£04 Dr. Herschel’s Experiments 
eaufe might bring on that indiftin&nefs which had been afpribed 
to the fmallnefs of the optic pencils, I continued thefe expe- 
riments with a variation in the apparatus, and ufed now an 
object lens of a different focal length ; the aperture and other 
particulars being as. in the 4th experiment. By this conftruc- 
tion, which gave me a pencil of the 724th part of an inch, I 
could fee obje£ts very well ; but though they appeared diftinctly, 
they were not fo fharp on the edges as one would wifh to fee 
them. This being compared with the 4th experiment, it ap- 
peared that, with equal pencils, unequal degrees of diftin&nefs 
may take place; and a pretty ftriking circumftance,., which lerved 
to lead me in the following experiments, was, that the fmaljeft 
power gave me the lead: diftindt image ; not with (landing, from 
former trials, the goodnefs of the lenfes I employed could not 
be doubted. 
Exp. 10 On an examination of circumftances it occurred to 
me, as indeed I had already before furmifed, that a certain 
proportion of aperture might be neceffary to a given focal 
length of an objeft-lens or fpeculum ; and that a failure in this 
point might probably bring on that indiftim&nefs which had 
been afcribed to the fmallnefs of the pencils. I11 order, there- 
fore, to put this to a trial, I ufed now an objeft-lens of 1,25 
focal length, with an aperture confined to., 01 ; the reft of the 
apparatus being as in the 3d, 4th, and 5th experiments. The 
pencil in this cafe was about the iooodth part of an inch; 
and though by a different conftruftion I had already feen very 
well with a pencil of not half that diameter, I found this to 
give me, as now I had reafon to expeft, a very indiftin£t 
pidlure, fo much fo indeed, that it could hardly be called a 
reprefentation of the objedh 
Exp* 
