for this experiment. A drop of colored water can be made to enter the tube by first heating the bulb a little by holding it in the fingers with the open end of the tube a little below the surface of the water. A bubble or two of air will be expelled, and the fingers may be removed from the bulb. As it cools a drop will be driven into the tube, and with a little painstaking it can be brought to any required place by cooling or heating the bulb. These movements can be shown with the pOT^e lumiere and a single lens, as shown in Fig. 17, or it can be put in HEAT. 145 front of the condenser of the lantern. A touch of the finger will heat the bulb sufficient to cause the drop to rise in the tube, and it may be made to descend by simply blowing upon the bulb, or by dropping a little water or ether upon it. Many of the pieces of apparatus for illustrating the expansion of metals by heat are so small that they may be readily projected. Thus Gravesand's Ring, Pyrom- eters, etc. The latter may have a small bit of mirror fastened to the end of the index, and the light so arranged that as the index rises, the beam will move upward. A rise in temperature of only a few degrees can be then shown, and the alcohol flame may be dis- pensed with ; the warmth of the hand or a little hot water answering the purpose. FORMATION OF CLOUDS. The condensation of liquid in the form of vapor mto minute globules and in the production of a shower of rain may be very well illustrated and projected for class purposes in the following manner : — Place about an ounce of Canada balsam in a Flor- ence flask and make it boil. At the top of the flask clouds of globules of turpentine will be seen hovering about, altering in shape very much like sky clouds, and the globules are large enough to be visible by the naked eye. If a cold glass rod be gradually introduced into the flask these clouds may be made to descend in showers. Lawson Tait in Nature. Another : Take a flask of one or two litres capacity j rinse it out with distilled water, and attach to the neck a cork and glass tube of about twenty or thirty centi- metres length. Place the glass tube in the mouth and 10 146 THE ART OF PROJECTING. exhaust, when a dense cloud will be formed ; then on blowing into the flask the cloud disappears. The cloud may be produced and dissolved as often as wished, and if a beam from the oxyhydrogen light be sent through the flask the experiment becomes very effec- tive. C y. Woodward in Nature, MAXIMUM DENSITY OF WATER. Take a small test-tube, not more than two or three inches long and half an inch in diameter, and through a tight-fitting cork thrust a small glass tube about three inches long, allowing it to project as much as two inches. Fill the test-tube with water at about 4° centigrade and cork it tight, so that the water will rise in the glass tube. See that there are no air bubbles beneath the cork. Mark the height of the water in the small tube by tying a thread about it. Project the whole with a lantern or with the p07'te lumiere. Now, if a small vessel contain- Fig* 109. ing hot water be brought up under the test-tube so that the latter dips in it, the expansion of the water will be indicated by the rise of the water in the tube, and the latter will overflow if it be sufficiently heated. Now, bring up under it in the same way a freezing mixture of ice and salt, or a mixture of equal parts of cold water and nitrate of ammonium. The water will contract in volume to its minimum, which should be indicated by the thread; then it will again expand until it freezes, the expansion again causing the water in the tube to overflow. The freezing mixture should be stirred constantly to hasten the work. HEAT, 147 THE GALVANOMETER. As many of the experiments in heat require the thermo-pile and galvanometer, the latter is treated of in this place rather than with electrical experiments. In the "American Journal of Science," Vols. II, III, V, IX and X, are given several ingenious arrangements for projecting the movements of a galvanometer needle, and if one desires to do extremely accurate work be- fore an audience he will do well to obtain some one of these forms. If, however, it is desirable only to ex- hibit qualitatively and with no great degree of precision the relation of heat to electricity, or the law of the galvanometer, etc., the following method will be found to answer, with the advantage of being extemporized in a few minutes : Make 2iflat coil about an inch square, of rather fine-covered copper wire having the ends of the wire a foot or more in length. Upon one side of this coil stick a bit of beeswax as large as a small marble, and through both wax and coil thrust half of a fine cambric needle. Press the wax down upon the middle of a piece of glass four or five inches square, and then holding the plate horizontal, suspend upon the needle point a small compass needle an inch or two long. This is now ready to place upon the upper con- denser c (Fig. 27) of the vertical attachment and then be projected. If a current from a battery or a thermo-pile be sent through the coil, the needle will be deflected. The needle will of course point towards the north, and that place will easily be noted upon the screen as zero. A small magnet brought into the neighborhood will serve to bring the north pole of the needle to any required place. If a circle with in- scribed degrees should be drawn upon the glass by 148 THE ART OF PROJECTING. either of the methods described upon pages 31 or 32, the movement of the needle can be noted in degrees. If the needle is too short to reach the numbers upon the glass, it can have a fine straight bristle made fast to its ends with a little mucilage. With the thermo-pile connected with the galvanom- eter, the sensitiveness of the former may be shown by presenting the hand to one face of it, or it may be breathed upon or blown upon with a common hand bellows. Let fall a drop of water, of ether, and of alcohol upon the face. The evaporation cools it. The heat generated by percussion may be exhibited by providing a leaden bullet which should have at first the same temperature as the thermo-pile, which may be known by putting it upon the pile, handling it with a pair of small tongs. It should not move the needle. Then strike it once with a hammer so as to indent it considerably, and with the tongs quickly put it again upon the face of the pile. It will indicate a higher temperature. The heat generated by friction may be shown by rubbing a stick upon the floor and then bringing it to the pile as in the other case. See Tyndall's work on Heat for a method of show- ing heat from the