490 PROCEEDINGS OF THE SOCIETY. 



puzzled by these different pictures, presented to the mind at the same 

 time ? Wheatstone applied himself to the study of this question ; and 

 in the course of his investigations it occurred to him that the dissimi- 

 larity of the two pictures was really the cause of the sense of pro- 

 jection ; and that though we have this sense with a single eye, it is in 

 such case by no means so unmistakable. He therefore reasoned in 

 this way ; if you draw two pictures of an object, one as it appears to 

 the right eye, and the other as it appears to the left, and then throw 

 the images of these dissimilar pictures upon the two eyes respectively, 

 you will get a solid effect. The original form of the Stereoscope was 

 a reflecting apparatus, consisting of two mirrors placed together at a 

 right angle, so that each reflected the image of its own picture direct 

 to its own eye ; and with this instrument Wheatstone found that two 

 mere outlines of a solid, drawn as already described, and reflected so 

 that each was seen only by the eye for which it was drawn, resulted 

 in the production of a perfect perception of the solid form. No one 

 welcomed this discovery more than Sir David Brewster; who said 

 that it was the greatest that had been made in vision since the time of 

 Newton. This combination of two dissimilar pictures is the funda- 

 mental principle of the Stereoscope, In the form of that instrument 

 now familiar to you all, a pair of small photographic pictures, taken 

 in different perspectives, are brought one before the right eye, the 

 other before the left, by two halves of a double-convex lens placed 

 back to back, so as to act both as prisms and as magnifiers. The 

 points of view from which the two pictures are taken, are generally, 

 I believe, about 15° apart ; that being the usual angle of convergence 

 of the axes of the eyes at the ordinary reading distance. The late 

 Mr. Claudet, who paid a great deal of attention to this subject in 

 relation to portraiture, tried various angles ; and having taken pictures 

 at 5°, at 10°, at 12°, at 15°, and at 20°, he found that 5° gave very 

 little projection, 10° was more satisfactory, but 12° was much better ; 

 and for people with nearly approximated eyes it was found to be 

 sufficient ; but for most people, 15° was required to bring out the full 

 stereoscopic effect, whilst if he widened the angle to 20° all the pro- 

 jecting parts came out with ludicrous exaggeration. (I have an early 

 stereoscopic photograph of an equestrian statue of Napoleon, showing 

 this exaggeration in a very marked degree, the two pictures having 

 been taken at too wide an angle.) 



As an illustration, take a truncated pyramid which is placed end-on 

 before one eye, as is shown in fig. 81 a ; with that eye alone jou would 

 be unable to measure the relative distances of its parts, and the 

 borders of its base and truncated top would appear like two squares 

 symmetrically placed one within the other. But if placed in front 

 of the nose, the right eye would see more of the right side of 

 the pyramid (as in the fig. 81 c), whilst the left eye will at the same 

 time see more of the left side of it (as in fig. 81 h) ; and if these two 

 pictures are put into the Stereoscope, and each is seen at the same time 

 — the one by the right eye, and the other by the left — the apparent 

 solidity of the figure is brought out perfectly; that is, these two dis- 

 similar pictures, viewed simultaneously, suggest to the mind a con- 



