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XXVIII. Investigation of the Caufe of that IndifinSJnefs of 
Vifion which has been afcribed to the fmailnefs of the Optic 
Pencil. By William Herfchel, LL.D. F.R.S. 
Read June 22 1 17 86, 
S OON after my fir ft eflays of ufing high powers with, the 
Newtonian telefcope, I began to doubt whether an opinion 
which has been entertained by feveral eminent authors* “ that 
“ vifion will grow indiftinft, when the optic pencils are lefts 
“ than the 40th or 50th part of an inch, ,v would hold good in 
all cafes. To judge according to fc rigid a criterion* I per- 
ceived that I was not in titled to fee diftin&ly with a power 
much more than about 320, in a 7-feet telefcope which bore 
an aperture of 6*4 inches ; whereas in many experiments on 
double ftars I found myfelf very well pleafed with magnifiers 
that far exceeded fuch narrow limits. This induced me, as it 
were, by way of apology to myfelf, for feeing well where I 
ought to have feen lefs diftin&ly, to make a few experiments 
on the fuhjeft of the diameter of optic pencils. It occurred to 
me* that an opinion which limits them to any given fize can- 
not be fupported by theory, which does not determine on fuh- 
jefts of this nature* but mu ft be decided, like many other 
phyficai questions relating to matters of fa£t, by careful expe- 
riments made upon the fubjedt. The way* therefore, to come 
at truth, in a cafe which feemed to me of confiderable im- 
portance, lay ftill open to me, as it had done to former ob- 
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