Dr. Herschel’s Experiment s 9 Sec. 501 
fevers ; and I thought myfelf authorifed, according to a Car- 
tefian maxim (JDubia etiam pro falfis habenda), to fuppofe, for 
a while, the fize of optic pencils, requifite for diftind vifion, 
intirely undecided. 
The firft opportunity I had of making the propofed experi- 
ments was in the year 1778, and the refult of them proved fa 
decifive that I have never fince refumed the fubjed ; and had it 
not been for a late converfation with fome of my highly 
efteemed and learned friends, I might probably have left the 
papers, on which thefe experiments were recorded, among the 
reft of thofe that are laid alide when they have afforded me the 
information I want. But a doubt Teeming ftill to be enter- 
tained on the fubjed of the fmallnefs of the optic pencils, it 
may now be proper for me to communicate thefe experiments, 
that it may appear how far the conclulions I have drawn from 
them are warranted by the fads on which I fuppofe them to 
reft. 
Experiments with the naked eye . 
Exp. 1. Through a very thin plate of brafs I made a minute 
hole with the fine point of a needle ; its magnified diameter, 
very accurately meafured under a double microfcope, I found 
to be ,465 of an inch, while under the fame apparatus a line 
of ,05 in length gave a magnified image of 3,545 inches. 
Hence I concluded, that the real diameter of the perforation 
was about the I52d part of an inch. Through this fmall 
opening, held clofe to the eye, I could very diftindly read any 
printed letters on which I made the trial. Proper allowance 
muft be made for the very inconvenient fituation of the eye, 
which by the.nmifual clofenefs to the paper cannot beexpeded 
T t t 2 to 
