Mr. A. Claudet on some Stereoscopic Phenomena. 399 



The reason of this difference between the effect of the ground glass 

 and that of the paper is, that through the surface of the ground glass, 

 composed of innumerable molecules of the greatest transparency, 

 only deprived of their original parallelism by the operation of grinding, 

 but acting as lenses or prisma disposed at all kinds of augles, the rays 

 refracted by the various parts of the lens continue their course in 

 straight lines in passing through these transparent molecules, and 

 are visible only when they coincide with the optic axes, being in- 

 \isible in all other directions ; that, in short, they are not stopped by 

 the surface of the ground glass ; while the paper being perfectly 

 opaque, stops all the rays on their passage, by which the image of the 

 oljject remains fixed on the surface. Each molecvde of the paper 

 becoming luminous, sends new rays in all directions ; and from what- 

 ever direction we look on the paper, we always perceive at pnce all the 

 images superposed, so that each eye seeing the two perspectives 

 mingled, the process of convergence according to the horizontal di- 

 stances of the same points of the various planes, cannot have its play, 

 and no stereoscopic effect can take place, as is the case with the 

 ground glass, which presents to each eye an image of a different per- 

 spective. 



The author explains that he has ascertained these facts by several 

 experiments, the most decisive of which consists in placing before 

 one of the marginal openings of the lens a blue glass, and a yellow 

 glass before the other. The object of these coloured glasses is to 

 give on the ground glass two images, each of the colour of the glass 

 through which it is refracted. 



The result is two images, superposed on the ground glass, one 

 yellow and the other blue, forming only one image of a grey tuit, 

 being the mixture of yellow and blue, when we look with the two 

 eyes at an equal distance from the centre. But when shutting 

 alternately, now the right eye and then the left eye, in the first case 

 the image appears yellow, and in the second it appears blue. 



If while looking with the two eyes (the opening on the right of 

 the lens being covered with the yellow glass, and the opening on the 

 left with the blue glass) we move the head on the right of about 6°, 

 the mixture of the two colours disappears, and the image retains 

 only the blue colour ; on the other hand, if after having resumed the 

 middle position, which shows again the mixture of the two colours, 

 we move the head on the left of 6°, the mixture disappears again, 

 and the image retains only the yellow colour. 



This proves evidently that each eye sees only the rays which, when 

 after having been refracted by any part of the lens, and continuing 

 their course in a direct line through the ground glass, coincide with 

 the optic axes, while all the other rays are invisible. 



The consideration of these singular facts has led the author to 

 think that it would be possible to construct a new stereoscope, in 

 which the two eyes looking at a single image could see it in perfect 

 relief, such a single image being composed of two images, of different 

 pers|)ectives superposed, one visible only to the right eye and tlie 

 other to the left. This would be easily done by refracting a stereo- 

 scopic slide on a ground glass, through two semi-lenses separated 



