352 
field of fourteen inches. In viewing large ob- 
jects for which great perfection is not required, 
we find it advantageous to interpose a ground- 
glass at about half or three quarters of an inch 
distance beneath them ; thisis easily adapted to 
the under side of the stage. It serves to pro- 
duce an extremely equable diffusion of the 
light over a large field, and to deaden the glare 
which is occasioned by the direct admission of 
so large a quantity ; but, if applied to objects 
which require to be seen with great distinct- 
ness, it will be found to produce a kind of, fog 
which seriously impese the power of the micro- 
scope; and with objects of any difficulty it is 
quite inadmissible. In viewing objects, how- 
ever, of the largest size that the microscope can 
receive, by a good diffused daylight, the ground- 
glass is never required. A bright cloud, oppo- 
site to the sun, is then the best source of illu- 
mination. For the purpose of effecting these 
and other adjustments of the mirror, we have 
found it convenient to have the tube which 
carries it sprung, so as to slide rather loosely on 
the stem, and to secure it in any particular 
situation by means of a large milled head which 
screws on one end of the tube, and clamps it 
upon the stem. 
The plane mirror, with a condensing lens 
between it and the stage, is preferred by many 
to a concave mirror; and for certain objects it 
is, without doubt, superior. For ordinary use, 
however, we prefer the concave mirror, for the 
following reasons :—Its effects are obtained by 
a single adjustment ; whereas, in the use of the 
plane mirror and lens, two adjustments are re- 
quired: and if (as we shall presently state to 
be often desirable) the mirror be thrown quite 
out of the axis of the optical part of the instru- 
ment, it is difficult to adjust the condenser 
with correctness.. Further, in order to obtain 
light enough for a field such as we have men- 
tioned, the condensing lens must be nearly 
three inches in diameter, and thus becomes 
very cumbrous and inconvenient. For objects 
of a high class the concave mirror should of 
course not be employed; but for these the 
common condenser is by no means adapted, 
and must be put aside for a superior one, such 
as we shall presently describe. The plane mir- 
ror and condenser enable the observer, how- 
ever, to obtain an additional variety of illumi- 
nation, which is often advantageous; and by a 
simple modification, proposed by Mr. Varley, 
the light of a bright cloud may be artificially 
imitated by them. The means of doing this 
consist in covering the surface of the plane 
mirror with carbonate of soda or pounded glass, 
by which the direct solar rays are reflected very 
much as by a white cloud. We have also seen 
a plaster of Paris mirror employed for the same 
purpose, and with good effect, where, on ac- 
count of the transparency of the object, it was 
necessary to reduce the amount of light sent 
through it, without interposing any screen that 
should produce indistinctness. 
It is often very desirable to throw the centre 
of the mirror a good deal to one side of the 
axis of the body of the microscope, so that the 
reflected rays may fall very obliquely upon the 
MICROSCOPE. 
object, and cause its prominences and ‘depres- 
maes to exhibit tales of much greater depth’ 
than can ever be seen with more direct light. 
No microscope, in which there is nota provision 
for this movement, can be edas having 
its resources properly developed ; and we have 
seldom seen one which comes * to our 
of the degree in which it should be permitted. — 
In general, the mirror-frame is im ; 
fixed to the sprung tube which carries it; and 
thus it can only be turned out of the axis at a 
angle which will evidently interfere with its u 
In our own microscope, the mirror-frame is 
connected with the tube by a stalk of an inch 
in length, so that the centre of the mirror is 
three inches from the stem. This, of . 
involves a lengthening of the stage, and of the 
arm which carries the body, in order that the 
centre of the apertures of all three may be in 
the same line; but the disadvantage hence 
resulting is easily avoided by increasing the 
strength of these nak: The riety of illaiite 
nation which may be given by a mirror fitted 
in the manner we have described, is very preat ; 
and some very curious and unexpected pheno- 
mena are not unfrequently disclosed by means 
of it. For example, the field may be rendered 
almost dark, by turning the centre of the mirror 
considerably out of the axis, so that none of tl 
rays reflected by it pass up the body of 
microscope; whilst objects of great delicac 
will frequently appear brilliantly illuminated, 
on account of their retention of ra ny the 
light which is passing obliquely through them. 
In this at be we die often Sen enabled | 
see an immense number of the minutest ani- 
malcules (monads) rapidly moving through 
water, in which, with direct light, none but the 
larger ones could be distinguished ; and the 
interest of the spectacle is heightened by the 
phosphorescent glow which the anima Ss 
appear to have when thus illuminated.* A 
similar effect may be produced without the 
use of the concave mirror, by causing a dire 
pencil of rays from a lamp or candle to be 
thrown very obliquely upon the object by 
means of a condensing lens. ij i>-tn 
In the examination of many. delicate objects 
with high powers, direct light will often 
found more advantageous than reflected. T 
may be obtained with facility, by placing a 
lamp or wax-candle behind the stage, irror 
being turned out of the way. In the me 
the direct light of a bright cloud will often be 
found to produce an extremely beautiful effect; 
+ 
but in order to attain this without an inconve- 
nient position of the head, the microscope § 
must agp etal to such a height, t whi 
the stem is horizontal, or even inclined in 
position contrary to the usual one, the eye-piece ~ 
may be at a convenient height for the eye. y 
different modes of illumination best suited to” 
different objects can only be found out by ex- 
perience, since achromatic objectives vary in 
their relative effects with each. 
pes 
* This method of viewing objects biel fo 
noticed, a few years since, as a new discovery; it — 
had long, however, been familiar to ourselves, — 
and, we belieye, to most other scientific observers, 
