THE MODERN EVOLUTION OF THE STEREOSCOPE. 



By A. LOCKETT. 



The valuable aid offered by the stereoscope to the 

 scientific investigator is again attracting notice. 

 In astronomy, photo-surveying, the surgical applica- 

 tions of radiography, photo - micrography, the 

 treatment of certain diseases of the eyes, and in 

 many other new and often unexpected ways, 

 stereoscopy places a fresh power in the hands 

 both of the inquirer and the expert. Even the 

 cinematograph is now made to give moving 

 pictures in natural relief — a development opening 

 out a wide vista of possibilities. 



The stereoscope itself has been greatly improved 

 of late years, and a better understanding of the 

 principles on which it depends has led to the 

 production of new models embodying important 

 modifications. This being so, the time seems not 

 inopportune for a brief sketch of the modern 

 evolution of stereoscopic apparatus and some 

 consideration of those factors that have to be 

 studied in order to secure the maximum satisfaction 

 of optical and physiological requirements. Clearly 

 what is needed is the best practical performance, 

 together with the greatest possible convenience and 

 the least complication. These different qualities 

 may not be obtainable together, but that will 

 evidently be the most perfect instrument which 

 enables the truest balance to be struck between 

 them. 



Like telescopes, most stereoscopes may be divided 

 into two classes, those, namely, that utilise reflection 

 and those depending upon refraction. A third class 

 which relies upon neither, but includes several 

 instruments or systems of unique character, may, 

 perhaps, most conveniently be considered first. 



The earliest stereoscope, Elliott's — designed in 

 1834, though not constructed till 1839 — dispensed 

 with all optical intervention, the two pictures, or 

 rather diagrams — for photography was then non- 

 existent — being viewed direct by the observer. 

 The instrument thus possessed two excellent 

 features ; but, unfortunately, it was necessary to 

 cross the axes of the eyes in a somewhat 

 unnatural manner, which called, in many cases, 

 for a little troublesome practice before it could be 

 accomplished. On that account Elliott's stereo- 

 scope never came into general use. 



A modified form of the Elliott instrument, devised 

 by the writer in 1912, may be of interest. As shown 

 by Figure 346, it consists of a tapering box, A, having 

 two oval openings at the front for the observer's 

 eyes and a rectangular opening at the back. In 

 the diagram the top is removed for clearness of 

 illustration. The box is blackened inside, and to 

 the bottom is attached a narrow piece, B, on which 

 slides a holder similar to those used on the ordinary 



American stereoscopes. The two photographs must 

 not, as is usual, be transposed in mounting, but 

 should be allowed to remain just as they are printed 

 from the negative, the picture for the right eye being 

 on the left and that for the left eye on the right. 

 To use the apparatus it is held by the handle, D, and 

 the stereograph is moved to and fro, keeping one eye 

 shut, until one picture appears to fill up the aperture 

 in the back. On opening the other eye the com- 

 bined images will then usually be seen in stereo- 

 scopic relief, or, if not, a few further trials will ensure 

 this result. The principle involved is obvious. 

 Owing to the fact that nothing can be seen except 

 through the back opening, placed midway between 

 the two pictures, the right eye can only perceive the 

 left-hand picture, while the left eye beholds only the 

 right-hand one. At the same time they are both 

 mentally referred to an apparently identical position, 

 so that they coalesce and give the effect of natural 

 relief. 



For a reason to be explained later, another unique 

 method, the parallax system of F. E. Ives, though of 

 much later date (1903), may not inappropriately be 

 dealt with next. By this highly ingenious method 

 each stereograph carries its own viewing arrange- 

 ment, so that no separate instrument is required. 

 In making the negative, the two slightly different 

 images are superposed on each other. This may be 

 done in various ways, but that commonly adopted is 

 to use on the camera a single plano-convex lens of 

 about three inches diameter, placing behind this a 

 diaphragm having two small openings two and a 

 half inches apart. A screen ruled with fine vertical 

 lines, about one hundred to the inch, is interposed 

 between the lens and the plate at such a distance 

 from the latter that it splits the two images into 

 a series of adjacent lines, every alternate line con- 

 sisting of a portion of one image only, while the 

 intermediate lines represent parts of the other 

 image. From the resulting negative, a transparency 

 is made and is bound up with a line screen of the 

 same spacing as that with which the photograph was 

 taken, kept at a similar distance by a cardboard 

 mask. On holding the bound-up transparency at 

 the correct distance from the eyes, usually about 

 twelve inches, perfect and beautiful stereoscopic 

 relief is perceived. Figure 348 illustrates, on a 

 much enlarged scale, the precise action of the ruled 

 viewing screen, A being the transparency, B the 

 screen, and C and D the eyes of the observer. It is 

 obvious that either eye sees only the intermediate 

 portions forming its own proper picture, while the 

 two pictures are at the same time superposed and 

 united. For the best result, the opaque lines of the 

 taking screen should be twice the width of the 

 transparent interspaces, as this prevents the parallax 



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