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

 perspectives superposed, one visible only to the right eye and the 

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

 scopic 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 ground glass placed before an ordinary stereoscope at the 

 distance of the focus of its semi-lenses, the slide being strongly 

 lighted, and the eye seeing no other light than that of the picture on 

 the ground glass. The whole being nothing more than a camera 

 having had its lens cut in two parts, and the two halves sufficiently 

 separated to produce at the focus the coincidence of the two opposite 

 sides of the stereoscopic slide placed before the camera. 



