Mr. Young's Observations on Vision. 
175 
greater density of the central parts. Supposing, with Hauks- 
bee and others, the refraction of the aqueous and vitreous hu- 
mours equal to that of water, viz. as 10000 to 74 65, the ratio 
of refraction of the crystalline in the eye will be as 10000 to 
8806, and it would collect parallel rays at the distance of 
1226 thousandths of an inch : but the distance of the retina 
from the crystalline is 550 thousandths, and that of the an- 
terior surface of the cornea 250 ; hence (by Smith, Art. 367,) 
the focal distance of the cornea and aqueous humour alone 
must be 2329. Now, supposing the crystalline to assume a 
spherical form, its diameter will be 642 thousandths, and its 
focal distance in the eye 926. Then, disregarding the thick- 
ness of the cornea, we find (by Smith, Art. 370,) that such 
an eye will collect those rays on the retina, which diverge 
from a point at the distance of 12 inches and 8 tenths. This 
is a greater change than is necessary for an ox's eye, for if it 
be supposed capable of distinct vision at a distance somewhat 
less than 12 inches, yet it probably is far short of being able 
to collect parallel rays. The human crystalline is susceptible 
of a much greater change of form. 
The ciliary zone may admit of as much extension as this 
diminution of the diameter of the crystalline will require; and 
its elasticity will assist the cellular texture of the vitreous hu- 
mour, and perhaps the gelatinous part of the crystalline, in re- 
storing the indolent form. 
It may be questioned whether the retina takes any part in 
supplying the lens with nerves ; but, from the analogy of the 
olfactory and auditory nerves, it seems more reasonable to sup- 
pose that the optic nerve serves no other purpose than that of 
conveying sensation to the brain. 
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