crystallization of sodium sulphate. The same thing may be shown with the air thermom- eter sunk into the solution, which may be projected with lantern or porte lutniere by preparing the solution in a beaker, fixing the air thermometer in it with a drop of colored water in it, and projecting the whole upon the screen by means of a large lens. The crystalliza- tion itself will be seen, as well as the manifested heat, when it reaches the bulb of the thermometer. Mix in a test-tube resting upon the face of the ther- HEAT. 49 mo-pile, a few drops of water and sulphuric acid about equal parts : the heat evolved will illustrate the origin of heat from chemical reaction. A few crystals of nitrate of ammonium in a test-tube may have an equal bulk of water poured upon them ; the cold produced is from the absorption of heat dur- ing liquefaction. Interpose between the source of heat and the ther- mo-pile various things, such as rock-salt, a solution of iodine in bisulphide of carbon, glass, crystals of various kinds, tubes filled with gases and vapors of various sorts. Also, project a solar spectrum with a part of the same beam that projects the galvanometer by the method described upon page 112. Move the thermo- pile through the various colors, and note the degree indicated by the galvanometer, particularly beyond the red end of the spectrum. The thermo-pile should be placed where the Fraunhofer lines are seen best upon a small screen placed temporarily to receive it. Many experiments on this subject will be found in Tyndall's work on Heat, which one will find himself able to repeat with satisfaction. CALORESCENCE. Let the light from theporte lumierCy or from the elec- tric or lime light, be sent through a vessel containing bisulphide of carbon in which some iodine has been dissolved : the solution will be jet black and will stop every light ray, but will permit the rays of greater wave length to freely traverse it. A lens may now be inter- posed and the obscure rays treated in every way like luminous rays. With a very powerful beam platinum foil may be raised to incandescence in the focus of the ISO THE ART OF PROJECTING. lens, and with a less powerful one pieces of wood and paper may be ignited. A transparent solution of common alum is opaque to the same rays that are so easily transmitted by the iodine solution. A test-tube filled with water placed at the focus of the obscure rays in a minute or two may be made to boil ; an air thermometer will scarcely be affected at that place. MAGNETISM. With the vertical attachment to the lantern the phenomena of magnetism are best shown. 1. Have two or three small magnetic needles mounted upon needle points thrust through pieces of cork, so as to turn freely. Place one upon the upper face of the condenser to the vertical attachment, and project it sharply upon the screen. A piece of iron or another magnet brought into its neighborhood will disturb it, and every motion will be plainly noticeable as well as the direction of the exciting body. 2. Place two of these needles near to each other, but not so near as to touch, and give to one of them a twirl so that it revolves upon its support. It will soon set the other revolving and it may be stopped itself after setting the second one going, and afterward be again started while the other one stops. 3. Place a third, quite small one not more than half an inch long in the neighborhood of the other two, and again set the one whirling. 4. The magnetic phantom. Lay a small magnet an inch or two long upon the upper condenser ; and upon the magnet lay a piece of clear glass three or four inches square. Project the magnet, and then scatter from a small sieve, or gently MAGNETISM. 151 with the thumb and finger, fine iron filings upon the glass. The filings will arrange themselves in the familiar lines called the magnetic phantom, and the whole being magnified to ten feet or more in diameter makes a very striking pictyre. 5. The elongation of an iron rod when strongly magnetized, may be shown by placing a small helix around the iron rod of the common pyrometer made for showing the longitudinal expansion of a rod by heat. To the end of the index finger that sweeps over the quadrant affix a small bit of plane mirror not more than one fourth of an inch square. So adjust the light to this small mirror that the reflection from the latter will fall upon the most distant part of the room ; the farther away the better. When the current of elec- tricity is sent through the helix the rod will be slightly elongated, but the slight tilting of the mirror may become a displacement of two or three inches at a distance of thirty feet. DIAMAGNETISM. The electro-magnet 'for demonstrating diamagnetic phenomena need not be over three or four inches in length, and the poles an inch apart. Objects to be tested may be suspen- ded by a thread between the poles, and the whole projected either in a beam of parallel rays or in front of the focus of a lens. In the latter case the whole will be seen in pro- file, but perfectly distinct. The fol- lowing experiments may be projected with such a magnet if a battery of Tig. 110, three or four cells be used : — 152 THE ART OF PROJECTING. 1. Suspend oblong pieces of various metals half an inch in length, and note whether they set themselves equatorially or axial ly between the poles. Iron, nickel, platinum, bismuth, antimony, zinc, tin, lead, silver, copper, alum, glass, sulphur, sugar, bread, paper, charcoal, are good substances to experiment with. 2. Suspend a cube of copper between the poles, and twist the thread so that the copper will rotate rapidly by torsion. It will quickly be brought to rest when the current is made to pass. 3. Fill small very thin tubes with liquids, and sus- pend them in the same manner. Try solutions of iron, cobalt, water, alcohol, turpentine, and salt. 4. Place the magnet upon the upper condenser of the vertical attachment, and upon its poles place a watch-glass containing a little water or sulphuric acid ; project the water in the watch-glass, and notice the distribution of light upon the image of the water. Now complete the circuit. The water will change its form slightly and the light will be differently refracted, thus making it quite visible. Salts of iron or nickel will scatter the light like a concave lens. 5. Hold the flame of a candle between the poles. 