C % 9 6 3 
XV. Observations on Vision. By David Hosack, M. D. Com- 
municated by George Pearson, M. D. F. R- S. 
Read May 1 , 1794- 
By what power is the eye enabled to view objects distinctly 
at different distances ? As the pupil is enlarged or d.m.mshed 
according to the greater or less quantity of light and in a 
certain degree to the distance of the object, it would readi y 
occur that these different changes of the pupil would account 
for the phenomena in question. Accordingly anatomists an 
philosophers, who have written upon this subject, have gene- 
rally had recourse to this explanation. 
Amusing myself with these changes of the pupil as a matter 
of curiosity, by presenting to the eye different objects a > 
ferent distances, I soon perceived that its contraction and 
dilatation were irregular and more limited than had ‘ ^ 
posed ; i. e. that approaching the object nearer the eye, with,. 
Hertain distance, the pupil not only ceased to contract, but 
became again dilated ; and that beyond a few yards distance, 
it also ceased to dilate : these circumstances immediately oc- 
curred as objections to the above explanation ; for were 
from the contraction and dilatation of the iris alone that we 
see objects at different distances, I naturally concluded . 
should operate regularly to produce its effects i^fftaview 
an object at a few yards distance it be enlarge 
