201 
Dr. Hosack's Observations on Vision. 
a larger or smaller aperture for the rays to pass through, but a 
power of altering their direction, which the change in the 
shape of the eye had rendered too convergent. The same 
fact is also observable in those who squint ; the pupil in both 
eyes equally contracts and dilates, but still the vision of one 
eye is less perfect than the other. Another principle upon 
which it has been attempted to explain this power of the eye, 
is a supposed change in the convexity of the crystalline lens ; 
the ancients had some obscure notion of it, but it has been 
lately pursued by Mr? Thomas Young, in a paper published 
in the Philosophical Transactions of London for 1793. He 
has endeavoured to demonstrate the existence of muscles in 
the crystalline lens, and by their action to account for distinct 
vision at different distances. This opinion deserves here the 
more particular examination, having met the attention of the 
Royal Society, and thereby likely to influence the general 
opinion upon this subject. 
That we may not mistake the meaning of the author, I beg 
leave to premise his description of the structure of the lens. 
“ The crystalline lens of the ox, JJ he observes, “ is an orbicular 
“ convex transparent body, composed of a considerable number 
of similar coats, of which the exterior closely adhere to the 
“ interior ; each of these coats consists of six muscles, inter- 
“ mixed with a gelatinous substance, and attached to six mem- 
“ branous tendons. Three of these tendons are anterior, 
“ three posterior ; their length is about two-thirds of the 
“ semidiameter of their coat ; their arrangement is that of 
“ three equal and equidistant rays meeting in the axis of the 
“ crystalline ; one of the anterior is directed towards the 
“ outer angle of the eye, and one of the posterior towards the 
MDCCXCIV. D d 
