﻿of Binocular Vision. 189 



individual by constant practice from the earliest childhood, that 

 a single act of volition accomplishes them all. Under ordinary 

 circumstances they are so indissolubly associated that neither 

 can be accomplished without the others. But the experiments 

 described above prove that under certain circumstances the first 

 two at least may be completely dissociated. In these experiments, 

 when the image is first obtained, the optic axes, the lenses, and 

 the pupil are all consensually adjusted for vision at the distance 

 of the image ; and hence the image must be indistinct, for the 

 rays diverge from an entirely different distance. But gradually 

 the lenses adjust themselves to the actual divergence, i. e. for 

 rays diverging from the real object, while the optic axes remain 

 adjusted for the distance of the image. The difficulty experi- 

 enced in dissociating these two adjustments causes the interval 

 of indistinctness. The perception of the difference between the 

 image and a real object is the sense of this dissociation. Con- 

 sensual movements have been perhaps brought about by the ne- 

 cessities of single and distinct vision; Helmholtz has shown* 

 that other consensual movements may be dissociated when the 

 necessities of single vision require it ; these experiments show 

 that the consensual adjustments of the eye may be dissociated 

 when the necessities of distinct vision require it. 



I was now anxious to determine what part was taken by the 

 pupil. Is the contraction of the pupil more intimately associated 

 with the axial or the focal adjustment ? This question has been 

 discussed by E. H. Weber, Cramer, and Donders f. Weber be- 

 lieves it is directly associated with the axial adjustment, Cramer 

 and Donders with the focal adjustment. To test this question, 

 while I was obtaining the image and making it clear, an assist- 

 ant standing behind and a little to one side observed my pupil 

 reflected in a small mirror conveniently placed. After gazing 

 intently at the real object until the pupil was steady, as soon as 

 I converged the optic axes so as to obtain the image No. 1, the 

 pupil was observed to contract decidedly, but as the image became 

 clear it again expanded to its original size. Again, at the moment 

 of obtaining the second image the pupil contracted still more 

 strongly, but as soon as the image became clear it again ex- 

 panded nearly, if not entirely, to its original size. The same 

 phenomena were observed for each of the images, only that in 

 the nearest images, when the convergence of the optic axes was 

 extreme and the first contraction very great, the pupil did not 

 return entirely to its original dimensions. 



I then made similar experiments on the image beyond the 



* Proc. Roy. Soe. April 1864. 



t Donders, " Accommodation and Refraction of the Eye," Transactions, 

 p. 574. 



