of Binocular Vision. 203 



therefore the eve is so constructed and moved as to restrict as 

 much as possible both distinct vision and single vision. Thus as 

 in monocular vision the more elaborate structure of the central 

 spot of the retina restricts distinct vision to the visual line, and 

 the focal adjustment still further restricts it to a single point in 

 that line, so also in binocular vision, axial adjustment restricts 

 single vision to the horopter, while rotation restricts the horop- 

 ter to a single line. 



Conclusions. 



The most important conclusions arrived at in this paper may 

 be briefly summed up as follows : — 



(1) The axial and focal adjustments of the eye are not so in- 

 separably associated as is generally supposed; but, on the con- 

 trary, when distinctness of vision requires it they may be com- 

 pletely dissociated*. 



(2) In this dissociation the contraction of the pupil associates 

 itself with the focal in preference to the axial adjustment. 



(3) In optic convergence there is a rotation of both eyes on 

 the optic axes outward, and this rotation increases with the degree 

 of convergence. 



(4) In inclining the visual plane downward, the rotation of 

 the eyes for the same degree of convergence decreases until, when 

 the visual plane is inclined 45° downward, the rotation becomes 

 zero for all degrees of convergence. Below the inclination of 

 45° the rotation is inward. In turning the eyes upward, except 

 in cases of strong convergence, the rotation also decreases 

 slightly but does not reach zerof ; in strong convergence it in- 

 creases as stated by Meissner. 



(5) Besides the rotation produced by optic convergence, there 

 is also a decided inclination of the vertical line of demarcation 

 upon the horizontal line of demarcation, which increases with 

 the degree of convergence. This change in the relation of these 

 two lines is probably the result of distortion of the ocular globe. 



(6) As a necessary consequence of the rotation of the eyes, 

 for all degrees of convergence in the primary visual plane the 

 horopter is a line inclined to the visual plane, the lower end 

 nearer the observer ; but whether the inclination increases or 

 decreases with distance I have not been able to determine with 

 certainty. It probably increases with distance. 



(7) In inclining the visual plane below the primary position, 

 the inclination of the horopteric line becomes less and less until, 



* While these pages were passing through the press, I discovered that 

 in this conclusion I had been anticipated by Donders and others. All 

 previous experiments, however, were made by means of glasses. Mine 

 were made with the naked eye. 



t See this statement modified in note on p. 190. 



P 2 



