C i°s J 
divided between three glades, the aberration will be 
found to be but the ninth part of what w r ould be 
produced from a Tingle glafs ; becaufe three times the 
cube of one is but one ninth of the cube of 3 . Whence 
it appears, that, by increafing the number of eye- 
glades, the indidinCtnefs, which is obferved near the 
borders of the field of a telefcope, may be very much 
diminifhed, tho’ not intirely taken away. 
The method of correcting the errors arifing from 
the diderent refrangibility of light is of a diderent 
confideratiQn from the former ; for, whereas the 
errors from the figure can only be diminifhed in a 
certain proportion to the number of glades, in this 
they may be intirely corrected, by the addition of 
only one glafs ; as we find in the aftronomical tele- 
fcope, that two eye-glades, rightly proportion’d, will 
caufe the edges of objects to appear free from colours 
quite to the borders of the field. Alfo in the day- 
telefcope, where no more than two eye-glades are ab- 
folutely necedary for ereCting the objeCt, we find, by 
the addition of a third rightly fituated, that the co- 
lours, which would otherwife confufe the image, are 
intirely removed : I fay intirely removed ; but this is 
to be underftood with fome limitation ; for tho’ the 
diderent colours, which the extreme pencils mud 
necedarily be divided into by the edges of the eye- 
glafles, may in this manner be brought to the eye in 
a direction parallel to each other, fo as, by the hu- 
mours thereof, to be converg’d to a point in the 
retina ; yet, if the glades exceed a certain length, the 
colours may be fpread too wide to be capable oi 
being admitted thro’ the pupil or aperture of the eye j 
which is the reafon, that, in long telefcopes, con- 
O druCted 