6. Blow small soap bubbles with oxygen and with illuminating gas, and hold them as close to the poles as possible or drop them so they will rest upon both. 7. Heat a coin and place it just beneath the poles, and then drop a piece of iodine upon the coin. The heat will volatilize the iodine, and the purple vapor will be repulsed. ELECTRICITY. Most of the experiments in electricity which can be showp by projection require the use of the galvanom- eter, such for instance as give evidence of the existence RLECTRICnV, 153 of electrical currents, their direction and strength. These will only need the arrangement already described under the head Galvanometer. For other experiments, such as that of the electric light, there will be needed some one of the many fixtures for holding the carbons to be ignited. . If this can be put into a lantern the carbons may be projected at once upon the screen by removing the objective and drawing the carbons back until the image appears plainly upon the screen. This image will be made much sharper by putting a dia- phragm with about an inch aperture over the conden- sers, which in this case serves for an objective. For the projection of spectra precisely the same conditions need to be observed as for the lime light: — Some regulator in the lantern, a slit in the focus of the condensers, an objective to project the slit and the prism in the focus in front of the objective. The spec- trum of metals is easy with this arrangement. Make a small cavity 'n the end of the lower carbon stick, and place a small bit of the metal whose spectrum is wanted in it ; then bring down the upper carbon upon it so as to complete the circuit and then raise it a little, the metal will be at once fused and volatilized, emit- ting its characteristic light, which will appear upon the screen as bright bands. Silver, copper, zinc, iron, and mercury give good spectra among the more common elements. For the successful working of this method of spec- trum analysis, not less than forty cells will be needed, and fifty are decidedly better than forty. DECOMPOSITION OF WATER. This is effected by sending a current of electricity from three or four cells through water that has been 154 THE ART OF PROJECTING. slightly acidulated by the addition of a little sulphuric acid. The terminals of the wires in the water are usually made of strips of platinum to prevent other chemical reactions from taking place. For projection, an excellent way is to insert two test-tubes filled with the acidulated water, and introduce them into the tank already described, having previously fixed the two platinum terminals through the rubber bottom as Fig, lit. shown in Fig. iii. When the current is sent through these wires the bubbles will rise rapidly and soon fill the hydrogen tube. This tank is of course to be pro- jected in the ordinary way, either with lantern or parte lumiere^ in which case the bubbles will appear very large and the water will appear to be in great commo- tion. In place of water fill the tank with a solution of acetate of lead, and without the test-tubes project the tank and make connection with the battery of two or three cells as before : the crystallization of the lead will at once begin and rapidly grow upon one of the termi- nals j reverse the current, and the formed crystals will ELECTRICITY. 155 dissolve while others will grow upon the other terminal. The same thing can be done still better by filling the horizontal tank for the vertical attachment with the solution of lead acetate, and then bending a piece of platinum wire or of tin wire around the interior of the tank. Then, on inserting another wire at the centre of the solution, and making connection with two or three cells so as to make the centre wire the negative and the hoop the positive pole, a beautiful growth of metallic crystals will shoot out from the centre and spread out over the entire field. In place of the solution of lead use a strong solution of the bichloride of tin, using a tin hoop in the solution. Crystals of tin will shoot out and appear in great beauty. These solutions in the horizontal tank should not be more than an eighth of an inch deep. HEATING BY THE CURRENT. Make a small coil of platinum wire, and thrust the ends of the wire through the rubber of the tank, as Fig. 112. shown in the engraving, Fig. 112. Fill the tank with water, and having projected the whole, send the current 156 THE ART OF PROJECTING. through the wire. If the current is sufficiently great the wire coil will be heated at once, and a convection current will at once show itself in the water, the heated water next to the wire rising rapidly to the top. The effect will be still more marked if a drop or two of some one of the aniline dyes be let fall from the surface over the wire. Its greater density will carry it at once to the bottom ; but when the current is sent through the wire, the movements in the water will be rendered very plain. The bichloride of tin or the sulphate of zinc will also answer the same purpose. CHEMISTRY. Most of the chemical reactions that are usually ex- hibited before classes in the recitation or lecture-room can be shown in a much more satisfactory way by means of the apparatus for projection than in the ordi- nary way. The method is moreover both cheaper and easier; cheaper, because each experiment requires but a few drops of the substance in a test-tube or the tank, instead of the large quantity necessary for many to see at once, and easier, because the preparation needed for experiments upon an extended scale is always tedious and tiresome. One who uses the tank (Fig. 20) for the first time for projection, say of sil- ver, in a solution as dilute as two or three drops of the nitrate to the tank full of water, will be surprised at the prodigious precipitation brought about by the addi- tion of a single drop of hydrochloric acid introduced upon the end of a glass rod. Great heavy clouds roll and tumble about upon the screen, looking as though they might weigh tons. CHEMISTRY. 