&g6 PHILOSOPHICAL TRANSACTIONS. [aNNO I789. 



and least refrangible rays, at the several humors of the eye, and thence inferred 

 the diffusion of the rays, proceeding from a point in an object, at their falling 

 on the retina, and the external angle that such coloured image of a point upon 

 the retina corresponds to. 



He took the dimensions of the eye from M. Petit, as related by Dr. Jurin ; 

 and the specific gravities of the aqueous and vitreous humors having been found 

 to be nearly the same with that of water, and the refraction of the vitreous 

 humor of an ox's eye having been found by Mr. Hauksbee to be the same as 

 that of water, and the ratio of refraction out of air into the crystalline humor 

 of an ox's eye having been found by the same accurate experimenter to be as 1 

 to .68327, he took the refraction of the mean refrangible rays out of air into 

 the aqueous or vitreous humor, the same as into water, as 1 to .74853, or 

 1.33595 to 1 ; and out of air into the crystalline humor as 1 to .68327, or 

 1.46355 to 1. Hence he found, according to Sir Isaac Newton's 2 theorems^ 

 related at part 2, book 1 of Optics, p. 113, that the ratio of refraction of the 

 most, mean, and least refrangible rays at the cornea, should be as 1 to .74512, 

 .74853 and .75197 ; at the fore surface of the crystalline as 1 to .91 173, .91282, 

 and .91392; and at the hinder surface of the crystalline as 1 to I.0968I, 

 1.09550, and 1.09420. Now taking, with Dr. Jurin, 15 inches for the dis- 

 tance at which the generality of eyes in their mean state see with most distinct- 

 ness, he found the rays from a point of an object so situated, will be collected 

 into 3 several foci, viz. the most, mean, and least refrangible rays, at the res- 

 pective distances behind the crystalline, .5930, .6034, and .6141 of an inch, 

 the focus of the most refrangible *rays being .0211 inch short of the focus of the 

 least refrangible ones. 



Also, assuming the diameter of the pencil of rays at the cornea, proceeding 

 from the object at 15 inches distance, to be 4- of an inch in a strong light, 

 which is a large allowance for it, the semi-angle of the pencil of mean refran- 

 gible rays at their concourse on the retina will be 7° 12', whose tangent to the 

 radius unity, or .1264, multiplied into .0211 inch, the interval of the foci of 

 the extreme refrangible rays, gives .O02667 inch for the diffusion of the different 

 coloured rays, or the diameter of the indistinct circle on the retina. Now, 

 having found that the diameter of the image of an object on the retina, is to 

 the object, as .6055 inch, to the distance of the object from the centre of 

 curvature of the cornea ; or the size of the image is the same as would be 

 formed by a very thin convex lens, whose focal distance is .6055 inch ; and 

 consequently a line in an object which subtends an angle of l' at the centre of 

 the cornea, will be represented on the retina by a line of -jVtt inch. Hence 

 the diameter of the indistinct circle on the retina before found, .OO2667, will 

 answer to an external angle of .OO2667 x 5678' = 15' 8^', or every point in an 



