a 
MICROSCOPE. 343 
‘that a considerable general advantage is hence 
derived. We can regard no microscope as 
complete, without an eye-piece of this kind, 
with a set of ordinary objectives of low powers; 
for it will certainly do what no other combi- 
nation with which we are acquainted can effect. 
The great improvements recently made in the 
construction of achromatic objectives, and the 
unquestionable fact that, for exhibiting the 
‘Minute details of objects, they are infinitely 
superior to all other kinds, have had, we think, 
atendency to blind microscopists to the ad- 
vantages afforded by other combinations, where 
it is desired to obtain a view of the general 
arrangement of the parts of a large object, 
rather than to investigate its minutiz. 
We should recommend, therefore, that every 
achromatic microscope should be fitted with at 
least two Huyghenian eye-pieces, adapted ex- 
ressly to the achromatic objectives; and that 
it should also have a meniscus eye-piece, with 
a set of ordinary object-glasses of long focus. 
And the best substitute for such a microscope, 
at least for the purposes of anatomical or phy- 
siological research, we believe will be found 
in Mr. Holland’s doublet microscope, which 
should be furnished with his eye-piece for 
doublets already described, and with a me- 
niscus eye-piece and ordinary object-glasses of 
low power. In this last form of compound 
microscope, there is the further advantage, 
that the high magnifying power of the doublets 
and triplets employed as objectives renders 
them available as simple microscopes; and 
this cannot be said of achromatic object-glasses, 
which have not yet been usually made of 
shorter focus than one-tenth of an inch, and 
which are not, therefore, of much use in them- 
selves. No one, however, can be regarded as 
entitled to form positive conclusions in regard 
to difficult questions of microscopic enquiry, 
until he has availed himself of the very best 
means of observation at his command; and 
these are certainly to be found only in achro- 
Matic microscopes of the highest class. 
For viewing large opaque objects, achromatic 
objectives of low power are often very useful, 
on account of the large quantity of light they 
admit, which supersedes the necessity of arti- 
ficial illumination ; this is a particular advan- 
tage in anatomical investigations, in which it 
is often especially necessary to avoid the re- 
flection of condensed light from the surface of 
the object, on account of the confusion which 
is thereby occasioned. 
The achromatic objectives at present usually 
made on the continent consist of sets of three 
or more, of which one, two, or three may be 
used at once. In this manner considerable 
variety of power may be gained; but the 
highest degree of perfection in the performance 
must be sacrificed to obtain it, since no single 
objective consisting of two lenses only can be 
thoroughly corrected, and each combination 
ought tobe corrected for itself alone. The 
best achromatics made by British artists consist 
of combinations of two or three compound 
lenses, which cannot be separated; and thus 
Fig. 165. 
° 
Section of the English achromatic combination. 
every required power must be furnished by a 
distinct combination. The expense of a mi- 
croscope fitted with the requisite number of 
these, however, is a great bar to its general 
employment. Other combinations have been 
constructed, therefore, in which the lens next 
the object may be removed, so as to diminish 
the magnifying power considerably ; and the 
corrections are so adjusted as to be nearly the 
same when the two or when three compound 
lenses are used together. The difference be- 
tween the performance of the best of these, 
and that of those most perfectly adjusted, is 
not, for general purposes, of much importance. 
Two sets of these separating lenses,—a high 
and a low one,—giving four powers, therefore, 
which may range from an inch and a half to 
one-eighth of an inch focus, will adapt the 
microscope, with the eye-pieces we have men- 
tioned, to a great variety of purposes. 
The power may be further varied by length- 
ening the body of the microscope, by drawing 
out the eye-piece, which should always be 
made capable of this kind of movement. This 
operates by increasing the distance from the 
object-glass of the image formed by it, and 
therefore augmenting the size of the image; 
the object must of course be brought some- 
what nearer on the other side. We have al- 
ready stated that the length of the body cannot 
be much increased with advantage; but a mo- 
derate variation will be found useful in many 
ways. It enables the magnifying power to be 
‘adjusted to almost any point intermediate 
between those given by the different objectives. 
Thus, one may give a power of 80 diameters, 
and another a power of 120; by using the first, 
and drawing out the eye-piece, the power may 
be increased to 100. Again, it is often very 
useful to make the object fill up the whole, or 
nearly the whole, of the field of view. This is 
especially the case, when it is itself not very 
transparent, and requires a strong light to 
render its details visible; in which condition 
a glare entering around its edges would very 
much interfere with its distinctness. When 
opaque objects, also, are being viewed by con- 
densed light, in the modes hereafter to be 
stated, it is often extremely desirable to make 
them, or the discs on which they are mounted, 
fill up the whole field. In either case the 
