68 On the UNEQUAL 
is not a perfect union of the rays at the focus of the objedt-glafs, 
but at the retina. This is to be effe@ted by fo difpofing the rays. 
at their emergence from the eye-glafs, that the humours of the 
eye fhall 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 iffue, 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 
thall 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 perfeét 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 pafling the edge of the iris, are inflected, that is to. 
fay, they are made to deviate from their retilineal 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 indiftin€inefs 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 muft not, however, be diminifhed too far; 
for if this is done beyond a certain degree, the diftin&tnefs will: 
be quite deftroyed, as was firft obferved by HuGENtus. 
I sHALL now recapitulate, and prefent in one view, the con- 
tents and fcope of this difcourfe. 
Tue unequal refrangibility of light, as difcovered and fully 
explained by Sir Isaac Newron, fo far ftands its ground un- 
controverted, 
