MEMCAL L1ISMA1ISY T H E SCIOPTICON MANUAL, EXPLAINING LANTERN PROJECTIONS IN GENERAL, SCIOPTICON APPARATUS IN PARTICULAR. INCLUDING MAGIC LANTERN ATTACHMENTS, EXPERIMENTS, NOVELTIES, COLORED AND PHOTO-TRANSPARENCIES, MECHANICAL MOVEMENTS, ETC. BY L. J. MARCY, OPTICIAN, No. 1340 CHESTNUT STREET, PHILADELPHIA. SIXTH EDI.TI-O-N. PHILADELPHIA: JAMES A. MOORE, PRINTER, 1222 AND 1224 SANSOM STREET. 1877. Entered, according to Act of Congress, in the year 1877, BT L. J. MARCY, In the Office of the Librarian of Congress, at Washington, D. C. PREFACE. YISIBLE illustrations are so highly esteemed among educationalists of the present day, that the announce- ment that a greatly improved form of Magic Lantern has made its appearance is very favorably received. Between a desire for such an instrument, and the lack of definite information concerning it, many very natu- rally manifest both their interest and their caution by searching letters of inquiry. The inventor has en- deavored, in hasty epistles, to state all the possibilities and impossibilities of the Sciopticon, and of lights, lenses, lantern slides, and tanks; but he finds it increasingly difficult to give each correspondent, individually, a full philosophical exposition. For such, therefore as desire a, more detailed account than can be given in a hasty letter, or than is contained in the circular, which it is always a pleasure to forward on application, a Sciopticon Manual has been prepared, to serve as a guide-book to bli iv PREFACE. the various interesting portions of the Magic Lantern Held in general, as well as to the Sciopticon in particular. It treats of the Optical Image without a lens, with a lens, in the camera obscura, in the eye, in the photo- graphic camera, and on the screen ; of the peculiarities of lenses, and the corrections required hy lenses; of the peculiarities of the Sciopticon, and its construction and management; of dissolving views, phantasmagoria, and the ghost ; of lantern slides in all their variety ; of photo- graphing slides by the wet-plate process, b}^ the dry-plate process, by Marcy's Photographic Printing Apparatus, by the Sciopticon, and other processes; of how to paint slides, and of ho\v to perform chemical experiment- to which is appended a catalogue, arranged to assist purchasers in making satisfactory selections. Thus this Manual may take the place of private cor- respondence to a considerable extent, allowing in let- ters more space for business, and tor an interchange of new ideas, with a view of making the Manual in subse- quent editions more interesting, and the Sciopticon more useful. It was at first my intention to give space to the sub- ject of chemical lights, but reflect ing that in this direc- tion there is no lack of printed matter, and that the in- terest felt in the Sciopticon is owing mainly to its giving PREFACE. results with little trouble, I have concluded to omit 1/n> gs, which would increase the bulk of the Manuul, without a corresponding addition to its usefulness. PREFACE TO FIFTH EDITION. THE lime light, in an improved form, having been in- troduced into the Sciopticon, it has become expedient to append to the Sciopticon Manual, a description of the apparatus and directions for its use. The demand for Lantern projections is steadily on the increase. A fine photograph (arid what can be finer?) projected upon a largo screen, before a thousand spec- tators, gives, it is safe to say, ten thousand times the satisfaction that one alone with his stereoscope receives from it. The appreciation is cumulative. "The more the merrier," is the philosophy of it. The Sciopticon with its oil lamp, rather than with its lime light, continues to be the choice of the many, be- cause its use is convenient and inexpensive. There are purposes and occasions however for which the lime light is a necessity. The gas therefore has now received its full share of attention. Much of the added matter is intended to assist those who have a Sciopticon, to pro- vide themselves with interesting objects for exhibition, without resort to a large assortment of exoensive slides. VI P li F ACE. PREFACE TO SIXTH EDITION. MARCY'S SCIOPTICON is now no new thing. It has taken its place as a standard projecting Lantern, and is accepted as a great advance in this direction. As it becomes better known, and less in need of explanations, the Sciopticon Manual might be contracted rather than enlarged, only that something more may be said to advantage of its new attachments and new uses. It will ba seen that the Triple Jet, the Gas Sciopticon, the Gas Microscope, etc., have been improved since the illustrations prepared for the Fifth Edition were drawn. The Automatic Cut-off is yet new to many, who will stand in need of a fuller explanation in print. The description and illustrations of the Franklin Insti- tute