157 ACIDS AND ALKALIES. Nearly fill the tank with water and add a few drops of blue litmus solution; then dip a glass rod into a weak acid solution of any convenient kind and gently stir the litmus solution with it : it will turn red in the neighborhood of the rod. After washing the rod, dip it into an alkaline solution of ammonia or potash, and again stir the solution in the tank. Blue clouds will form in the red sky upon the screen until the whole is again a beautiful blue. In place of litmus solution use a solution made by boiling purple cabbage. Acid turns this red, and an alkali turns it green. Such changes may be effected a number of times in succession in the same solution. Nearly fill the tank with sulphate of soda, in which is put either litmus or cabbage solution to color it ; the latter is the best. After projecting it as a blue solu- tion dip the terminals of a battery of three or four cells into it. Decomposition will begin and the acid and alkaline reactions will be observed about the poles. REACTIONS AND PRECIPITATION. Fill the clean tank nearly full of pure water and add a drop or two of a solution of nitrate of silver and stir it well. Then dip the glass rod into very dilute hydro- chloric acid. Very dense clouds of chloride of silver will form and fall to the bottom of the tank. Add a few drops of strong ammonia water, and the cloudy solution will again become clear. A small piece of carbonate of lime or of soda placed in the tank containing a very dilute solution of hydro- chloric acid gives up its carbonic acid in apparently large quantities 158 THE ART OF PROJECTING. To water made slightly acid, add enough litmus solution to turn it red and project it ; then drop a lump of carbonate of ammonia into it. It will dissolve rapidly with effervescence, and the solution will be made blue about the crystal, and if there is enough of it the whole solution will ultimately become blue. The gradual solution of substances in water may be nicely shown by filling the tank with pure water and dropping a crystal of alum or sulphate of zinc or sul- phate of copper into it. Where the substance is dis- solved the solution will be denser, and its refractive powers changed, which will be manifest by gently stir- ring it with a glass rod. A dilute solution of copper sulphate may be placed in the tank. With a pipette, force into the solution some ammonia water : A dense precipitate will at first be formed, which will afterwards be dissolved if am- monia enough has been added, leaving the solution a beautiful blue color. A few drops of sulphuric acid will reproduce the precipitate and destroy the color ; and when the solution again becomes clear, a few drops of ferrocyanide of potassium added will produce a brownish-red bulky precipitate, which will present a fine appearance upon the screen. In like manner all of the characteristic reactions of inorganic chemistry may be projected, and often with much less expenditure of materials than would be used if large vessels were employed to demonstrate the same things. One who has projected a number of these phenomena will find no diflaculty in projecting any reaction that may be observed in a test-tube. Pictures of chemical apparatus, of processes, etc., will be very convenient for projection when instruction is given in chemistry. ELECTRIC LIGHTS. '39 ELECTRIC LIGHTS FOR PROJECTION. Since this book was first published Electric lighting has become a great industry, and most remarkable advances have been made in the economy of pro- duction of electricity, and in the devices for its utiliza- tion. Compare the statement made on pages 9 and 10 with what any one may see in any city and in hundreds of towns here and in Europe. Arc lights of great steadiness are made by many makers, and the carbons ^adapted to them are plentiful and to be had for a few cents apiece. Consequently one may now have an arc light for projection experiments in al- most every place. A regulator is not specially needed, for the carbons burn but slowly, about an inch an hour, and hand regulation does not much inter- fere even with extended lec- tures, while the brilliancy of the pictures surpasses many for Projection. times the best possible with the oxyhydrogen light, but an automatic regulator is a great convenience. The ordinary electric lamps are so made as to feed Ha-wkkkksk's Electric Lamps i6o THE ART OF PROJECTING. by the movement of the upper carbon alone, and this will not answer at all for lantern work. Both carbons must move. The annexed cuts represent electric arc lamps designed to do this. Marcy has also an electric lamp in which the upper car- bon is inclined so as to present the concave surface of the glowing carbon to the condense*, which device ap- pears to work well. A good electric arc gives light equal to about a thousand stand- ard candles, while very ordinary ones give five or seven hundred. For most teachers' uses, however, the steady projection of trans- parencies is seldom needed, but an electric light for common purposes that may be had by simply turning a switch is highly desirable. There is said to be an ad- vantage to be derived from combining the arc light with the incandescence of lime, Queen's Electric Lamp. the latter giving a degree of steadiness and a brighter light with a given current than would be had without it. The block of lime has a hole through it large enough to allow the carbons to move loosely in it. Near the middle of the block, on one side, a hole is cut through to meet the other, and it is opposite to this hole that the carbons are to touch and the arc be formed, shin- ELECTRIC LIGHTS. i6i ing through the front hole as through a window. The lime soon gets white hot, and adds its luminosity to that of the carbons. If there be a degree of' un- steadiness in the arc itself the lime does not so quickly Electric Lamp in Lantern. cool, and the field is kept bright until the current is fully established again. TO PROJECT THE ARC LIGHT. It is only necessary to place an ordinary lens three or four inches in diameter and a foot focus — that is, the ordinary projecting lens described on page 25 — l62 THE ART OF PROJECTING. near the light and between it and the screen, and focus it in the way indicated on page 25. The incandescent carbons will show beautifully and between them the moving bluish arc. For this experiment the white wall of a room, in other directions than where the screen may be, will be found to be a good surface to receive the image upon. The source of light is so bright that the most distant place in the room will show it plainly enough, and the more distant the image is the larger it will be. The incandescent filament may be projected in a similar manner, and will show as an inverted, glowing loop. The common incandescent electric light does not give light enough to enable one to use it in a lantern. Most of them give a light of but fifteen or twenty candles. Those that give more have a filament so long that its use in a lantern is quite impracticable, not alone on account of size of the bulb, but because the source of the light is from so large an area that definition is impossible. Lamps may be made, though, having the luminous filament reduced to a small area, like a coil, thus : When this can be done so that the luminous area does not much exceed an inch in diameter, a very good source of light is provided. But if common fila- ments are made into this shape they must be supplied with a much larger current than they are usually sup- plied with, and they will not, therefore, last so long. A filament about six inches long is intended to give about sixteen candles' light or nearly three candles ELECTRIC LIGHTS. 