out them. The advantages procured by the magnifying power,
would be exceedingly circumfcribed, if they were not accompanied
by the latter : for if the fame quantity o f light is diffufed
over a much larger furface, it’s force is proportionably diminilhed;
and therefore the objeft, though magnified, will be dark and
obfcure. Thus, fuppofe the diameter o f the ob ject to be enlarged
ten times, and confequently the furface one hundred times, yet,
i f the focal diftance o f the glafs was eight inches, (provided this
was poffible) and it’s diameter only about the fize o f the pupil of
the eye, the object would appear one hundred times more obfcure
when viewed through the glafs, than when it was feen by the
naked e y e ; and this even on the fuppofition, that the glafs tranf-
mitted all the light which fell upon it, which no glafs can do.
But if the glafs was only four inches focal diftance, and it’s
diameter remained as before, the inconvenience would be vaftly
diminifhed ; becaufe the glafs could be placed twice as near the
objeft as before, and would confequently receive four times as
many rays as in the former cafe, and we fhould therefore fee it
much brighter than before. By going on thus, diminifhing the
focal diftance of the glafs, and keeping it’s diameter as large as
poffible, we {hall perceive the objeft proportionably magnified,
and yet remain bright and diftinft. Though this is the cafe in
theory, yet there is a limit in optical inftruments, which is foon
arrived at, but which cannot be paffed. This arifes from the
following circumftances. *
l. The quantity of light loft in paffing through the glafs.
2. The
Encyclopedia Br tannica, vol. viii. p. 5^35*
M i c r o s c o p i c a l E s sa y s . 51
2. The diminution in the diameter o f the glafs or lens itfelf, by
which it receives only a fmall quantity of rays.
3. The extreme fhortnefs o f the focal diftance of great magnifiers,
whereby the- free accefs of the light to the objeft we wifh
to view is impeded, and confequently the refleftion of the light
from it is weakened.
4. The aberration o f the rays, occafioned by their different
refrangibility.
T o make this more clear, let us fuppofe a lens made of fuch
dull kind of glafs, that it tranfmits only one half the light that
falls upon it. It is evident, that fuppofing this lens to be o f four
inches focus, and to magnify the diameter o f the objeft twice,
and it’s own breadth equal to that o f the pupil o f the eye, the
objeft will be four times magnified in furface, but only half as
bright as if it was feen by the naked eye at the ufual diftance; for
the light which falls upon the eye from the objeft at eight inches
diftance, and likewife the furface o f the objeft in it’s natural fize,
being both reprefented by 1, the furface o f the magnified objeft
will be 4, and the light which makes it vifible only 2; becaufe
though the glafs receives four times as much light as the naked
eye does at the ufual diftance o f diftinft vifion, yet one half is loft
in paffing through the glafs. The inconvenience, in this refpeft,
can only be removed fo far as it is poffible to increafe the tranf-
parency o f the glafs, that it may tranfmit nearly all the rays
which fall upon i t ; and how far this can be done, has not been,
yet afcertained.