VOL. LXXIX.] PHILOSOPHICAL TRANSACTIONS. 597 



object should appear to subtend an angle of about 15', on account of the differ- 

 ent refrangibility of the rays of light. 



Dr. M. now shows that this angle of ocular aberration is compatible with the 

 distinctness of our vision. This aberration is of the same kind as that which 

 we experience in the common refracting telescope. Now, by computation from 

 the tabular apertures and magnifying powers of such telescopes, it is certain that 

 they admit of an angular indistinctness at the eye of no less than 57'; therefore 

 the ocular aberration is near 4 times less than in a common refracting telescope, 

 and consequently the real indistinctness, being as the square of the angular 

 aberration, will be 14 or 15 times less in the eye than in a common refracting 

 telescope, which may be easily allowed to be imperceptible. 



Further, Sir Isaac Newton has observed, with respect to the like difficulty of 

 accounting for the distinctness with which refracting telescopes represent objects, 

 that the erring rays are not scattered uniformly over the circle of dissipation in 

 the focus of the object-glass, but collected infinitely more densely in the centre 

 than in any other part of the circle, and in the way from the centre to the cir- 

 cumference become continually rarer and rarer, so as at the circumference to be- 

 come infinitely rare; and by reason of their rarity are not strong enough to be 

 visible, unless in the centre and very near it. He further observes, that the most 

 luminous of the prismatic colours are the yellow and orange, which affect the 

 sense more strongly than all the rest together; and next to these in strength are 

 the red and green ; and that the blue, indigo, and violet, compared with these, 

 are much darker and fainter, and compared with the other stronger colours, little 

 to be regarded; and that therefore the images of the objects are to be placed not 

 in the focus of the mean refrangible rays, which are in the confine of green and 

 blue, but in the middle of the orange and yellow, there where the colour is most 

 luminous, that which is in the brightest yellow, that yellow which inclines more 

 to orange than to green. 



From all these considerations, and by an elaborate calculation, he infers, that 

 though the whole breadth of the image of a lucid point be -sV^h of the diameter 

 of the aperture of the object-glass, yet the sensible image of the same is scarce 

 broader than a circle whose diameter is ^-i-^th part of the diameter of the aper- 

 ture of the object-glass of a good telescope; and hence he accounts for the ap- 

 parent diameters of the fixed stars as observed with telescopes by astronomers, 

 though in reality they are but points. 



The like reasoning is applicable to the circle of dissipation on the retina of 

 the human eye; and therefore we may lessen the angular aberration, before com- 

 puted at 15', in the ratio of 250 to 55, which will reduce it to 3' 18'''. This 

 reduced angle of aberration may perhaps be double the apparent diameter of the 

 brightest fixed stars to an eye disposed for seeing most distinctly by parallel rays; 



