570 



result is a stereoscopic perception, as when we look through the 

 stereoscope at two images of different perspectives. 



It appears that all the different images refracted separately by 

 every part of the lens, are each only visible on the line of their 

 refraction when it corresponds with the optic axis, so that while we 

 examine the image on the ground glass, if we move the head we lose 

 the perception of all the rays which are not corresponding with the 

 optic axes, and have only the perception of those which, according to 

 the position of the eyes, gradually happen to coincide with the optic 

 axes. Consequently when we look on the ground glass perfectly in 

 the middle, the two eyes being equally distant from the centre, the 

 right eye sees only the rays refracted from the left of the lens, and 

 the left eye only those refracted from the right of the lens. 



If we move the head horizontally, as soon as we have deviated 

 about 6 from the centre on the right or on the left, in the first 

 position the right eye sees no image, and the left eye sees the image 

 which before was seen by the right eye ; in the second position the 

 inverse takes place, and of course in both cases there cannot exist 

 any stereoscopic illusion. 



When we examine on the ground glass the image of a solid pro-' 

 duced by the whole aperture of the lens, if we have taken the focus 

 on the nearest point of the solid, we remark, in looking with the two 

 eyes, that the image is stereoscopic, and as soon as we shut one eye 

 the illusion of relief disappears instantly. 



The stereoscopic effect is beautifully brought out by the image of 

 a group of trees ; and when<experimenting in an operating room, it is 

 rendered quite conspicuous if we take the image of an object having 

 several planes very distinct, such as the focimeter, which the author 

 has described in a former' memoir (see Phil. Mag. for June 18ol). 



If without altering the focus we examine the same image with the 

 pseudoscope, the effect is pseudoscopic. But if the focus has been 

 set on the most distant plane of the focimeter, the effect is pseudo- 

 scopic, and it becomes stereoscopic in looking with the pseudoscope. 



The image loses its relief when it is produced only by the centre of 

 the lens. The stereoscopic and pseudoscopic effects are therefore 

 as much less apparent as the aperture of the lens has been more 

 reduced, and they are the more evident if the image is produced by 

 two apertures on both extremities of the horizontal diameter of the 