Gas Holders will interest Managers of Educational Institutions. It is believed that all who are alive to the advantages of photographic projections will find the Sciopticon Manual a valuable Hand-book. CONTENTS OF MANUAL. Introduction, xi CHAPTFR I. The Camera Obscura The Inverted Image, Color and Shading, Motion, Size, Size on the Retina, Size in the Photographic Camera, Magnified Image, Inferences, Experimental Verification, Indistinctness, Convex Lens, Stops, CHAPTER II. The Corrections required by Lenses, The Form of Lenses, Pencils of Rays Spherical Aberration, Distortion, Curved Field, Chromatic Aberration, Unequal illumination, Corrections in the Eye, The Landscape Lens, The Portrait Objective, 24 CHAPTER III. The Construction of the Sciopti- con, No Loss of Light from using an Objective of Low Power with a Concentrated Light, 24 Advantages of Concentrated Light 25 The Condenser, 26 The Reflector, 26 Various Modes of Lantern Illu- mination 27 The Size proper for the Illumin- ated Di?c, 28 Analysis of the Sciopticon 29 Packing, 32 Rules for Operating the Sciopti- con 33 Recapitulation, 36 The Screen, 37 Dissolving Views 39 Directions for Producing the Dis- solving Effect, 41 The Phantasmagoria, 44 Ancient Magicians, 45 CHAPTER IV. 50 Picture Slides, The Standard Size for Lantern Slides, 51 The Sealed Picture, 52 Statuary, 53 (vii) Vlll CONTENTS OF MANUAL. The Slip Slide 54 The Lever Slide 55 Revolving Figures 55 The Chromatrope 66 Eidrot ope 57 Moving Waters 57 Long Slides 68 Dioramic Painting, with Mov- ing Figures 58 The Ghost 58 The Tank 59 Precautions about Slides 60 CHAPTER V. PHOTOGRAPHY. Glass Positives for the Magic Lantern 61 The Camera 62 Solutions The Negative 64 The Lens 65 Dry Plate Process 67 Collodio-Chloiide Process 71 Marcy's Photographic Print- ing Apparatus 72 Sciopticon Process 74 Woodbury Photo-Relief Slides. 76 CHAPTER VI. COLORING SLIDES. Water Colors, etc 78 Easel Brushes, etc 79 Statuary 82 Silhouettes, Natural Objects, etc 82 CHAPTER VII. CHEMICAL EXPERIMENTS. Tank and Pipette 84 Chemical Reactions 85 Crystallizations 86 Miscellaneous Experiments... 87 CHAPTER VIII. DESCRIPTIVE LECTURES. Bible Pictures... .... 95 PA8B Sunday-school Pictures 97 Extracts 98 Bible Lands 103 Egypt 110 Views of interest in all parts of the World 119 Nursery Tales 1 19 Composition Pirturts 121 8 tat ua ry 1 22 Movable Slides 123 Scientific Slides 123 CHAPTER IX. THE SCIOPTICON AND ITS USES. Description of the Sciopticon, 125 Magic Lantern, from 1650 to 1870 126 Traveling by Magic 128 Sciopticon for Sunday-schools, 130 Science at Home 130 Various Experiments 132 CHAPTER X. THE TEACHER HIS OWN ARTIST. A Process of Drawing and Painting Magic Lantern Slides 135 List of Apparatus and Mate- rials 135 A New Copying Camera 139 Diaphanie 141 Enamel Slides 142 Prepared Glass 142 Sample of Size and Shape 143 CHAPTER XI. NOVELTIES. Chimney Cap 144 Sciopticon Curtain 144 Tinters 145 New Slide Stop 145 A New Departure 14G Eidrotrope HO Wheel of Life 147 Danciug Skeleton 147 Dance of the Witches !< Effect Slides 149 CONTENTS OF CATALOGUE. IX CHAPTER XI.. THE LIME LIGHT. PACK Flame Illuminnti m lal Vaiietit-s in Lime Light 151 No. 1, Alcohol Burner 152 No. 2, House Gas Concentric Burusr 153 Dr unimond Light 154 No. 3, Mixed Jet , 155 Dangers Peculiar to Mixed Jet, 155 Annoyances Peculiar to the Mixed Jet 156 Marcy's Triple Jet 158 Oil Sciopticon with Triple Jet. 160 Lime-light Sciopticon 161 Triple Jet Adjustments... 161 Lime-light Adjustments 163 Ratio of Spaces 164 Changing the Scene 166 The Automatic Cut-off. 167 Patent Slide Carrier 169 Both Seiopticons with an in- terchangeable Set of Lenses.. 170 Dissolving Cock 171 Preparation of Oxygen Gas .... 172 Preparation of Hydrogen Gas. 174 Pressure Boards 177 Self-Condensing Cylinders 178 MultuminParvo 181 An Economical Form of Gas Holder for the Lime Light... 182 Other Gas Contrivances 186 Lantern Attachments 188 Microscope Experiments 189 Holman's Siphon Slide 190 Gas Microscope, by Henry Mor- ton, PH. D 191 Sciopticon Microscope Attach- ment 194 The Megascope 195 Sciopticon Kaleidoscope 197 Reflecting Polariscope 199 Refracting Polariscope 200 CONTENTS OF SCIOPTICON CATALOGUE. INTRODUCTION. Notice, Sixth Edition Where to find Explanations, Peculiar Advantages of the Oil Sciopticon Peculiar Advantages of the Gas Sciopticon How to Secure the Fullest Ad vantages of Both Late Improvements Lantern Projections as an Educational Appliance. . . PAGE ii iii iv xiii xiv PAGB Comparison of the Different Sources of Illumination., xviii The Slide Question- Plain or Colored ? xxi Nature or Art? xxiii Strong or Faint ? xxiii The Bad and the Good xxiv What's in a Name? xxiv Descriptive Lectures xxv Sciopticon Catalogue A r - rangement xxvi PRICE LIST OF SCIOPTICON APPARATUS. PACK .. 