of Stereoscopic Phenomenon. 333 



4. With reference to the experiment itself, it should be men- 

 tioned that the stereoscopic curve displays itself with particular 

 distinctness in comparison with the single images when the 

 plane of rotation is parallel to the incident rays and consequently 

 the axis of rotation is perpendicular, and when the apparatus is 

 regarded from behind and on one side. The single curve is, 

 however, still more clearly discernible when there are two differ- 

 ent sources of light ; these give rise to distinct curves, from the 

 comparison of which the stereoscopic effect is increased. 



It hardly needs to be observed that, instead of the parallel 

 rays of the sun, any other source of light with diverging rays 

 may be employed, and that the above calculations, when the 

 source of light is not too near, are still sufficiently accurate. 

 Moreover the equation of the curve described by the point of 

 reflexion when the light proceeds from a point L at the distance 

 r x from 0, and OL makes with the axes of x, y, and % the 

 angles a lt /3 P y lf may easily be shown to be as follows : 



r-LsinLOB, _ r sin APR 



v^ + p^+Ir^cosLOR" VrH^+2r^osWE J 



where for LOR and AOR the values found above must be sub- 

 stituted. 



5. Though it is stated above that the two images generally 

 unite into one, it should be observed that this property depends 

 on the extent to which it is possible to bring them into super- 

 position. Images which in size or form are too different cannot 

 be stereoscopically united. As, according to the formula, p is 

 directly proportional to r (the distance of the eye from the origin), 

 stereoscopic images will be more easily perceived in proportion 

 as the distances of the two eyes from the origin are equal; but 

 even w r hen these distances are the same, the angles may be so 

 different that the images cannot, or can only with great diffi- 

 culty, be superimposed. 



6. The conclusion that may be drawn with certainty from the 

 experiment is as follows. Since two points of the separate curves 

 which unite to form one point of the steroscopic image are not 

 seen at the same moment with both eyes, it is impossible for the 

 eyes to be so accommodated that corresponding points throw 

 their images on corresponding points of the retinse of the two 

 eyes. That the eyes arrange themselves by anticipation is not 

 credible ; and even if they did, for every revolution of the cylinder 

 only one point would appear single, the rest double. 



It is moreover impossible that the two curves that exist in the 

 two eyes can be compared as wholes, and received point by point 

 on corresponding retina-points ; for since the images are only 

 virtual, that is, only produced by the endurance of the impres- 



