Stereoscope

“ In its most general form the Stereoscope is a binocular instrument, that is, is applied to both eyes; but in two of its forms it is monocular, or applied only to one eye, though the use of the other eye, without any instrumental aid, is necessary in the combination of the two plane pictures, or of one plane picture and its reflected image. The Stereoscope, therefore, cannot, like the telescope and microscope, be used by persons who have lost the use of one eye, and its remarkable effects cannot be properly appreciated by those whose eyes are not equally good. ”

“ In these observations, it is understood that the observer obtains distinct vision of the pictures in the stereoscope, either by the adjustment of the moveable eye-tubes, if they are moveable, as they ought to be, or by the aid of convex or concave glasses for both eyes, either in the form of spectacles, or separate lenses placed immediately above, or immediately below the semi-lenses in the eye-tubes. If the eyes have different focal lengths, which is not unfrequently the case, lenses differing in convexity or concavity should be employed to equalize them. ”

“ When the stereoscope is perfectly adapted to the eyes of the observer, and the general union of the pictures effected, the remote parts of the picture, that is, the objects seen in the distance, may be under refracted, while those in the foreground are over refracted, so that while eyes which have the power of convergence beyond the picture, unite the more distant objects which are under refracted, they experience much difficulty in uniting those in the foreground which are over refracted. ”

“ We have already seen, in the history of the stereoscope, that in the binocular vision of objects, each eye sees a different picture of the same object. In order to prove this, we require only to look attentively at our own hand held up before us, and observe how some parts of it disappear upon closing each eye. This experiment proves, at the same time, in opposition to the opinion of Baptista Porta, Tacquet, and others, that we always see two pictures of the same object combined in one. ”

“ Every one who has tried the experiment with an ordinary stereoscope, and a large, miscellaneous collection of stereographs, knows how wearying it is, and how in some cases distinct vision is found impossible. To indicate the real differences between normal vision and that which is attained in most stereoscopes, it will be necessary first to study the development of this instrument. ”

“ The instrument first devised by Professor Wheatstone, was what is termed a reflecting stereoscope, and was expensive to make and cumbrous to use. It was modified by Sir David Brewster, by the substitution of prisms for reflectors, and was thus made cheaper and more portable. The refracting form of stereoscope is so familiar to most people, that it really needs no description. It will only be necessary to mention that the prisms used in the eye-pieces are made by cutting a double convex lens in two, and reversing the halves. ”

“ To the engineer and the mechanist, and the makers of instruments of all kinds, the stereoscope will be of inestimable value. The difficulty of representing machinery is so great that it is not easy to understand its construction or its mode of operation from plans and perspective views of it. The union of one or two binocular pictures of it, when thrown into relief, will, in many cases, remove the difficulty both of drawing and understanding it. ”

“ Among the binocular diagrams, consisting of white lines upon a black ground, which have been executed in Paris, there is one representing the apparatus in which a ray of light, polarized by reflexion from a glass plate, passes through a crystallized film perpendicular to the plane of the paper, and is subsequently analysed by reflexion from another plate at right angle to the following plate. This diagram, when placed in relief by the stereoscope, gives as correct an idea of the process as the apparatus itself. ”

“ Every picture, whether taken photographically or by the eye, is necessarily a picture seen by one eye, or from one point of sight; and, therefore, a skilful artist, who fully understands the principle of the stereoscope, might make a copy of any picture as seen by the other eye, so approximately correct as to appear in relief when united with the original in the stereoscope; but the task would be a very difficult one, and if well executed, so as to give a relievo without distortion, the fortune of the painter would be made. ”

“ With this figure the reader may attempt the experiment of seeing the stereoscopic effect without the stereoscope. When he has succeeded in doing this, or when he fuses the images together by placing a simple stereoscope over the page, he will find the result very singular; for he will receive the impression of a solid crystal of some dark polished substance—black lead, for instance—placed on a surface of the same material. The edges of the solid will appear to have a certain lustre, such as one sees on the edges of a real crystal. ”

“ Then there is that wonder of refreshment, the stereoscope. One comes back from a half hour there in a Swiss valley as into a new world, with the dust all blown away. A stereoscope costs little, and views are not expensive,—that is if you are content with one or two at a time, which is the real way to buy them; choosing, considering, carefully selecting only those you cannot possibly go home without! ”

“ The Stereoscope, a word derived from στέρεος, solid, and σκόπειν, to see, is an optical instrument, of modern invention, for representing, in apparent relief and solidity, all natural objects and all groups or combinations of objects, by uniting into one image two plane representations of these objects or groups as seen by each eye separately. ”

“ The microscopic stereoscope will, in like manner, give accurate magnified representations in relief of grains and seeds of all kinds, and by comparing these with the representations of those of a standard form and quality, the purchaser may be enabled to form a better idea of their excellence than if he saw them with his own eyes, or had them in his own hands. ”

“ Such are the stereoscope and the photograph, by the aid of which form is henceforth to make itself seen through the world of intelligence, as thought has long made itself heard by means of the art of printing. The morphotype, or form-print, must hereafter take its place by the side of the logotype, or word-print. The stereograph, as we have called the double picture designed for the stereoscope, is to be the card of introduction to make all mankind acquaintances. ”

“ As the eyes themselves form a stereoscope to those who have the power of quickly converging their axes to points nearer than the object which they contemplate, it might have been expected that the first attempt to make a stereoscope for those who do not possess such a power, would have been to supply them with auxiliary eyeballs capable of combining binocular pictures of different sizes at different distances from the eye. ”