1 Oil Sciopticon, etc Gas Sciopticon 2 Gas Bags, Generators, etc 2 Chemicals, Chemical Apparatus, 3 Separate Parts of Apparatus 3 Lantern Attachmeats 4 Combinations and Estimates... 5 Dissolving Views 6 CONTENTS OF CATALOGUE. PRICE LIST OF COLORED MAGIC LANTERN SLIDES. l.-Choice Selected Slides II. Sacred History 9 III. Holy Land and Egypt 9 IV. Ancient Greece and Rome, 11 V. Complete Illustrations t o the Text of the Holy Bible.... 12 VI. Views of Interest in Dif- ferent Parts of the World 17 VII. American History 20 VIII. Franco German War... 21 IX. Views in Sets, Conveying Moral Lessons 22 1. The Pilgrim's Progress 22 2. Christiana and her Child- ren 22 3. The Drunkard's Progress.. 22 4. The Bottle (Cruikshank).... 23 5. The Bottle (Comic) 23 6. Stomach of the Drunkard in Different Stages of Disease.. 23 7. The Gambler's Career 23 8. The Ten Corrmandments.. 23 {). The Lord's Prayer 24 10. Four Scenes from the Life of a Country Boy 24 11. Shakspeare's Seven Ages... 24 12. Masonic, and other Lodge Pictures 21 13. TamO'Shanter 24 14. New Tale of a Tub 24 15. Renard, the Sly Fox 25 16. Fables of Lafontaine 25 17. The dusaders 25 18. The Ill-Fated Ship 25 19. Schiller's Song of the Bell.. 25 20. Rip Van Winkle 26 X. Miscellaneous Pictures 26 XI. Dissolving Views 28 XII. Chromatropes, etc 30 CLASS PAGE XIII Original Leaf Designs.. 32 XI V.-Statuary and Bas Reliefs, 32 XV. Select Painted ComijSlip Slides 33 XVI. Paintings with Levers, Comic 34 XVII. Dioramic Paintings, with Moving Figures 35 XVIII. Paintings Illustrating Nursery Tales 35 XIX. Fine Engravings 36 SCIENTIFIC DEPARTMENT XX. Mammalia 37 XXI. Birds 38 XXII. Reptiles.and Fishes.... 38 XXIII. Insects 39 XXIV. Araclmida, Crustacea.. 39 XXV. Botany 46 XXVI. Flowers and Plants... 41 XXVII.-Physical Geography. 42 XXVIII. Astronomy 42 XXIX. Geology 44 XXX. Natural Phenomenon.. 45 XXXI. Anatomy and Physi- ology 45 XXXII. Microscopic A n a t- omy 45 XXXIII. Optics 46 XXXIV. The Microscope and its Revelations 46 XXXV. Crystallography 47 XXXVI. Spectrum Analysis.. 47 XXXVII. Geological Land- scapes, etc 48 XXXVIII. Society Emblems. 48 XXXIX. Dissolving E ff e c ts, etc 50 PRICE LIST OF PLAIN GLASS SLIDES. CLASS PAGE | C XL. American Views 55 ^ XLL Foreign Views XLII. -Centennial Slides XLIII. Scientific Sciopticon Slides INTRODUCTION. THE SCIOPTICON (pronounced Si-op-ti-con), is by far the most convenient and easily managed of any form of Magic Lantern. Its ridge of wide, intensified double flame, lying lengthwise in the axis of the condensing lenses, gives it much greater efficiency than any other lamp-illuminated lantern. All who have become acquainted with this new in- strument, see in it the accomplishment of what has long been greatly desired by those who appreciate the value of visible illustrations as a means of imparting instruc- tion and of affording rational amusement. Confessedly, the medieval magicians with their lan- terne magique effected little good by their incantations and ghostly spectres. But modern educators have higher aims and better means at hand. Their lenses are greatly improved in form and quality. The pho- tographer secures images of all that is interesting or (xi) Xll INTRODUCTION. beautiful in nature and art. Literature and the sciences teem with pictorial illustrations, from which choice se- lections can bo easily copied for lantern slides. And now the Sciopticon, with its own peculiar light for all ordinary occasions, and with the oxy-hydrogen light for occasions extraordinary, comes in to show up what is thus made ready. In form and construction the Sciopticon is very unlike that relic of the middle ages, the old magic lantern. Those who are interested in the philosophy involved in it, in the peculiarities pertaining to it, in the practical management of it, in making and selecting slides for it, in performing scientific experiments with it, and in pro- moting the interest of education by it will do well to inquire within. SCIOPTICON MANUAL. CHAPTEK I. THE CAMERA OBSCVBA. A picture formed by rays of light from the several parts of an object as seen at A (Fig. 1), is called an image ; and the cham- ber in which it is formed, and from which all light is ex- cluded, except what enters a small hole as at S, is called a camera obscura. This dark chamber claims attention here because its properties, which are common to the eye, the Sciopticon, and all forms of the camera, are seen on a broad scale, and may be readily verified by inexpensive experiments. To avoid confusion the pencils of light are represented in Fig. 1 without marginal rays need- ing to be focused by a convex lens. Explaining the fol- lowing properties in connection with a diagram without 2 1. 