571 



lens. This mode of conducting the experiments presents the most 

 decided manifestation of the whole phenomenon. 



But it must be remarked, that if the image is received on a trans- 

 parent paper instead of ground glass, it does not in any case present 

 the least illusion of relief. The surface of the paper has the property 

 of preserving to both eyes the same intensity of image from whatever 

 direction the rays are refracted on that surface, and at whatever angle 

 the eyes recede from the centre to examine the image. In fact, all 

 the various images refracted through every part of the lens and coin- 

 ciding on the surface of the paper, are visible at whatever angle they 

 are examined. 



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 prisms disposed at all kinds of angles, 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- 

 visible 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 

 object remains fixed on the surface. Each molecule 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 once 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 it 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 



VOL. VIII. 2 T 



