354 
to enter the objective, we cannot but think that 
a source of error still remains, and that the 
most perfect image possible, formed by an 
achromatic object-glass, of an object which is 
artificially illuminated, can only be produced 
when the rays from the source of light take 
exactly the same course as if they proceeded 
from the object itself. That they may have 
this course, they must be made to converge 
upon the object by a condensing lens, whose 
focus for parallel rays (if the illuminating point 
be very distant) shall be the same as the acting 
focus of the objective. A different condenser 
would thus be required for every objective ; 
but the ccpuate of these would be sufficient 
to preclude their general employment. The 
same condenser might be employed, however, 
for several low powers; the highest having 
their own expressly adapted to them. We 
may mention it as a fact for which we cannot 
very well account, that we have been able to 
obtain a very beautiful and distinct illumina- 
tion by the use of an aplanatic-doublet con- 
denser, receiving its rays from the ground glass 
globe of the common table-lamp, which would 
not seem to furnish any of the conditions that 
we have dwelt on as of theoretical impor- 
tance. 
Opaque objects may be illuminated in two 
principal ways, either by a light cast obliquely 
upon them by a condensing lens, or by rays 
thrown upon them perpendicularly by a silver 
speculum fixed to the object-end of the body, 
which receives them from the mirror below, 
Both these modes have their peculiar advan- 
tages, and will be found by experience to be 
applicable with advantage to different classes 
of objects. No general directions on the sub- 
ject can, however, be given. The condenser 
to be used by lamp-light for large opaque ob- 
jects should be a bull’s-eye or hemispherical 
lens of four inches in diameter; by this, 
even from a common candle or flat-wicked 
lamp, a sufficient light may be attained for 
almost any purpose, and with an Argand lamp 
a very powerful illumination is obtained. For 
the parallel rays of day-light, however, an 
ordinary double-convex lens of the focus of 
two inches or more may be employed; this 
may be mounted upon a separate stem and 
foot, as the bull’s-eye should always be, or it 
may be attached to some part of the micro- 
scope itself. We have seen foreign instru- 
ments in which it was fixed to the object-end 
or nose of the body; a construction which we 
deem essentially bad, inasmuch as it then re- 
rinse to be readjusted every time that the 
ocal distance is changed by the substitution of 
one objective for another. A better mode in 
our opinion is to attach it to the side of the 
stage by a jointed arm possessing a pin which 
may be fitted into one of three or more holes 
drilled in the side of the stage with sufficient 
tightness to remain in any position. If the 
wire to which the lens-frame is attached be 
made to through a sprung socket con- 
nected by a ball-and-socket-joint with the 
jointed arm, an immense variety in position 
may be readily given to the condensing lens, 
MICROSCOPE. 
which will render a separate mounting for it 
unnecessary. Ae 
The metallic speculum, or Lieberkuhn, for 
throwing down rays directly upon the obj 
is sometimes attached to the objective i 
sometimes to a tube which may be drawn down 
over it to the required amount. The former con- 
struction is the most perfect, each magnifier 
having its own speculum; but the latter is_ 
most economical, as one peraies serves for 
many objectives. When the latter \ 
tion is employed, the tube should be marked 
with the numbers of the magnifiers at the 
points at which it may be best adjusted for 
each, so that, when the object is in the focu: 
of the objective, it may be in full receipt of 
the rays reflected from the speculum. In this 
mode of illumination the size of the concave 
mirror will be found to have a considerable 
influence ; and the capability of adjusting also 
its distance from the s lum gives a 
variety to the mode of illumination. It can 
never be too fully kept in mind, that, in the 
examination of doubtful objects, no really satis- 
factory result can be attained, until they have 
been viewed in every possible way. When the 
object is not perfectly apaqiie, or does not fill 
up the whole of the field of view, it should 
always be placed on an opaque disc, which 
should be large enough to interrupt all the rays 
passing directly upwards from the mirror to the 
eye. The colour of this disc may be advan- 
tageously varied for certain objects, but in 
general we consider a dead black the best for 
giving them effect. Asa general rule it may 
be remarked, that the fewer the rays entering 
the eye except through the object, the more 
perfect will be the view of it; and if there be 
anything approaching to a glare around an 
opaque object, whether the light di- 
rectly from the mirror, or be reflected back 
from the too-bright surface of the dise, it is 
equally injurious, and will occasion a mis- 
tiness over the object itself. It is evident, how- 
ever, that the size of the disc must be governed 
by that of the field of the objective em d; 
for, if too large, it will interrupt too much of 
the light impinging on the speculum. A piece 
of black glass, mounted upon a long strip of — 
common window-glass, will be 
useful where the object is such that it 
be laid upon it when the microscope is 
clined. hen the object requires to be 
in the stage-forceps, however, a disc of 
card may be placed behind it, so as 
it a back-ground. And for some 
Opaque objects, it is most advantageous to em- 
ploy little concave discs or cups, with th 
interior blackened, in front of which the ol 
jects are to be placed. Similar dark back- 
grounds are useful when oblique light is em-— 
ployed. vy 
Pare 
E 
aE 
i 
be 
' - 
III. MacGniry1nG POWER OF MICRO- 
SCOPES. 
The next point to which we shall advert is 
8 
J. 
tg ss n 
an 
H 
] 
the mode of estimating the magnifying pow 
of microscopes, and of measuring the real — 
size of objects under examination, Our esti- — 
ee Ri 