10 SCIOPTICON MANUAL. a lens, may help to correct the common impression that these properties and a convex lens arc inseparable. THE INVERTED IMAGE. Of the rays of light proceeding in all directions from every point in the object , as from all illuminated objects, just an assortment, that is to say a pencil from each point will enter the hole at S, just in lino to fall in reverse order upon the screen J., forming an inverted image. The picture results from admitting just an assortment and excluding all the rest. COLOR AND SHADING. Light from each of the several parts of the object B illuminates with its peculiar color and relative intensity each corresponding part of the imago, so that it is seen in natural light and shade, and in natural colors. The photographer can fix the relative shading, but ho cannot, as yet, fix the colors. MOTION. If, for example, the bird moves to a higher porch, the pencils of light will fall to a lower place on the screen, and so any movement of the object which alters the direction of the pencils of light, will give a reverse movement to the image. SIZE. By inspecting the angle of extreme rays it will bo seen that the imago in this case is smaller than the ob- ject, because it is nearer the aperture; so in all cases, the relative size of the image depends on its relative distance from the aperture. SCIOPTICONMANUAL. 11 SIZE ON THE RETINA. A A (Fig. 2) represent* the visible portion of the sclerotica of the human eye, which is a camera obscura in perfection. 1 I represents the iris (or window shutter), in the middle of which is the pupil (or aperture). As the retina is only about half an inch behind the optical centre, it follows that the images of distant objects upon it must be xfy- 9. very minute. For example, the figure of a man 6 feet high, seen at a distance of 40 feet, produces an image upon the retina the height of which is about y^th part of an inch. The face of such an image is included in a circle whose diameter is about y^th of the height, and therefore occupies on the retina a circle whose diameter is about the -j-j^th part of an inch ; nevertheless within this circle, the eyes, nose, and lineaments are distinctly seen. The diameter of the eye is about T ^th of that of the face, and therefore, though distinctly seen, does not occupy upon the retina a space exceeding ^(j^iuirD^^ f a square inch. How infinitely delicate must be the structure of the retina or canvas on which this exquisite miniature is delineated to receive and transmit details so minute with such marvellous precision ! SIZE IN THE: PHOTOGRAPHIC CAMERA. A man G feet high, standing for his picture 10 feet from a camera tube whose lenses require the screen of ground-glass adjusted to 5 inches, gives a 3 inch picture. For we have (in inches) 120 : 5 : : 72 : x hence x = * = 3. 12 SCIOPTICON MANUAL. MAONIFIEI* IMAGE. This rule works both ways ; for suppose a screen to bo suspended in place of the man, the space being dark- ened, and suppose the three-inch inverted image to bo "fixed" and highly illuminated, then a secondary imago will be projected upon the screen ten feet from the lens, corresponding to the original object in size and position. INFERENCE 1. A good portrait objective for the camera is also suitable for a lantern objective ; for the lines of light and the angles are in both cases the same. INFERENCE 2. The light, if reflected from the three-inch picture, radiates so as to cover 100 times as much surface on the magnified image. How, as a very small fraction of this reflected light is re-reflected to the eye of the observer, it seems a hopeless undertaking, to make the opaque lan- tern practically useful in showing the images of small paper photographs, on a large scale, with any ordinary flames, however well arranged. INFERENCE 8. With an intense light at a point behind the three-inch transparency, converged by a condenser, so as to enter the objective through all points of the picture, the mag- nified image is illuminated with incident rays concentra- ted, and its exhibition becomes a success. INFERENCE 4. Additional light outside this point (as some recom- mend), would not fall in line with the objective so as to improve the illumination; while the additional heat and diffused light would be very objectionable. SCIOPTICON MANUAL. 