154 ANNUAL OF SCIENTIFIC DISCOVERY. 



that phenomenon; and his experiments and researches have disclosed the 

 singular and unexpected fact, that although only one image seems depicted 

 on the ground glass, still each eye perceives a different image; that in 

 reality there exist on the ground glass two images, the one visible only to 

 the right eye, and the other visible only to the left eye, that the image 

 seen by the right eye is the representation of the object refracted by the 

 left side of the lens, and the image seen by the left eye is the representa- 

 tion of the object refracted by the right side of the lens. Consequently, 

 these two images presenting two different perspectives, the 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 cor- 

 responds with the optic axes ; 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 arc not corresponding with the optic axes, and have only the per- 

 ception 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 e^ve 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 produced 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 disap- 

 pears 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 PiJil. May. for June, ISol). 



If, without altering the focus, we examine the same image with the pseudo- 

 scope, the effect is pseudoscopic. But if the focus has been set on the most 

 distant plane of the focimetcr, the effect is pseudoscopic, 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 extrem- 

 ities of the horizontal diameter of the lens. This mode of conducting the 

 expcrimenis presents the most decided manifestation of the whole phenom- 

 enon. 



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

 paper instead of ground glass, it does not hi any case present the least illu- 

 sion 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 



