J. LeConte on Binocular Vision. 69 
versa. The large majority of the best physicists and physiologists, 
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 
@ question in doubt. It has been attributed by some to the 
n ? 
images of external objects made by reflection from the anterior 
surface of the crystalline lens, and the changes in form an 
size, which they undergo when the eye is adjusted for near ob- 
Jects, have definitely settled the question in favor of a change 
wm 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 ques- 
tion 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 ef- 
fort I could see perfectly distinctly at five inches. At the pre- 
sent 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 ac- 
quired considerable and perhaps very unusual facility in mak- 
ing experiments on binocular vision and in analyzing my vis- 
ual impressions. The following experiments which I have 
practised from boyhood, are interesting not only as a beautiful 
illustration of the laws of binocular vision, but I believe as 
throwing some light on the subject of adjustment and also 
Upon the difficult subject of the Horopter. 
f a plane surface checkered or otherwise figured in regular 
pattern, such as an oil floor-cloth, a tesselated pavement or a 
AR wall, be placed before the eyes at the distance of sev- 
eTal feet 
