342
MICROSCOPE.
referred to, whom we believe to have had great
practical experience in the matter, “ for pro¬
ducing these results. Close study of the for¬
mulae for achromatism given by celebrated ma¬
thematicians will do much ; but the principles
must be brought to the test of repeated expe¬
riment. Nor will the experiments be worth
any thing, unless the curves be most accu¬
rately measured and worked, and the lenses
centred and adjusted with a degree of preci¬
sion which, to those who are familiar only with
telescopes, will be quite unprecedented.” We
are not favourable to the use of a very high
magnifying power in the eye-piece; and we
believe that it will be discontinued in propor¬
tion to the perfection attained in object-glasses.
We have seen microscopes of foreign construc¬
tion in which only object-glasses of compara¬
tively long focus were employed, and the re¬
quired power was made up by the great con¬
vexity in the eye-glass ; but the performance of
these was not to be compared to that of instru¬
ments of British manufacture. Our own expe¬
rience leads us to think that there are very few
objects of which more can be made out with a
deep than with a shallow eye-piece,—the dimi¬
nution in distinctness and loss of sight being
nearly sufficient to counterbalance the gain
derived from increased power. Hence the mag¬
nifying power of an instrument is by no means
to be regarded as an indication of its excel¬
lence ; for that is to be considered as the best
which, ceteris paribus, will show the most
with the lowest power. It may be scarcely an
exaggeration to affirm that there are few objects
of which the details may not be as well made
out by an achromatic microscope magnifying
but 100 diameters, as by the best ordinary
microscope magnifying 1000 ; and there are
many objects shown with the greatest readi¬
ness by the former, which are totally inscru¬
table by the latter.
Next in order of optical perfection to the
achromatic microscope with the Huyghenian
eye-piece, we are disposed to rank the doublet
microscope, invented by Mr. Holland. This
gentleman has proposed to adapt to his dou¬
blets and triplets a compound body, con¬
taining an eye-piece somewhat resembling the
Huyghenian, but differing from it in having the
lenses fixed at a distance equal to the whole
sum of their foci. “ By this increase of dis¬
tance, light and defining power are gained,
although the magnifying power and the field
of view are diminished ; but at the same time
the latter is rendered very perfect.” Having
ourselves had a microscope constructed upon
this principle, we can speak in very high terms
of its performance. The field of view will
appear very small to those accustomed to the
use of eye-pieces of high power; but every
part of it is brilliantly illuminated, and diffi¬
cult test-objects are exhibited by it with a
sharpness and definition which we have seldom
seen equalled. For objects which require to
have a large surface in view at once, such a
microscope is inappropriate ; but for the pur¬
pose of minute examination of those in which
the parts may be studied independently, we
regard it as the best substitute for the achro¬
matic microscope; and we can strongly re¬
commend it to those who are debarred by the
price of the latter instrument from possessing
themselves of it. In employing doublets or
triplets of high power as objectives in such
a microscope, care must of course be taken
(as when they are used singly) to avoid injuring
them by contact with the object. For the most
difficult test-objects, a triplet of ith of an inch
focus should be employed; and the powers
should successively diminish down to a dou¬
blet of |th of an inch, which may be advanta¬
geously employed for opaque objects. It is
necessary to state that the performance of this
microscope will very much depend upon the
attention paid to the illumination of the
object; on this we shall hereafter treat in
detail.
We shall next speak of eye-pieces which
are intended to increase the size of the field in
the most advantageous manner, without regard
to the perfection of minute details. This ob¬
ject is ordinarily attempted by the substitution
of two plano-convex lenses, or double-convex
lenses of low curvature, for the single lens of
the eye-glass. It has been proposed to make
the same change in the field-glass ; but, in our
opinion, the loss by reflexion from two addi¬
tional surfaces is by no means compensated by
the diminution of aberration. We are our¬
selves, however, in the habit of employing an
eye-piece, which we regard as greatly superior
to that in ordinary use. It consists of a me¬
niscus having the concave side next the eye,
and a convex lens having the form of least
aberration, with its flattest side next the ob¬
ject, nearly resembling, therefore, Herschel’s
aplanatic doublet. The field-glass is a double-
convex lens of the form of least aberration.
With this eye-piece we are enabled to obtain a
field of 14 inches diameter (measured at the
usual distance—10 inches) equally distinct and
well illuminated over every part, and admi¬
rably adapted for the display of sections of
wood, wings of insects, and objects of a simi¬
lar description, and also for opaque objects.
When employing it for these purposes, we
much prefer the use of ordinary double-convex
objectives to achromatic lenses ; for the latter,
being adjusted for a much smaller field, pro¬
duce an image which is only distinct in the
centre ; and the former, being of low power,
may have an aperture quite sufficient to admit
the requisite amount of light. Even with
deeper objectives, the performance of this eye¬
piece approaches much more closely to the
effect of an achromatic microscope, than would
be supposed by those who have only seen the
ordinary one ; and, when made on a small
scale, it may be advantageously substituted for
Mr. Holland’s for all but the most difficult
objects. The two additional surfaces are of
course disadvantageous, by reflecting some of
the fainter rays proceeding from the more
delicate markings ; but the increased magnify¬
ing power is gained with so little aberration,
