Dr. Hosack's Observations on Vision. 
205 
the power of accommodating the eye to the different distances 
of objects ; at present, I believe the contrary fact is almost 
universally asserted.* 
Besides, if the other powers of the eye are insufficient to 
compensate for the loss of this dense medium, the lens, a glass 
of the same shape answers the purpose, and which certainly 
does not act by changing its figure. I grant their vision is 
not so perfect ; but we have other circumstances upon which 
this can be more easily explained ; which will be particularly 
noticed under the next head. It may not be improper also to 
observe, that the specific gravity of the crystalline compared 
with that of the vitreous humour, and of consequence, its den- 
sity and power of refraction, is not so great as has been ge- 
nerally believed. Dr. Bryant Robinson, by the hydrostatic 
balance, found it to be nearly as 1 1 to 16. I have also exa- 
mined them with the instrument of Mr. Schmeisser, lately 
presented to the Royal Society, and found the same result; of 
consequence the crystalline lens is not so essentially necessary 
for vision as has been represented; especially as it is also pro- 
bable, that upon removing it, the place which it occupied is 
again filled by the vitreous humour, whose power of refraction 
is nearly equal. At the same time we cannot suppose the 
lens an unnecessary organ in the eye, for nature produces 
" Et lente ob cataractam extraeta yel deposita oculum tamen ad varias distantias 
„ . ’ “ n ? bl1 ' viro vit!eo abs 't” ”>>» experimento quo earn facltatem recupera- 
.. Tk' T ' n,m *“ nC 0b diminutas vires qu* radios uniunt, seger lente vitrea opus 
habet, eadem tamen lens in omni distantia sufficit H aller, El. Phys. 
“ La lentille Cristalline n’est cependant point de premiere necessite pour la vision 
1 Aujourd’hm, dans Iteration de la cataracte on l’enleve entierement, et la vision 
n en sou re point.”— De la Metherie Vues Physiologiques. See also De la 
Hire, Hamberger Physiolog. 
