200 
Dr. Hosack/s Observations on Vision. 
Most writers upon this subject refer this power of the eye 
to the contraction and dilatation of the iris. Within certain 
limits this would, upon first examination, as already observed, 
appear to be the case, since the pupil enlarges as the object is 
further removed from the eye, and again contracts as it is 
brought near. The extent of this principle I have already 
pointed out ; but I suspect we also err in attributing to the 
difference of distance what are only effects of different quanti- 
ties of light, a circumstance in which it is the more easy to 
commit error as they are generally proportionate one to the 
other ; i. e. as the object is near we require a less degree of 
light, and to exclude what is superfluous the iris contracts ; 
but as it is more distant, a greater quantity of light becomes 
necessary, and the iris dilates : thus far we see the use of the 
enlargement or diminution of the pupil, as the object is more 
or less distant. But distinct vision does not consist in the 
quantity of light alone, though too much or too little would 
obscure the image. 
It is also necessary that the rays which flow from the ob- 
ject should fall upon the retina in a certain direction, to form 
a distinct picture ; but surely the greater or less quantity of 
light, the greater or less number of rays, which it is only the 
property o & f the iris to diminish or increase, cannot alter the 
direction. 
But there is still another argument to prove, that the con- 
traction or enlargement of the pupil is not of itself sufficient 
to produce distinct vision at different distances, viz. that the 
myopes, whose pupil contracts and dilates as in other eyes, 
are still unable to adapt the eye to different distances; and the 
means by which this is remedied certainly does not consist m 