13 EXPERIMENTAL VERIFICATION. These properties of the camera obscura, thus far con- sidered, may receive more lively illustrations by actually darkening a room and admitting light through, say an inch hole. A room with but one window, and that looking from the sun, and towards objects illuminated by sunlight, is to be preferred. A lens, if one is used, of long focal distance (nearly flat) gives more room for spectators before the screen. The images, if the lens has short focus, may be better seen on the back of a semi-transparent screen by transmitted light, as they are seen on the ground-glass in a photographic camera. These moving pictures of busy life and wavy trees, of curling smoke and floating clouds, are peculiarly pleasing and beautiful, as well as suggestive of im- portant principles in optics. INDISTINCTNESS. Fig. 1 fails of showing the divergence of each pencil of light to the size of the aperture as seen at c (Fig. 3) ; Fig. 8. a property which renders the image indistinct, from the consequent overlapping of the blunt ends, so to speak, of innumerable pencils. CONVEX EENS. In accordance with the law of refraction, rays as from d (Fig. 4) are bent towards a perpendicular in entering the convex lens /, and from a perpendicular in 14 SCIOPTICON MANUAL. emerging from it. As the result of these refractions they meet at /. The converging power of lenses, of uniform substance and density, is in proportion to their Fig. 4. degree of convexity. For a lens to sharpen the image, the screen must be adjusted to the focal distance. STOPS. To get on the same plate something like distinct images of objects at various distances, a " stop " is used by the photographer, which, though it necessitates long exposure, secures " depth of focus." This expedient of having a small aperture is also resorted to for lessening the defects or aberrations of lenses, just as the aperture b (Fig. 3) is made small to lessen the greater defect of having no lens. Stops are not used in the Sciopticon objective, because all portions of the picture-slide are in the same plane, and because sharpness produced by stops is always at the expense of light. CHAPTEE II. THE CORRECTIONS REQUIRED BT I.EXSES. The corrections required by lenses (as well as every- thing photographic), is well set forth in Dr. Yogel's Handbook of Photography. The use of such diagrams, as are here appropriated, is kindly allowed by the Ameri- BCIOPTICON MANUAL. 15 can publishers, Benerman & Wilson, Of course one may successfully operate the Sciopticon, or even excel in photography, without a critical knowledge of lenses; but a very short, connected showing of their properties, with diagrams, will doubtless prove acceptable to many who use the Sciopticon, or who are interested in pho- tography. THE FORM OF LENSES. The convex, or converging lenses. 1, 2, and 3 (Fig. 5), called biconvex, plano-convex, and meniscus, are thicker Fig. 5. in the centre than on the margin. The concave, or dis- persing lenses, 4, 5, and 6, called biconcave, plano-concave, and concavo-convex, are thinner in the centre than on the margin. A line through the centre of these lenses, from side to side, would show the axis of each lens. PENCILS OF RATS AND THEIR ILLUSTRATIONS. A pencil of rays considered in reference to its direction and the points in the image which it illuminates, may be represented by a simple straight line, as in Fig. 1 ; but in most cases, when the action of lenses on its rays is considered, it must be shown as a bundle of rays, as in Fig. 4. The pencil in Fig. 6 differs from df in Fig. 4, in having middle rays represented as well as marginal, and 16 SCIOPTICON MANUAL. in having them proceed from a point too distant to be shown. The rays of a pencil from a point 100 times further from the lens than is the image, are about paral- lel, and their focus is called the focus of parallel rays, or principal focus. A real pencil is composed of i nnumerable rays, and such pencils from innumerable points in the object meet and cross at the lens on their way to cor- responding points in the image, and wonderful to tell, no one is switched from the track for another, and there are no collisions. An explanation of one answers for countless millions. SPHERICAL ABERRATION. It is seen (Fig. 6) that the marginal rays d d must be more refracted, or bent, than the more central rays / /, Fig. 6. in order to meet the axial rays at / , and so it is seen that the margin of the lens G D has a greater refracting angle than the more central portions. But the trouble is, the refracting at the margin is overdone, so that the rays d d meet the axial ray at / 8 instead of at/ r Hence if a ground-glass has been placed at/ t , the marginal rays which have intersected the axis at / 8 will form a circle of dispersion about f t . The diameter of this circle is called the lateral aberration, and the distance between / 8 and/ t is called the longitudinal aberration. As a con- SCIOPTICON MANUAL. 