20.2 Prof. J. LeConte on some Phenomena 



same as in the object. I conclude, therefore, that in this posi- 

 tion of the eyes the horopter is at right angles to the plane of 

 vision; and since there is no rotation of the eye, the horopter 

 in this position expands into a surface. Below this inclination 

 the horopter again becomes a line, but inclined now the other way, 

 i. e. the upper end towards the observer. In turning the eyes 

 upward toward the eyebrows, I have found the rotation, except 

 in cases of strong convergence, less than looking straight for- 

 ward. I conclude, therefore, that in this position the horopteric 

 line inclines less to the visual plane than it does when the visual 

 plane is in its primary direction*. 



The points in which my experiments do not confirm Meissner 

 are (1) the increasing inclination of the horopteric line with in- 

 creasing convergence, (2) the increasing rotation of the eye as 

 well as inclination of the horopteric line under all circumstances 

 in turning the eye upward. Again, I believe that Meissner is 

 also wrong in supposing that the horopter is a plane when the 

 eyes are depressed 45°. In this position it is a surface, but not 

 a plane. It is clear that the images of points situated to the 

 right and left of the point of sight and in the same plane with it 

 caunot fall on corresponding points of the two retinas. As to 

 the form of this surface, I feel myself unequal to the task of its 

 mathematical investigation ; and its experimental investigation 

 presents, I believe, insuperable difficulties. 



We have seen that the eye in convergence rotates on the optic 

 axis. The question naturally occurs, Is this rotation to be re- 

 garded in the light of an imperfection of the instrument (of 

 which there are several examples in the structure and mechanism 

 of the eye), and should the law of Listing be regarded as the 

 ideal of ocular motion, though an ideal seldom or never realized 

 in nature ? or does the rotation of the eye subserve some useful 

 purpose in vision ? I believe there is no doubt that the latter 

 view is the correct one ; for there seem to be special muscles 

 which are adapted for this rotation, and the action of these 

 muscles is consensual with the adjustments of the eye and the 

 contraction of the pupil. This purpose I explain as follows. A 

 general view of objects in an extended field is absolutely ne- 

 cessary to animal life in its highest phases, but an equal distinct- 

 ness of all objects in this field would only distract the attention ; 



* As stated in note on p. 190, eyes certainty differ in this respect. In 

 my own, if convergence be small, the outward rotation decreases with the 

 elevation of the visual plane, becomes zero, and is even converted into an 

 inward rotation ; the inclination of the horopter, therefore, decreases, 

 becomes perpendicular, and even inclines the other amy. In some other 

 eyes the outward rotation increases whatever be the convergence; in this 

 case, of course, the inclination of the horopter increases as stated by 

 Meissner. 