“ As an amusing and useful instrument the stereoscope derives much of its [Pg 4] value from photography. The most skilful artist would have been incapable of delineating two equal representations of a figure or a landscape as seen by two eyes, or as viewed from two different points of sight; but the binocular camera, when rightly constructed, enables us to produce and to multiply photographically the pictures which we require, with all the perfection of that interesting art. ”

“ In selecting a stereoscope, first look that the glasses be large and heavy, and of perfectly clear glass, and see that they are wide enough
for the eyes to come opposite the middle of them, producing no feeling of being shut in by dark objects at the side. Many instrument-makers seem unaware that heads differ in width.
Then, placing a picture in the rack, see that the diaphragm hides no part of it when the rack is drawn up to a short focus. Near-sighted people have rights as well as others. ”

“ This influence over our judgment is beautifully shewn in viewing, with one eye, photographs either of persons, or landscapes, or solid objects. After a little practice, the illusion is very perfect, and is aided by the correct geometrical perspective and chiaroscuro of the Daguerreotype or Talbotype. To this effect we may give the name of Monocular Relief, which, as we shall see, is necessarily inferior to Binocular Relief, when produced by the stereoscope. ”

“ The invention of the stereoscope by Sir Charles Wheatstone, F.R.S., 1838, and improved by Brewster, was characterised by Sir John Herschel as “one of the most curious discoveries, and 61 beautiful for its simplicity, in the entire range of experimental optics,” led to a more general appreciation of the value of the conjoint use of both eyes in conveying to the mind impressions of the relative form and position of an object, such as the use of either eye singly does not convey with anything like the same precision. ”

“ There are few who think of him as an inventor, when they use the convenient and compact stereoscopes that have been multiplied in tens of thousands, until now no home is too humble, no father too poor, to delight his little ones with phantom scenes of beauty, brought by the sunbeam and the stereoscope from places that their eyes will never behold. ”

“ Although a perfect stranger to you, and living on the reverse side of the globe, I have taken the liberty of writing to you on a small discovery I have made in binocular vision in the stereoscope. I find by taking two ordinary carte-de-visite photos of two different persons' faces, the portraits being about the same sizes, and looking about the same direction, and placing them in a stereo-scope, the faces blend into one in a most remarkable manner, producing in the case of some ladies' portraits in every instance a decided improvement in beauty. ”

“ ALL of the forms under which the stereoscope has come into general use have been devised with a view to creating to the utmost the illusion of natural binocular perspective by reproducing as nearly as possible the conditions of natural vision. That this end is not successfully attained is painfully felt by those who linger too long over an attractive collection of stereographs. To secure comfortable vision the muscles of the eyes must suffer no unusual strain. ”

“ A friend complained to me that the stereoscope she had bought, on my recommendation, was worthless. I had taken it on faith, supposing that all "Holmes" instruments must be good; but a short examination showed that the glasses were rather flat, and were placed so near together that the rays came to the eyes through the central part of the lens, and of course without refraction, and hence the eyes were not deceived. ”

“ In order to construct the instrument independently of the stereoscope, we have only to combine two equal Kaleidoscopes, with their reflecting mirrors equally inclined to each other, and place at the eye-end of them two semi-lenses or quarter lenses, at the distance of two and a half inches, and having their focal lengths equal to the length of the stereoscope. ”

“ The proper illumination of the picture, when seen by reflected light, is also a point of essential importance. The method universally adopted in the lenticular stereoscope is not good, and is not the one which I found to be the best, and which I employed in the first-constructed instruments. The light which falls upon the picture is prevented from reaching the observer only by its being incident at an angle greater or less than the angle of reflexion which would carry it to his eyes. ”

“ We may become more strongly conscious of this difference between an object of our knowledge and an object of our impression, if we remember a well-known instrument, the stereoscope. The stereoscope, which was quite familiar to the parlor of a former generation, consists of two prisms through which the two eyes look toward two photographic views of a landscape. ”

“ They are in themselves less portable, and require a larger stereoscope; and we believe, no person whatever, who is acquainted with the perfection and beauty of the binocular slides in universal use, would either incur the expense, or take the trouble of using pictures of a larger size. ”

“ If the lenses of the binocular camera, when they are whole lenses, be made to separate a little, so that the distance between the centres of their inner halves may be equal to 21⁄2 inches, they become a lenticular stereoscope, in which we may view the pictures which they themselves create. The binocular pictures are placed in the camera in the very place where their negatives were formed, and the observer, looking through the halves of his camera lenses, will see the pictures united and in relief. ”

“ Berkeley’s theory of the size of the horizontal moon,—a theory to which the stereoscope enables us to apply another test, for if we make one or more of these circles less bright than the rest, no change whatever will be produced in their apparent magnitude.
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CHAPTER XIV.
APPLICATION OF THE STEREOSCOPE
TO PURPOSES OF AMUSEMENT.
Every experiment in science, and every instrument depending on scientific principles, when employed for the purpose of amusement, must necessarily be instructive. “Philosophy in sport” never fails to become “Science in earnest.” ”

“ New geometrical drawings were executed for them, and binocular pictures taken by the sun were lithographed by Mr. Schenck of Edinburgh. Stereoscopes of the lenticular form were made by Mr. Loudon, optician, in Dundee, and sent to several of the nobility in London, and in other places, and an account of these stereoscopes, and of a binocular camera for taking portraits, and copying statues, was communicated to the Royal Scottish Society of Arts, and published in their Transactions. ”