17* Mr. Young's Observations on Vision. 
eye. But their action would not be sufficiently regular, nor 
sufficiently strong ; for a much greater pressure being made 
on the eye, than they can be supposed capable of effecting, 
no sensible difference is produced in the distinctness of vision. 
Others say that the muscles shorten the axis : these have 
still less reason on their side. 
<& 
Those who maintain that the ciliary processes flatten the 
crystalline, are ignorant of their structure, and of the effect 
required : these processes are yet more incapable of drawing 
back the crystalline, and such an action is equally inconsis- 
tent with observation. 
Probably other suppositions may have been formed, liable 
to as strong objections as those opinions which I have enu- 
merated. 
From these considerations, and from the observation of 
Dr. Porterfield, that those who have been couched have no 
longer the power of accommodating the eye to different dis- 
tances, I had concluded that the rays of light, emitted by ob- 
jects at a small distance, could only be brought to foci on the 
retina by a nearer approach of the crystalline to a spherical 
form ; and I could imagine no other power capable of pro- 
ducing this change than a muscularity of a part, or the whole, 
of its capsule. 
But in closely examining, with, the naked eye in a strong 
light, the crystalline from an ox, turned out of its capsule, I 
discovered a structure which appears to remove all the diffi- 
culties with which this branch of optics has long been ob- 
scured. On viewing it with a magnifier, this structure be- 
came more evident. 
The crystalline lens of the ox is an orbicular, convex. 
