a Difficulty in the Theory of Vifion , 259 
to be -1th of an inch in a ftrong light, which is a large allow- 
ance for it, the femi-angle of the pencil of mean refrangible 
rays at their concourfe upon the retina will be f 12', whofe 
tangent to the radius unity, or ,1 264 multiplied into ,0211 
inch, the interval of the foci of the extreme refrangible rays, 
gives ,002667 inch for the difflifion of the different coloured 
rays, or the diameter of the indiftindr circle upon the retina. 
Now, I find, that the diameter of the image of an object upon the 
retina is to the ohjedt as ,6055 inch to the diftance of the ob- 
jedt from the center of curvature of the cornea ; or the fize of 
the image is the fame as would be formed by a very thin con- 
vex lens, whofe focal diftance is ,605 ; inch, and confequently 
a line in an objedl which fubtends an angle of i / at the center 
of the cornea will be reprefen ted on the retina by a line of 
th inch. Hence the diameter of the indiftindt circle on 
5 o 7 o 
the retina before found, ,002667 will anfwer to an external 
angle of ,002667 x 5 6 y 8 7 = 1 5' 8 /y , or every point in an objedt 
fhould appear to fubtend an angle of about if, on account of 
the different refrangibility of the rays of light. 
I (hall now endeavour to fhew that this angle of ocular aber- 
ration is compatible with the diftindtnefs of our vifion. This 
aberration is of the fame kind as that which we experience in 
the common refradling telefcope. Now, by computation from 
the tabular apertures and magnifying powers of fuch telefcopes* 
it is certain that they admit of an angular indiftindlnefs at the 
eye of no lefs than 57'' ; therefore the ocular aberration is near 
four times lefs than in a common refradling telefcope, and con- 
fequently the real indiftindlnefs, being as the fquare of the 
angular aberration, will be 14 or 15 times leis in the eye than 
in a common refradling telefcope, which may be eafily allowed 
to be imperceptible. 
- ' Moreover,- 
