NATURAL PHILOSOPHY. 155 



centre to examine the image. In fact, all the various images refracted 

 through every part of the lens and coinciding on the surface of the paper, arc 

 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 invisible 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 lumi- 

 nous, sends new rays in all directions ; and from whatever 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 distances 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 ray an image of a 

 different perspective. 



The author explains that he has ascertained these facts by several experi- 

 ments, the most decisive of which consists in placing before one of the mar- 

 ginal openings of the lens a blue glass, and a yellow glass before the other. 

 The object of these colored glasses is to give on the ground glass two images, 

 each of the color 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 gray tint, 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 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 colors disappears, and the image retains only the blue color; on the 

 other hand, if after having resumed the middle position, which show again 

 the mixture of the two colors, we move the head on the left of 6, the mix- 

 ture disappears again, and the image retains only the yellow color. 



This proves evidently that each eye sees only the rays Avhich, 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 perspectives superposed, one 

 visible only to the right eye and the other to the left. This would be easily 

 done by refracting a stereoscopic slide on a ground glass, through two 

 semi-lenses separated enough to make the right picture of the slide coincide 

 with the left picture at the focus of the semi-lenses. The whole arrangement 

 may be easily understood; we have only to suppose that we look through a 