163 to the inch of filament. By increasing the current the light increases very rapidly, so that by doubling it the light may be made equal to 100 candles or more, that is, sixteen candles or more, to the inch of filament. When filaments are made tubular, like Bernstein's, they may be made much shorter. Such an one, hav- ing a length of three inches, bent into a U form may give a light equal to 300 candles, — 100 candles to the inch, — and this answers for projections where the de- tails of the picture are not too minute. It will not answer well for micro- scopic projections, but for common transparencies works well enough. When such a lamp is placed in the lantern and moved towards the con- denser, the light upon the screen will increase to a maximum, when the en- larged image of the fila- ment will appear, and the disc will not be uniformly lighted. The lamp should therefore be drawn back a little to secure a uniform field. This will be at the sacrifice of some of the light, but the brightness Bernstein's Electric Lamp. 1 64 THE ART OF PROJECTING. is then equalled by only a very good oxyhydrogen lime light. Electric-light plants are now to be found in most cities and large towns, and in a short time will be found in every town and village. It will therefore be possible for every one to use electricity for his source of light for projecting. Different electric-light companies use currents of different strengths for their service, and at present there is nothing like uniformity among them. As an electric lamp needs to be adapted to the current it is supplied with in order that it should give its proper amount of light, the maker of it must know what current the lamp is to be supplied with. The lamp filament is a conductor of electricity, and as such is subject to Ohm's Law, namely, - = C, when E is the difference in electric potentials between the terminals of the lamp, R the resistance of the filament, and C the strength of the flowing current. The electric energy in the lamp equals EC, and is reckoned in units called watts. Ordinary incandescent lamps require three or four watts per candle, but by increasing the current through them the luminosity increases at a higher rate, and may easily be made a watt per candle. This shortens the life of the lamp, but for lantern purposes that is of but little consequence. That is to say, a lamp run at the rate of one volt per candle will last fifty or one hundred hours. It will always be prudent to have two or three lamps at hand. In case the one in use should suddenly collapse another may instantly be substituted and with no awkward delay. I shall assume here that every lamp used for lantern projec- tions will be so adapted to the current provided for it as to yield a candle for a watt, thus, EC ^ watts = ELECTRIC LIGHTS. 165 candle power, and the value of C will always be known. Let W equal candle power required, then W E = — . The difference of potentials E equals the candle power divided by the current provided. As R = -, the resistance of the filament may be computed, remembering that the resistance of the filament while hot is but about one-half of what it is when it is cold. Suppose the electric-light company in the neighborhood provides a ten-ampere current, what must be the resist- ance of the lamp in order to give say 300 candles ? E = ^^^^ 30 volts must be the difference of potentials and R=3ft = 2 ohms must be the resistance of the filament while hot. It will be five or six ohms when cold. In this way one may adapt his lamp to currents of other degrees of strength. In ordering a lamp, how- ever, it will always be best to specify the current strength at command and the candle power wanted. SPECTRA OF THE ELEMENTS. By making the terminals of an induction coil of different metals, sparks from them will give their char- acteristic spectra. Arrange then a lens so as to pro- ject the spark upon the screen, as if the spark were a common object. Then near the focus of the lens place the prism so as to deflect the rays. The dispersion will at once be apparent as there will be as many images of the spark as there are visible rays. The zigzag form of the spark will be duplicated in each bright line. If the terminals of a condenser like a leyden jar be con- nected to the terminals of the induction coil as is usual for brightening the spark, the latter will be shortened very much, and the spectrum made brighter, 1 66 THE ART OF PROJECTING. appearing more like colored spots upon the screen, than characteristic lines. Very small fragments of metals or. other conducting substances may be used in this way. It is not, however, to be understood that such spectra can be made large like those produced by the oxyhydrogen or electric light. They may, how- ever, be shown as spectra a foot long and the lines two or three inches long, and thus be useful in places where the more pretentious ways of projecting spectra are not to be had. If an induction coil capable of giving a spark two or three inches long is not to be had, a common electrical machine will answer ; for the elements to be employed may be fastened into retort stands, and separated an inch or two as if simply to pass sparks from one to the other, these connected by wires to the Holtz or other similar electrical machine. The sparks may be pro- jected as in the case with induction coils. TO PROJECT AN ELECTRIC SPARK, Suppose, from a Holtz machine. So place the machine that the spark between the terminals shall be parallel to the screen to receive the image. Take a lens with a foot focus and with as large a diameter as possible, say four or five inches, and mounted in a broad frame (Fig. 15). If this be placed at the proper distance and height from the terminals of the electric machine, a spark will be projected on the screen much magnified, all its zigzag lines amplified. There is no difficulty in making an ordinary three or four inch spark appear to be six or eight feet long if the screen be fifteen or twenty feet away. It will be necessary to have the room quite dark, and also, to have the screen shielded from the light of the spark, but the frame of ELECTRIC LIGHTS. 