17 sequence of this want of coincidence between the foci of the central and marginal rays the picture on the screen, or ground-glass, will appear blurred and ill defined. We can conceive of a lens with a gradually lessening degree of convexity towards the margin, causing the foci to coincide, but lenses cannot well be ground in this form. The crystalline lens in the eye is supposed to cause the foci to coincide by an increase of density towards its centre, but such an arrangement of matter would be impracticable in art. Much is gained by re- versing the lens, for spherical aberration is four times as great when the parallel rays enter its plane surface, as when they enter its convex surface. Much is gained by a combination of lenses so that the refracting angle may be less in each. Were the mar- ginal rays d d cut off by a stop, the aberration would be less, as we can see by tracing them in the diagram, but the illumination would also be less by so much. DISTORTION. When we focus with a single lens with a front stop C Fig. 7. sharply on a square, A (Fig. 7) the resulting picture will not appear square, but barrel-shaped, as at B. When we 18 SCIOPTICON MANUAL. substitute a lens with the stop in the rear, the curves will be reversed, as at C. This property is based on the fact that the marginal rays of the field of view strike the lens under a larger angle than the central rays, and consequently suffer a greater refraction. Of the simple form Of lenses, the meniscus, with its concave side to the object, shows it the least. But it is best overcome by a combination of lenses with central stops. CURVED FIELD. This error is not caused by spherical aberration, for it occurs with all perfectly aplanatic lenses, but by the curve of the image, as is shown by the arrow, Fig. 8. Fig. 8. "When the ground-glass is placed at a a, only the central part will appear sharp ; when at b 5, only the points of the arrow will appear well defined. This error is avoided by combination of lenses with suitable curves, and by stops. SCIOPTICON MANUAL. 19 CHROMATIC ABERRATION, OR DISPERSION OF COLOR. White light is separated by a prism into the seven primary colors ; violet, indigo, blue, green, yellow, orange, red. As a lens is analogous to a system of prisms, and as violet is more refrangible than red, the violet rays v v (Fig. 9) will intersect the axis closer to the lens than the red rays r r. This error is corrected by combining a concave lens of flint-glass with a convex lens of crown- glass, so as to neutralize their contrary dispersions. The concave flint-glass lens / (Fig. 12), which has great dispersive power in proportion to its curves, diverges the violet more than the red, while the convex crown-glass lens converges the violet more than the red, so we have in both combined an achromatic convex lens. As the chemical rays are in the violet end of the spectrum, the photographer may succeed with an im- Flg. 9. perfectly corrected lens by having the sensitive plate a little nearer the lens than the focus of luminous rays on the ground-glass would indicate. Lenses without chro- matic aberration are called achromatic. The term aplanatic means without wandering, and may apply to lenses corrected of both spherical and chromatic aber- ration. 20 SOIOPTICON MANUAL. UNEQUAL ILLUMINATION. We can see that the brightness of an image decreases from the centre towards the margin. The diameter of the pencil g g passing through the lens parallel to its axis, is of the same size as the opening of the stop , and exceeds the diameter of the oblique bundle of rays. Besides, the oblique rays lose considerable light by re- flection, which may in part be re-reflected upon the image, detracting from its distinctness. With a short exposure in the camera, this unequal illumination causes an under- exposure at the margin. In the Sciopticon it is even exaggerated by the reflector, but we generally wish the objects occupying the central portion of the "field of view" to stand out more clearly in the illuminated disk. CORRECTIONS 1JS THE EYE. Spherical aberration and distortion in the eye are corrected (it Is believed) by the greater density of the crystalline lens about its centre than towards the margin, where the refracting angle would otherwise be too great. Chromatic aberration is corrected by the combined BCIOPTICON MANUAL. 21 P. action of the crystalline lens and the vitreous and aqueous humors. As the retina, R R R (Fig. 11), is con- cave, the centre of concavity being the optical centre, there is no need of adjusting the focus to a flat field. The eye is readily, for the most part unconsciously, ad- justed, so that an object upon which we fix our attention is at once in the centre of the field of view, and is focused according to its distance. These five troublesome properties enumerated in this chapter, are thus, in the eye, harmoniously reconciled. In art we lack the peculiar crystalline lens, and the con- cavity of field. Making amends for this lack interferes with other corrections. Efforts of various makers to effect the best compromise for particular kinds of work has given rise to lenses, in variety too numerous here to particularize. THE LANDSCAPE LENS. This simple achromatic lens (Fig. 12) is the oldest photographic lens in existence. It is composed of the A c ' i f \i i I- . 