8o8 Dr. maskelyne’s Account of 
But when the prifras are placed in the manner fhewn 
in fig. 4. the images at the point of contact, as the prifms 
are removed from the obje< 5 t-glafs towards the eye-glafs, 
will be enlightened with more than half the rays that 
fall on the objeCt-glafs, and will be moft enlightened 
when the prifms are brought to the focus itfelf ; for the 
point ic of the image 2zvie will be enlightened by all 
the rays ee that fall on the objeCt-glafs between b and f, 
and the point w of the image we will be enlightened by 
all the rays ww which fall on the objeCt-glafs between a 
and g. But the difference of the illuminations is not 
very confiderable in achromatic telefcopes, on account of 
the great aperture of the objedl-glafs ; as the greateft 
fpace fg is to the focal diftance of the objeCt-glafs, as the 
fum of the fines of the refractions of the prifms is to the 
radius. 
There is a third way, and perhaps the beft, of placing 
the prifms, fo as to touch one another along their fides 
which are at right angles to the common feCtions of their 
refracting planes. In this difpofition of the prifms, the 
images will be equally enlightened, namely, each with 
half the rays which fall on the objett-glafs, wherever 
the prifms be placed between the objeCt-glafs and eye- 
glafs. 
From 