1 67 the lens may be sufficient. If it is not, a screen of paste-board or something similar may be extemporized. FLOATING MAGNETS (mAYER's EXPERIMENT). Magnetize six or eight cambric needles so that their points will all have similar poles. Thrust these needles through small vial corks so that when placed in a dish of water they will float with similar poles up. Thus placed they will repel each other and move as far apart as possible. Bring a small bar magnet over them so that the adjacent pole will be the opposite of that of the upper ends of the needles. The needles will be attracted by it and approach it, but repelling each other they will arrange themselves in certain symmetrical order, which will depend upon the number of the floating magnets. If there be but three of them they will assume a triangular form. If there be four, a square. If five or six, there will be two or three posi- tions of stability. To project these motions and forms it will be necessary to have a glass tank similar to the one described on page 47 for cohesion experiments with the vertical attachment to the lantern. The tank must, of course, be deep enough to allow the needles to float freely about. The needles may be short — an inch long. If the corks be half an inch in diameter and quarter of an inch thick, they will float in three- quarters of an inch of water without danger of over- turning. The controlling magnet need not be a heavy one. One made of a stout knitting-needle will answer, and it will be best for projecting purposes if the con- trolling pole be bent at a right angle for two inches of its length. This will allow proper movement of it in the field without obscuring the field by large shadows. (See Mayer's experiments, Amer. Jour, of Science 1878). l68 THE ART OF PROJECTING. LESSENING CHROMATIC ABERRATION. When a double convex lens is used as an objective as described on page 25, the parts of the picture upon the screen near the margin of the disc will be seen to have many of the lines brightly colored with spectrum tints. At a distance of fifteen or twenty feet from the screen these spectral colors do not give much trouble, but nearer they are oftentimes objectionable. By using a compound objective, such as is made for lanterns, and especially for photographic work, all this may be avoided ; but such compound objectives cost considera- ble. If one cannot afford such a lens, he can use two similar lenses having the same or nearly the same focal length. Use one of these for a condenser, placing the picture close to it, and permit the converging rays to pass through the middle of the lens used as an objective. The trouble willlargely.be prevented, especially if the objective be covered, except a round or square hole at its middle, so that no light will pass to the screen except what goes through the orifice and the middle of the lens. A plano-convex lens two or three inches in di- ameter, and with a focus of eight or ten inches, so used, will do as well as a combination achromatic costing ten or twenty times as much. P'or microscopic projections, small objectives, such as are used for taking multiple tin-types, answer nearly as well as the more costly ones. They may be had with ratchet movements for about five dollars, and without the ratchet for much less. Their focal length is about an inch. BUBBLE COHESION. A group of soap bubbles in contact with each other cohere together, and their surface tension always ELECTRIC LIGHTS. 169 organizes them into a symmetrical arrangement, with plane sides at their junctions, instead of curved sides, as single ones have. Pour a few drops of soap solution upon a piece of window-glass, and with the finger spread it over a sur- face four or five inches square. Then with a common blow-pipe or glass dropping-tube blow bubbles upon the glass, starting them at any point. They had better not be blown more than an inch or two in diameter. If a second bubble be started at a point an inch or two from the first one, the two will rush together ; a third one will join to the two so that the interior angles at their junction will equal 120°. As others are added all will change their surfaces of adhesion and their relative positions. By placing the glass upon the vertical projector (pp. 42 and 43), the growth, motions, and symmetrical ar- rangement may be seen and studied by a hallful at once. VIBRATION OF FILMS. If the end of a tube like a glass lamp-chimney be dipped into a soap solution, a film will remain over the end when it is taken out of it. If now a beam of par- allel rays of light be directed upon this film, some of the light will be reflected from it. Place a lens four or five inches in diameter and ten or twelve inches focus so as to project this reflected light upon the screen or white wall. An enlarged image of the film will be seen upon which a series of spectral colors will appear. If a sound be made by the voice at the open end of the tube, the film will be thrown into vibrations similar in form to the air waves that produce them. These vibratory movements will show upon the screen as a curious network which will change for each different 170 THE ART OF PROJECTING. kind of sound. The changes in the patterns, combined with the many colors of the fihn, make interesting studies in acoustics. It is important that the tube upon which the fihns are made should be fixed while the projections are looked at, for otherwise the coruscations cannot be seen. It will be sufficient, however, to fasten it in a retort holder. As the light for the pro- jection is reflected from the surface, it will be best not to have the reflected beam more than about ninety degrees from the incident beam, otherwise the pro- jection will appear too oval, it will also be less distinct. LANDSCAPE PROJECTION. In the experiments to illustrate rectilinear move- ment of light, on page 81, it is remarked that the appearance of the landscape as shown on the walls of the room is much brighter when snow is upon the ground. The definition is made much better by making the orifice small, but then the light is so much scattered that the images are not very distinct. By employing a lens with as long a focus as possible, and placed at the orifice, a beautiful image of the external landscape will appear upon a screen at a proper distance, A lens with almost any focal length will show good definition, but the projection will be too small for any considerable number to see at once. A lens with six or eight feet length of focus will show a picture five or six feet square with all the details of the landscape easily discernible in a darkened room. SODIUM LINE IN SOLAR SPECTRUM. Having arranged the apparatus as shown in Fig. 89, for showing the more prominent Fraunhofer lines, ELECTRIC LIGHTS. 