494 PROCEEDINGS OF THE SOCIETY. 



as it then appeared ; he then covered up the other half, and made 

 another drawing. You can see for yourselves that these two figures 

 are two dissimilar pictures ; and I have found that they pair perfectly 

 well in the Stereoscope, bringing out the object in relief. I have at 

 home two photographs taken in the same way, showing by their 

 combination precisely the same result. 



Another very curious piece of evidence, furnished by the dissimi- 

 larity of the pictures given by the two lateral halves of a portrait- 

 lens, will strengthen my case. In 1857 Mr. Claudet brought 

 before the Koyal Society this very interesting fact: — "I have 

 noticed that when I hold my head in a certain position behind 

 the focusing ground glass, I see the sitter, not as a flat picture, 

 but as an image in relief. But this image is only to be seen 

 when my head is in a certain place ; if I move it to either side, 

 or either forwards or backwards, I lose the effect." He found 

 the explanation of it to be, that in that particular position the 

 picture taken by the left half of the lens came to his right eye, while 

 the picture formed by the right half came to his left eye ; whilst, if 

 he moved so that he broke the lines of these two images, he lost the 

 effect ; he further found that if he covered up either half of his lens, 

 the solid image gave place to a flat picture, proving the combination 

 of the two to be required to give the impression of solidity. He 

 found that the effect of relief was most decided, when rays forming 

 the picture were only allowed to pass through an aperture at each 

 end of the horizontal diameter of the lens, all the rest being stopped 

 out ; while the appearance of solidity was lost when only the central 

 portion of the lens was employed. Further, he found that the illusion 

 of relief is not produced when the image was received on translucent 

 paper instead of on ground glass ; the reason of this difference being 

 that, as all the molecules of the ground glass are in themselves 

 transparent, though their surfaces are turned into lenses or prisms 

 by grinding, some of the rays pass through it to the eyes ; whilst, 

 when the image is thrown upon paper, the rays are stopped by the 

 opacity of its fibres, each molecule of which, becoming self-luminous, 

 sends out its rays in all directions, so that one and the same 

 picture of the object is seen by both eyes. Mr. Claudet obtained 

 further proof of the correctness of his interpretation by placing a 

 blue glass before one of the marginal openings of his portrait lens 

 and a yellow glass before the other. The image seen when the eyes 

 were in a position to receive and combine the two pictures was of a 

 grey tint. But if one eye was closed, the image became blue ; and if 

 the other was closed, it became yellow, — the same effect being pro- 

 duced by moving the head to one side or the other.* Although 

 Mr. Claudet's view of this matter was denounced by Sir D. Brewster 

 as completely at variance with the laws of optics, yet he subsequently 

 succeeded in establishing it beyond question by the construction of 

 his Stereo-monoscope ; f in which the like effect was given by throwing 

 on the same part of a ground glass, by two separate lenses, two 



* Proc. Royal Soc, viii. (1856-7) p. 569. 

 t Ibid., ix. (1857-8) p. 194. 



