Physiological Perspective. 319 



yellow spot, and to diminish toward the margins. Let a pair 

 of circles, A and A f (fig. 5), be drawn, a few inches apart, and 

 below A a third circle, A", equal to each of the others. Bring- 

 ing the face as near to the page as is consistent with distinct 

 vision, let the right eye be directed to A' and the left to A. 

 A diminished elliptic binocular image is seen directly in front, 

 apparently in mid air ; on each side of which is a monocular 

 image, sensibly circular, or but slightly elliptic, distinctly 

 larger than the binocular image, but smaller than either circle 

 seen by normal vision. Even though distinct vision is attained 

 under these conditions, the ciliary muscles are strongly con- 

 tracted, consensually with the internal rectus muscles. On 

 closing the right eye, relaxation follows, while the direction of 

 the visual line for the eye remaining open is easily kept un- 

 changed. The right monocular image disappears; the elliptic 

 image apparently recedes and moves to the right, growing 

 larger in doing so; but an interval of one or two seconds may 

 elapse before normal monocular vision is restored, and now 

 both circles have recovered their original size. The experi- 

 ment is very easy. 



This process can be reversed, and made as gradual as is 

 desired, by slowly increasing the convergence of visual lines 

 and carrying it beyond the point necessary for binocular com- 

 bination. As soon as double vision is produced, the two inte- 

 rior images are perceived to be smaller than the exterior ones. 

 They continue to grow steadily smaller and apparently nearer 

 to the observer, until they coalesce in front ; but the diminu- 

 tion continues after they have been made to pass each other. 

 On the usually accepted theory, the apparent contraction is 

 referred to the illusive estimate of diminished distance deter- 

 mined by the crossing of the visual lines at the moment the 

 binocular combination is secured. But the falsity of this is 

 additionally shown by regarding the circle A" while A and A' 

 are binocularly combined. Although necessarily seen only 

 monocularly with each eye, the left-eye image appears directly 

 under the binocular image and of the same size, being much 

 smaller than the right-eye image, which appears under the 

 corresponding one of A. 



9. Physiology of Retinal Sensation. 

 These experiments, taken in connexion with those on ste- 

 reoscopy, conclusively show that not only is convergence of 

 visual lines not necessary for binocular vision and for the loca- 

 lization of objects in the field of view, but that even change in 

 such localization and in apparent magnitude can be secured 

 without binocular fusion, as a consequence of muscular tension 