171 ignite a piece of sodium as large as half a pea, and hold it while burning so that the light from the sun must pass through the flaming sodium that is immediately in front of the slit. The yellow of the sunlight will be stopped, and a large and densely black line will be seen in the place of the yellow in the spectrum. A very good way to ignite the sodium is to provide a soft pine stick, six or eight inches long and half an inch or more thick. Close to one end cut out a hole large enough to hold the bit of sodium to be used, and crowd this into it. The end of the stick can be lighted in a gas or alcohol flame, and then hastily moved to the position where it is needed. The inflamed wood will set fire to the sodium in a few seconds, when it will burn with a great flame and dense fumes, yet without endangering the hand. The yellow flame and the light from it will not seriously impair the appearance of the spectrum upon the screen. RELATION BETWEEN SIZE OF OBJECT, SIZE OF IMAGE, AND FOCAL LENGTH OF OBJECTIVE. It is often convenient to know how large a given picture will be upon the screen when projected, what kind of an objective to use to obtain a picture of a definite size, and so on. The following rule will enable one to know and provide such conditions. Let A represent the focal length of the objective ; let B represent the distance from the objective to the screen ; let C represent the diameter of the space to be projected; let D represent the diameter of the lighted space upon the screen. Then, as A : B : : C : D. Three of these will nearly always be known. Suppose the transparency to be projected be 3 inches in 172 THE ART OF PROJECTING. diameter, the lens to be one foot focus, the distance to the wall 25 feet. How large will the screen need to be to receive the image ? As i : 25 : : 3 : x = 75 inches = 6^ feet. Suppose the object to be one inch long. What must be the focal length of a lens to project the image 4 feet long when the screen is 20 feet away ? As 48 : I : : 240 : x = 5 inches. In this way one may pro- vide distance, screen, and lenses to suit his con- venience near enough for all practical purposes. VORTEX RINGS AND THEIR PHENOMENA. The phenomena presented by vortex rings are so interesting — some of them so surprising — and yet are so easily produced as to warrant giving some space to an account of their production. Aside from this the physical importance of their study is very great, seeing that Sir Wm. Thomson and others have seriously proposed to account for the properties of atoms of mat- ter by supposing the latter to be vortex rings of ether in the ether. The ring of steam often seen puffed from a locomotive, and rising in the air sometimes a hundred feet or more, is called a vortex ring or sometimes a smoke ring. Such rings are formed whenever a gas or a liquid is suddenly pushed through an orifice. If they be formed solely of air they cannot be seen, on account of their transparency, but they may be made manifest in other ways. Over the mouth of a glass or a tin funnel three or four inches in diameter, tie a piece of stout paper. Snap with the finger upon this stretched paper and a ring will be projected from the stem. If the latter be directed towards the face it will be easily felt, and if it be directed towards a candle flame it may blow it ELECTRIC LIGHTS. 173 out at a distance of three or four feet. With a larger funnel and stronger blow it may extinguish the flame at a greater distance. To make them visible it is neces- sary to mix the fumes of something, smoke for instance, with the air of which they are formed. Very good ones may be formed by' the mouth. Let the mouth be filled with smoke and the lips be pursed as if to produce the vowel 0, then tap the cheek with the end of the finger, and smoke rings an inch or two in diameter will be formed. Some smokers are able to project very large and dense ones from their mouth by a sudden forward thrust of the back of the mouth, a movement which has to be acquired by practice. For the production of vortex rings for the study of their behavior, it will be necessary to have made a box which may be kept filled with the visible vapor. One of the following shape and dimensions will be found to answer well. A box of wood about a foot cube, having a round hole about four inches in diameter cut in the middle of one side. A swinging hinged back, framed square, over which may be stretched tightly some stout cotton cloth, will close the box tightly enough when it is (iown. The cut represents the back of the box lifted up. It will be convenient to have two strips in front grooved so as to permit a slide to be inserted. Several slides may be made for this position, each one having its orifices through which the smoke may be pro- jected. The following are suggested as being useful. One with a round hole three inches in diameter. One 174 THE ART OF PROJECTING. with an oval hole three inches in its longer and two inches in its shorter diameter. One with two holes one inch in diameter and an inch and a half O O apart, the two holes horizontal. Oue with two holes like the last except that one is to be over the other. One with three holes each an inch in diameter, their centres two and a half inches apart. One with a hole two inches square. Two saucers or other crockery vessels presenting as large a fluid surface when filled as convenient, may be filled, one with the strongest ammonia water, the other with the strongest hydrochloric acid, and placed in the box and the back closed. The box will at once be filled with the dense white vapor of ammonium chloride. If the solutions be heated before being placed in the box the fumes will be denser still, and therefore better for this purpose. EXPERIMENTS. 1. Strike the cloth back of the box with the hand suddenly. A white ring five or six inches in diameter will be projected and will move several feet. If the smaller three inch hole be in front, the ring will be smaller and will move faster. 2. Produce a ring by swinging the back of the box an inch or two and letting it strike the box smartly. The ring will move with rapidity fifteen or twenty feet in the air, going in a straight line if there be no currents of air to deflect it or objects near to its path. 3. If the table be ten feet long or more and the box be at one end of it, so that the rings may move over the length of the table, a swift moving ring will come down to it and be broken, as if the table attracted it. To prevent this, tilt up the box, being careful about ELECTRIC LIGHTS. 17$ spilling the contents of the saucers if they are very full. 4. Make one ring to follow another so as to overtake it. If the axes of the two coincide, the forward one will expand, while the oncoming one will contract in diam- eter, permitting the latter to go through the forward and larger one, when each will assume its original dimension. 