The fecond obftacle to the perfe&ion of microfcopic glades, is
the.fmall fize o f great magnifiers ; by which means, notwithstanding
their near approach to the objeft, they receive a fmaller
quantity of light than might be expected. Thus-, fuppofe a glafs of
only one-tenth o f an inch focal diftance,fuch a glafs would increafe
the vifible diameter eighty times, and the furface 6400 times. I f
the breadth of the glafs could at the fame time be preferved as
great as the pupil of the eye, which we fhall fuppofe one-tenth of
an inch, the objeft would appear magnified 6400 times, and-eyery
part would be as bright as it appears to the naked eye. But if we
fuppofe the lens to be only A o f an inch diameter, it will then
only receive i o f the light which would otherwife have fallen
upon i t ; therefore, inflead o f communicating to the magnified
obje it a quantity o f light equal 106400, it would communicate
an illumination fuited only to 1600, and the magnified object
would appear four times as dim as it does to the naked eye. This
inconvenience can, however, be in a great degree removed, by
throwing a much larger quantity o f light on the object. Various
methods of effe&ing this purpofe will be pointed out in the
courfe of the work.
The third obftacle arifes' from the lhoitnefs o f the focal
diftarice in large magnifiers ; this inconvenience can, like the
former, be remedied in fome degree by artificial means of accumulating
ligh t; but ftill the eye is fo ftrained, as it muft be
brought nearer the glafs than it can well bear, which in fome measure
fuperfedes the ufe of very deep lenfes, or fuch as are capable
of magnifying beyond a certain degree. -
The fourth obftacle which arifes from the different refrangi-
biiity of the rays of light, and which frequently caufes fuch
deviations from truth in the appearance of things, that many have
imagined themfelves to have madeTurprizing difcoveries, and have
communicated them as fuch to the world jj when, in fa&, they
have been only optical deceptions, owing to the unequal refraction
o f'th e rays. In telefcopes, this error has been happily
corre&ed by Mr. Dollond’s valuable difcovery o f achromatic
glaffes ; but how far this invention is applicable to the improvement
of microfcopes, has -not yet been afcertained; and indeed
there is fome reafon for fuppofing, they cannot be fuccefsfully
applied to them; fo that this obftacle remains.yet to be remedied,
before microfcopes can be faid to have received their ultimate
degree of perfe&ion.
O f t h e M a g n i f y i n g P o w e r s o f t h e M i c r o s c o p e .
We have already treated o f the apparent magnitude of obje&s,
and {hewn that they are meafured by the angles under which they
are feen, and that this angle is greater or fmaller, according as
the obje& is nearer to, or further from, the eye; and con-
fequently, the lefs the diftance at which it can be viewed, the
larger it will appear ; but from the limits of natural vifion, the
naked, eye cannot diftinguilh an obje& that is very near to i t ; yet,
when affifted by a convex lens, diftin& vifion is obtained, however
fliort the focus of the lens, and confequently, how near
foever the obje& is to the e y e ; and the fhorter the focus of
the lens is, the greater will be the magnifying power thereof.
From thefe confiderations, it will not be difficult to eftimate the
magnifying power of any lens ufed as a fingle .miciofcope, for