p6 Mr. herschel on the 
than fufficient to judge well of objefls and difhnguifh them, 
from each other, fuch as a circle from a 1 qua re, triangle, oi 
polygon 
It has been obferved, that obje&s grow indiftinct when the 
principal optic pencil at the eye becomes lefs than the 40th or 
50th part of an inch in diameter. In the experiments that 
have been made upon this fubjedt it appeals to me, that the 
indiftindtnefs which is afcribed to the fmallnefs of the optical 
pencil may be owing to very different caufes . at lcaft it will 
be ealy to bring contrary expeiiments of extiemelv fmall pen— 
cils, not at all affected by this inconvenience ; for inftance, it 
is well known, that microfcopes, confining of a fingle lens or 
globule, are remarkable for diffindtnefs. We alfo know, that 
they have been made fo fmall as to magnify above 10,000 
times + . From this we may infer that their apertures, and 
confequently the diameters of the optic pencil at the eye could 
not exceed the 2500th part of an inch. I am therefore in- 
clined to believe, that we muff look for diffinffnefs in the per- 
fection of the object-fpeculum or objeft-glafs of a telefcope ; 
and if we can make the firff image in the focus of a fpeculum 
almoff as perfect as the real object, what fhould hinder our 
magnifying but the want of light ? Now, if the objea has light 
fufficient, as the ffars moff undoubtedly have, I fee no real'on 
why we fhould limit the powers of our inftruments by any 
theory. Is it not beff to have recourfe to experiments to find 
* By a fet of experiments, made in the year 177 4> f found, that I could dif- 
cover or perceive a bright object, fuch as white paper, again it the fky-light, when 
it fubtended an angle of 35" ; but could only diftinguifh it to be a circle, and no 
other figure, wlien it appeared under an angle of 2 ' 24". 
•j- See Padre della torre’s Method, &c. Scelta di Opufculi, 
how 
