﻿132 Prof. J. LeConte on some Phenomena 



impossible to converge the optic axes on any point without at 

 the same time adjusting the lens and contracting the pupil in 

 a manner suitable for perfect vision at that distance. Such 

 inseparably associated movements are called consensual move- 

 ments. 



The binocular adjustment is well understood ; there is no dif- 

 ference of opinion as to its necessity, nor the means by which it 

 is accomplished. But in regard to the focal adjustment, there 

 has been much difference of opinion among the best physiolo- 

 gists and physicists. Some have denied altogether the necessity, 

 and therefore the existence, of any adjustment — attributing the 

 phenomena which are usually explained by this means to mere 

 transference of attention from near to distant objects, or vice 

 versa. The large majority of the best physicists and physiolo- 

 gists, however, have for a long time regarded focal adjustment 

 as an optical necessity, and therefore a fact ; but the real nature 

 of this adjustment, and the means by which it is accomplished, 

 has been a question in doubt. It has been attributed by some to the 

 elongation of the eye by the action of the recti muscles, by others 

 to the change in the convexity of the cornea, by others to the 

 structure of the crystalline lens combined with contraction of the 

 pupil, by others to the pulling forward of the crystalline lens 

 by the ciliary muscle so as to elongate the chamber behind the 

 lens, and by still others to the change of form of the lens by 

 the action of the ciliary muscle. Recent very ingenious obser- 

 vations by Donders, Cramer, and Helmholtz upon the images of 

 external objects made by reflection from the anterior surface of 

 the crystalline lens, and the changes in form and size which they 

 undergo when the eye is adjusted for near objects, have definitely 

 settled the question in favour of a change in the curvature of the 

 lens. The mechanism by which this change is effected is not 

 clearly known ; but it is probable that it is effected by the action 

 of the ciliary muscle. 



Before giving some experiments which bear upon the question 

 of adjustment, I will state that my eyes are perfectly normal. 

 In youth and early manhood the natural distance for distinct 

 vision of small objects was eight inches ; but with effort I could 

 see perfectly distinctly at five inches. At the present time my 

 natural distance for fine print is ten inches, though with effort I 

 see distinctly at eight inches. Beyond this there is for me no 

 limit of distinct vision. My eyes define the edge of the moon 

 as perfectly as they do an object at the distance of ten inches. 

 Moreover, by long practice I have acquired considerable, and 

 perhaps very unusual facility in making experiments on binocu- 

 lar vision and in analyzing my visual impressions. The follow- 

 ing experiments, which I have practised from boyhood, are in- 