12. concave lens of flint-glass /, and the convex lens of crown-glass c. 22 SCIOPTICON MANUAL. Among the modified forms, the Dallmeyer Landscape Lens, which consists of three lenses" cemented together, a central one of flint-glass and two outer ones of different kinds of crown-glass, gives better results. The stop B B is generally one-fifth of the focal length distant from the lens, and consequently cuts off much of the light. In the earlier days of photography a person had to sit in front of such a lens, in a strong light, for several minutes. That in this way no artistically perfect pictures could be made is self-evident, and so it became necessary for portrait photographers to have a lens that would work satisfactorily with a larger opening. THE PORTRAIT OBJECTIVE. This invention is no accident, but the result of a thorough theoretical calculation. It is a double ob- jective with two unequal lenses, with or without central stops between. \ Fiff. 13. The front lens A consists of a biconvex crown, and an almost plano-concave flint-glass lens, cemented to- gether with Canada balsam. The back lens B consists SCIOPTICON MANUAL. 23 of an almost plano-concave flint, and a biconvex crown- glass lens, separated by a ring. It is seen that some marginal rays entering the lens A do not reach the lens J5, because of the length of the tube F F, which effects about the same result as the slight stop D D. The curvature of the field is somewhat exaggerated in the diagram, to remind us that an aplanatic lens cannot give a perfectly flat field without a stop. In this gene- ral form all portrait lenses mostly coincide, differing in regard to the focal length of the separate objectives A and B, the distance and size of the same, the position of the stops, &c. Considering Fig. 13 as representing a portrait ob- jective, the pencils JS S S proceed from an object com- paratively large and distant, forming a small image in the camera. Considering the diagram as representing a lantern objective, the order is reversed. A small transparency is in place of the curved line, which in its turn becomes the focus of incident rays, projecting upon a screen a comparatively large and distant image where the pencils S S S, if extended, meet in a focus of re- fraction. B is called the back lens in either case, as it is back next to the instrument to which the tube is attached. 24 SCIOPTICON MANUAL. CHAPTER III. THE CONSTRUCTION OF THE SCIOPTICON. THE PORTRAIT OBJECTIVE. This objective (Fig. 13 or 15) is made for the camera, and is known mostly in its relations to photography. An objective, however, that with large opening, will give proper direction to rays from a large object to a small image in the camera, will answer equally well in giving direction to rays from the small picture in the Sciopticon back to life-size on a screen; both object and image being in the conjugate foci in either case. THE PLAIN LANTERN OBJECTIVE. This objective, like the achromatic portrait objective (Fig. 13 or 15), has the advantage of a front and a back lens, A B (Fig. 14), so far apart that the tube serves as a stop for marginal rays with comparatively small loss of light. The front lens A is a meniscus of crown-glass, whose tube slides into a larger tube which holds the plano-convex crown- glass lens B. Arranged as in the diagram, the effect is scarcely inferior, so far as common observation goes, to that of the most expensive combinations. With the front tube reversed, so as to bring A near to B, the image is larger but less distinct. With only one lens the image is smaller. These different arrangements give the three powers commonly attributed to lenses mounted in this form. NO LOSS OF LIGHT FROM USING AN OBJECTIVE OF LOW POWER WITH A CONCENTRATED LIGHT. Were the picture p (Fig. 14) made luminous by light shining upon it, as in an opaque lantern, the light would radiate in all directions, and in accordance with the law SOIOPTICON MANUAL. of radiation would lose intensity with distance, and so a lens of short focus, being nearer, would bring more light Fig. 14. to bear on the image. But the light in fact proceeds from I, and what passes through the condenser and picture becomes a cone of light, with its apex in the objective. This cone of light must be regulated by the position or power of the condenser so as to fall within the compass of the objective. Except for some reflections from the surfaces of the glasses, the light would not illuminate the room at all in its passage, and there would not be even the small need there is of the hood t (Fig. 15). ADVANTAGES OF A CONCENTRATED EIGHT. Could the light proceed from a mathematical point behind a faultless condenser, the pencils a! V d (Fig. 14) would be without marginal rays, and there would be nothing for the objective to do, for its office is to bring the marginal rays of each pencil to coincide with the axial ray. Were we to adjust the aperture of our window shutter (Fig. 1), to the apex of the cone, there would be no rays for the shutter to cut off. The imago would be formed anywhere within reach of the light, either with or without lens or stop. But in reality even the most concentrated light occupies some space with countless radiant points, so near together however that S 26 SCIOPTICON MANUAL. their radiations cross at a very small angle. The rays which meet and cross at each point in the picture j?, become a pencil of diverging rays to be focused on the screen by the objective. We conclude, therefore, that the more concentrated the light, the nearer in each pencil will the marginal rays coincide with the axial ray, and the less will the imperfections of lenses become manifest. THE CONDENSER. The condenser is formed of combined lenses, because the refracting angles would be too great in a single lens of sufficient diameter and short focal distance. The simplest arrangement is where two plano-convex lenses are combined, with their curved surfaces inwards, as in Fig. 14 ; or at p q in Fig. 15. The shorter the focus of the condenser, the shorter, with a given objective, must be its distance from the light; it will collect more light, but it will be in more danger of breakage from the heat. In the Sciopticon the space between the lens q (Fig. 15) and the front of the flame is only about two and a half inches, but the glass 6r, and the air between it and q rising up and out at A, makes it perfectly secure. The condensing lenses in the Sciopticon are usually each 4 inches in diameter; but a 4J inch front condensing lens, p, is used to advantage for slides larger than the standard size, and to show fully the corners of some of the ordinary square transparencies. THE REFLECTOR. The reflector r (Fig. 14) is so adjusted that the light is in the centre of concavity. Each ray is reflected back to its starting-point, and continues in line with SCIOPTICON MANUAL. 27 the incident rays a b c. Some advocate a larger reflec- tor, a parabolic reflector, or reflectors at the sides; but unless the reflected light coincides with a b c, or nearly so, it is not transmitted by the objective lens. "With proper adjustments, the light J, with that from the reflector, is focused at the objective tube, of a com- parative size proportionate to its relative distance from the condenser; and the picture p is focused upon the screen at i of a comparative size proportionate to its relative distance from the objective. As represented in Fig. 14, the smallest diameter of the cone of light in the objective A B would be twice that of the point of light I, and the height of the image i would be twice that of the picture^. To project a picture to a great distance without too much enlargement, the objective must be of low power and carried forward; and the light should be from a point (as in the calcium light) to avoid loss, and should bo carefully adjusted to secure even illumination. The longitudinal ridge of light E (Fig. 15), with a medium objective gives uniformly good results without perplexing experimental adjustments. VARIOUS MODES OF LANTERN ILLUMINATION. The Hydro-Oxy-Calcium light, or lime made incan- descent by a jet of hydrogen and oxygen in flame upon it, is the most brilliant available light. Its concentrated form adds greatly to its value for the lantern. The Oxy- Calcium light produced by a jet of oxygen on to lime, through an alcohol flame, is much inferior to what is produced by the mixed gases. It is much used, because ono gas is easier to manage than two, and it is counted 28 SCIOPTIOON MANUAL. Bafer. A lantern with a calcium light of either kind is commonly called a Stereopticon. The Magnesium light is excellent in photography, but it gives off troublesome fumes, and for a continued lantern exhibition is too liable, even with well-regulated clockwork, to leave the lookers-on in sudden darkness. The Electric light is intense and concentrated, but it requires too much apparatus to be available. The above so-called chemical lights, are, if well man- aged, much brighter than flame, even at its best. The Sciopticon has a double flame, which is not only very bright, but gives much more distinctness to the image, by its standing edgewise to the condenser instead of broadside, as a single flame must, to prevent its casting a dark shadow on the disk. No lantern of any sort can compare with the Sciopticon in point of convenience. We may conclude that the Hydro-Oxy-Calcium light is best for exhibitions on a large scale, and as for the rest, the Sciopticon is desirable as combining efficiency and great convenience. THE SIZE PROPER FOR THE ILLUMINATED DISK. The image enlarges in area, and diminishes propor- tionately in brightness, as the distance of the lantern from the screen increases. A disk of six or seven feet is about right for figures, statuary,