IJ02 Dr, Herschel’s Experiments 
to fee with its common facility. Befides, the continual mo- 
tion of the letters, which is required on account of the fmall- 
nefs of the field of view, mu ft needs take up a confiderable 
time. 
Exp, 2. In fome other pieces of brafs I made fmaller holes; 
and among many, that were meafured with the fame accuracy 
as in the former experiment, I found one whofe magnified dia- 
meter was ,29 : hence the real diameter could not exceed the 
244th part of an inch. Through this opening I could alfo 
read the fame letters ; but the difficulty of managing fo as not 
to intercept all the incident light, as well as the very uneafy 
fituation of the eye, were fufficient reafons for not carrying the 
intended experiments any further under this form. Befides, I 
fhould hardly have allowed them to be fair, if, on a further 
contraction of the hole in the brafs plate, an indiftinCtnefs had 
come on ; as we might well have fufpeded at leaft two other 
caufes, befides the fmallnefs of the pencils, to contribute to 
fuch an imperfection ; viz. want of light, and a deflection of it 
on the contracted edges of the hole. 
Microfcopic Experiments . 
Exp. 3. I had now recourfe to a double microfcope, confift- 
ing, for fimplicity’s fake, of only two lenfes. The focal 
length of the eye-glafs, carefully afcertained by an objeCt half 
a mile off, being ,9 ; the diftance of the objeCt-glafs from the 
eye-glafs 9,36 ; and the aperture of the objeCt-glafs ,0405. 
Hence we compute that the diameter of the optic pencil, when 
it entered the eye, could not exceed the 23 2d part of an inch ; 
yet with this conftruCtion I faw very diftinCtly every objeCt I 
placed under the microfcope. 
Exp. 
