342 
referred to, whom we believe to have had great 
practical experience in the matter, “ for pro- 
ducing these results. Close study of the for- 
mule 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 
law alae surface in view at once, such a 
microscope is inappropriate ; but for the pur- 
pose of minute examination of those in which 
MICROSCOPE. 
the may be studied independently, we 
orl ng as the best substitute for the achro- 
matic microscope; and we can ly 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 
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 jth of an inch 
focus should be employed; and the 
should successively diminish down to a dou- 
blet of jth 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 s of ieces which 
are intended to afl 7 the inner the field in 
the most advantageous manner, without 
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 pro 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 nee 
the diminution of. aberration. e are nie 
selves, however, in the habit of employing an 
pd gw 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 distinet and 
well illuminated over every part, i 
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 | 
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, 4 
j 
q 
