﻿140 On some Phenomena of Binocular Vision. 



real object. As before, I looked intently first on the real object 

 at the distance of twelve inches until the pupil became steady. 

 So soon as I gazed beyond the object the pupil of course ex- 

 panded ; but as soon as the image became clear, it again con- 

 tracted to nearly its original size. In this last experiment the 

 pupil is apt to be unsteady. This might have been expected ; 

 for, as we have already said, it is much more difficult to obtain 

 this image clear, or to retain it when obtained. 



There is no doubt of the fact, therefore, that the contraction 

 of the pupil is most intimately associated with the focal ad- 

 justment. 



I believe that this principle of dissociation of consensual 

 adjustments explains perfectly certain phenomena of the stereo- 

 scope. It is well known that many persons experience diffi- 

 culty in seeing stereoscopic pictures distinctly even when the 

 two pictures are brought into perfect coincidence; and I be- 

 lieve all persons experience some fatigue to the eyes in look- 

 ing at stereoscopic pictures for a considerable length of time. 

 I have often felt both the difficulty and the fatigue, though 

 to a much less degree than most persons. The explanation 

 of this difficulty is as follows. We judge of distance, as is 

 well known, by the axial adjustment. If, then, the two pic- 

 tures are so taken that, in order to bring them together, the 

 visual lines must meet at a certain distance, say, fifty yards, 

 then the picture will be seen at that distance, and of course very 

 much enlarged. But in order to see the picture clearly, the 

 rays must come to the eye as if they diverged from the same 

 distance ; for the eyes are adjusted for that distance. To fulfil 

 this condition lenses are always used ; but it is obvious that a 

 given pair of lenses are suitable for one distance only. For all 

 other distances or degrees of optic convergence there must be 

 some degree of dissociation of the two adjustments ; and this is 

 both difficult and fatiguing to most persons. 



I have found that observations upon the images of the ruled 

 diagram are a most delicate means of determining both the rota- 

 tions of the eye and the position of the horopter. I hope in my 

 next communication to take up this most difficult subject. 



[To be continued.] 