5. Project one ring after another so that they may col- lide. Each will be seen to be deformed and each will vibrate, assuming oval shapes with axes at right angles to each other, thus indicating that the rings are elastic. 6. Project a ring so that it will pass near a suspended fibre of thread or other light body. The thread will appear to be repelled from the front and attracted by the back of the ring. 7. A ring formed by the oval hole will move forward like the round one, but will vibrate energetically, going through the phases mentioned in experiment 5. 8. The triangular hole will likewise give a vibrating ring, as will one generated with any other form of orifice, so that it is impossible to have a ring that will maintain any other form than the circalar one with its phases of vibration. 9. With the double aperture slide, two rings will be formed simultaneously, but instead of producing them as the larger ones were, they can best be made by a tap with the finger upon the cloth back near to its edge. The rings will be small but well formed, and move so slowly that their motions may be easily watched. 10. Observe that when the two are produced they ifivariably collide, they never move off parallel with each other. 176 THE ART OF PROJECTING. 11. After a collision of two such formed rings they may separate, but when they do they always move away from each other in a plane at right angles to the plane of collision. If the two holes of the slide are horizontal, the rings will bound from each other in a vertical plane. If the holes be vertical, they will bound away from each other horizontally. 12. If the rings do not rebound, they will each break at the point of contact and weld together into a single ring having twice the diameter, and move on in a right line from the front of the box, but vibrating like the ring formed by the oval orifice. 13. By using the slide with three holes the rings may rebound from each other after collision — for they will always collide as do those formed from the two holes — or they may all combine to form a single ring, each breaking apart at the point of contact with the others. 14. Observe that a ring always moves plane on — that is, never sideways or in other directions than at right angles to a plane through itself. (Of course a ring may be drifted about by currents of air, but such is not the proper motion of the ring.) 15. When a ring strikes upon a surface parallel with its own plane, its diameter increases indefinitely, while the cross section of the ring gets thinner and thinner. 16. Rings having sections of greater density than other parts often show vibratory motions of such parts. Two such on opposite sides of a ring will approach each other and combine midway, heaping up at that place, then each part' retreats from the other to meet upon the opposite side of the ring. A kind of peripheral vibration. If the motions be not very energetic, the denser parts may not separate more than 180 degrees. ELECTRIC LIGHTS. 1 77 17. A denser part of a ring may sometimes be seen to be travelling round the ring without apparent rotation of the ring itself, but this phenomenon I have not been able to reproduce at will. 18. Sometimes a spiral movement may be seen to be taking place — in and out'as well as round the ring. * 19. If one is provided with two boxes for forming these rings, they may be used in conjunction. Rings may be made to move towards and away from each other at any angle and at different velocities. Rings of different sizes moving towards each other in the same line present a singular phenomenon. The smaller one will go through the larger, and the one with the less velocity will be brought to a standstill in the air, while the other one goes on with lessened velocity. After the moving one has advanced a foot or two, the arrested one will again start up as if it had been pushed in the direction it originally had. Showing in a curious way that the forward movement of the vortex ring is necessitated by the motion that constitutes the ring itself. The liquids in the saucers may get too dilute to serve for experiment for all the above indicated ones. The ammonia water may get a crust of chloride formed on its surface which will need removal. If the liquids be freshly heated they will again serve for experiments for a few minutes. In order that a roomful may see the rings to good advantage, it will be better to have a dark background, and have the rings lighted by a beam of light parallel to the general direction of their motions. If the fumes are not quite dense, they may not be easily seen at the distance of fifteen or more feet. With such a series of experiments various properties of matter may be illustrated. For example, the external and internal 178 THE ART OF PROJECTING. energy of gases, free path motion, heat motion (5), momentum, attraction and repulsion (6) as due to motion, gravitative action (3), chemical action (12), elasticity (5). It is not to be understood that these phenomena are the same as those mentioned, only that they are strikingly like them in some respects. • PHAS. S. BOURNE, ^°^|s^^ MANUFACTURER OF LISSAJOUS FORKS (Illustrated on page oo) AND OTHER APPARATUS .FOR PROJECTION. ^^^Send for Circular. Established 1870. 8. HAWKRIDGE, HOBOKEN, N. J., (Successor to George Wale & Co.), PHILOSOPHICAL INSTRUINT MAKER At the Stevens Institute of Technology. A HOARDS OF MERIT: Silver Medal, American In- stitute, 1875. Bronze Medal, American In- stitute, 1875. Two Silver Medals, Cincin- 7inti Exposition, 1881. Ma^o Lantem.i and AcressoriM. Col lefieLanterns and Attachments, includinp Gas Miiroscope Po- larisiope. Apiiariitus for Total Keflection, Mapnetic Spectra, Spectrum Analysie. Tanks for Solar I'roniiiiences and Decompo- Bition of Water, (.hromotropei. Prisms, Nicol's Prisms. Uouble- Imape Prisms, Porte Lumieres, Heliostats, Spectroscopes.Spectro- meters, and Gratinfrs. Wale's Working Microscopes and Uni- versal Lenses. Apparatus for Blowpipe Analysis. Apparatus for illustration of Prof. A. M. Mayers Scientific Series. Mafrnesium Ribbon, Powderand Lamps. Alu- minum and Platinum. Catalogues sent on applleatlon. Jmes W. Queen I Co. 924 Chestnut Street, philadelphia. WW APPWIUS Foil PPJECIIOH, SPECTROSCOPES, m Magic Lanterns and Attachments, &c„ ac. PHYSICAL APPARATUS FOR SCHOOLS AND COLLEGES. GHEMIGAL^^' BLOWPIPE APPARATUS, BEST C. P. CHEMICALS. tiiom:a.s h^ll. MANUFACTURING ElectriGiQii^^'OptiGiaii. Manufacturer and 1 mporter of Telegraphie, Kleetrie, Magnetic, Galvanic, Opti- cal, and Jlrteorologleal iDHtruments. Chemicals, Chemical and Philosophical Ap>- paratus of all descrip- tions. Illustrate