50 6 Dr. Herschel’s Experiments 
fmaller than the 40th or 50th part of an inch. It did not ap- 
pear to be neceffary, nor would the fubjeCt of that Paper per- 
mit me to enter into a detail of experiments ; but having, in 
the courfe of my reading about that time, met with an account 
of fome very fmall globules made for microfcopic ufes, I con- 
tented myfelf with an inftanee of fmall pencils taken from 
them. I ftiall, however, now proceed juft to hint at a few 
inferences that may be drawn from thefe related experiments ; 
as, upon a mature confideration, we may find reafon to be- 
lieve they point out a cauie of indiftinCtnefs of vifion hitherto 
never noticed by optical writers; and which, when properly 
inveftigated, cannot but influence, and in fome refpeCts con- 
tribute to the improvement of, our theories in optics. For, 
admitting that every objedt-glafs or fpeculum, whofe aperture 
bears lefs than a certain ratio to its focal length, will begin to 
give an indiftinCt picture, it will follow, that while former 
opticians have been endeavouring to diminifti the aberrations 
ariling from the fpherical figure, and the different refrangibility 
of rays, by increafing the focal length, they have been unaware 
of expofing themfelves to the confequences of the caufe of 
indiftinCtnefs here pointed out. And till its influence (hall be 
well afcertained and brought to a proper theory, we muff: fuf- 
peCt that fuch tables as thofe which are given in our beft 
authors of optics, pointing out an aperture of lefs than 6 
inches for a glafs of 120 feet focal length (or a ratio of 1 to 
240) muft be far from having that degree of perfection which 
may yet be obtained. No wonder that telefcopes, made ac- 
cording to theories or tables, where one of the caufes of in- 
diftinCtnefs is unfufpeCted, and therefore left out of the account, 
can bear no fmaller pencil than the 40th or 50th part of an 
inch! If then, on one hand, by increafing our apertures we 
1 certainly 
