25B Dr. Maskelyne’s Attempt to explain 
upon the retina, and the external angle which fuch coloured 
image of a point upon the retina correfponds to. 
1 took the dimenfions of the eye from M. Petit, as related 
by Dr. Jurxn; and, the fpecific gravities of the aqueous and 
vitreous humors having been found to be nearly the fame with 
that of water, and the refraction of the vitreous humor of an 
ox’s eye having been found by Mr. Hawksbee to be the fame 
as that of water, and the ratio of refraction out of air into 
the cryftalline humor of an ox’s eye having been found by 
the fame accurate experimenter to be as 1 to ,68327, I took the 
refradion of the mean refrangible rays out of air into the 
aqueous or vitreous humor, the fame as into water, as 1 to 
974^53* or 1 *33595 to 1 5 and out of air into the cryftalline 
humor as 1 to ,68327, or 1 ,46 3 5 5 to 1. Hence I find, 
according to Sir Isaac Newton’s two theorems, related at 
Part II. of Book I. of Optics, p. 1 1 3 . that the ratio of refrac- 
tion of the moft, mean, and leaf refrangible rays at the cornea 
fhould be as 1 10,74512, ,74853 and ,75197; at the fore- 
fur face of the cryftalline as 1 to ,911 73? ,91282, and ,9 159 2; 
and at the hinder furface of the cryftalline as 1 to 1,09681, 
1, 09550, and 1,09420. 
Now, taking with Dr. Jurin 15 inches for the diftance at 
which the generality of eyes in their mean ftate fee with moft 
diftindnefs, I find the rays from a point of an objed fo fituate 
will be colleded into three feveral foci, viz. the moft, mean, 
and lead refrangible rays at the refpedive diftances behind the 
cryftalline ,5 930, ,6034, and ,6141 of an inch, the focus of 
the moft refrangible rays being ,0211 inch fhort of the focus 
of the lead refrangible ones. 
Moreover, affuming the diameter of the pencil of rays at 
the cornea, proceeding from the objed at 15 inches diftance, 
7 to 
V 
