[ I°+ ] 
grees of refrangibility of light, and from the figure 
of the fphere, which is not of a proper curvature 
for collecting the rays in a fingle point. The 
objeCt-glafs is chiefly affeCted by the firfl of thefe ; 
nor has there been yet any method difcover’d for rec- 
tifying that aberration fo, as in the leafl: to remove 
the indiftinCtnels of the image arifing from it. We 
are therefore reduced to the neceflity of contracting 
their apertures, which renders it impoflible to mag- 
nify much without very long glaffes. 
But the cafe is widely different with regard to the 
eye-glaffes ; for, tho they are very much affeCted by 
both the aberrations before-mention’d, yet, by a proper 
combination of fevered together, their errors may be 
in a great meafure corrected. If any one, for in- 
ftance, would have the vifual angle of a telefcope to 
contain 20 degrees, the extreme pencils of the field 
mud be bent or refraCted in an angle of 10 degrees; 
which, if it be performed by one eye-glafs, will caufe 
an aberration from the figure, in proportion to the 
cube of that angle: but if two glaffes are fo propor- 
tioned and fituatcd, as that the refraCtion may be 
equally divided between them, they will each of them 
produce a refraCtion equal to half the required angle: 
and therefore the aberration being in proportion to the 
cube of half the angle taken twice over, will be but 
a fourth part of that, which is in proportion to the 
cube of the whole angle ; becaufe twice the cube of 
one is but f of the cube of two ; fo the aberration from 
the figure, where two eye glaffes are rightly propor- 
tion’d, is but a fourth of what mull unavoidably be, 
where the whole is performed by a fingle eye-glafs. 
By the fame way of reafoning, when the refraCtion is 
divided 
