68 On the UNEgVJL 



is not a perfect union of the rays at the focus of the object-glafs, 

 but at the retina. This is to be effected by fo difpofing the rays 

 at their emergence from the eye-glafs, that the humours of the 

 eye mail accurately converge each of the pencils to one. point of 

 the retina. If we conceive a point of the retina to become a 

 radiant point whence the rays iiTue, the rays of different co- 

 lours, at their emergence from the cornea, will be inclined to 

 each other in a certain degree, on account of their unequal re- 

 frangibility, and will continue to diverge, till they arrive, we 

 {hall fuppofe, at the eye-glafs. Now, this is exactly the ftate in 

 which rays emerging from the eye-glafs, and tending towards 

 the eye, ought to be, in order to infure their perfect union at 

 that point of the retina from which the above mentioned rays 

 were fuppofed to radiate. 



Another caufe which operates in favour of telefcopic vifion, 

 is the fmallnefs of the pencil where it enters the eye. When 

 the diameter of the pencil is equal to that of the pupil, the 

 rays, in paffing the edge of the iris, are inflected, that is to 

 fay, they are made to deviate from their rectilineal courfe, fome 

 of them being bent towards the iris, and others from it, and 

 thus throw a fcattered light round the image on the retina. 

 The radiation of the bright fixed ftars proceeds partly from 

 this caufe. This fource of indiftinctnefs is totally removed in 

 a telefcope, where the diameter of the pencil, at its entrance 

 into the eye, is fo much lefs than the pupil, that none of the 

 rays pafs near enough the iris to fuffer any inflection. The 

 fize of the pencil muit not, however, be diminifhed too far; 

 for if this is done beyond a certain degree, the diftinctnefs will 

 be quite deftroyed, as was firft obferved by Hu genius. 



I shall now recapitulate, and prefent in one view, the con- 

 tents and fcope of this difcourfe. 



The unequal refrangibility of light, as difcovered and fully 

 explained by Sir Isaac Newton, fo far flands its ground in- 

 controverted, 



