MICROSCOPE. 353 
When condensed light is employed with 
“4 powers, great care is necessary in order 
to bring out the best effects of a microscope, 
with difficult objects. These remarks apply to 
the simple as to the compound forms of the 
instrument. We do not ourselves consider the 
ordinary condenser as of much more value for 
the higher than for the lower class of trans- 
parent objects; and we think that it may be 
discarded from the instrument without dis- 
advantage. Great assistance may be obtained, 
however, from a well-constructed condenser, 
in the resolution of the more difficult class of 
microscopic objects. That which was pro- 
posed by Dr. Wollaston for use with his 
doublet consists of a tube about six inches 
long, having at the lower end a diaphragm 
with a circular perforation about three-tenths 
of an inch in diameter, through which light, 
proceeding from a radial point or surface, is 
reflected by a mirror below it. At the upper 
end of the tube is a plano-convex lens, about 
three-fourths of an inch focus, with its plane 
side next the observer; the object of which is 
to form a distinct image of the circular per- 
foration in the plane of the object, which 
should be at about eight-tenths of an inch 
from the lens. We consider this instrument 
to be theoretically faulty; inasmuch as the 
point from which the illuminating rays di- 
verge, and the limiting aperture are not co- 
incident, so that pencils brought to a focus for 
the former are not for the latter. The length 
of the tube, again, is an inconvenience, espe- 
cially in the case of small microscopes. Never- 
theless, it is much superior to the ordinary 
form of condenser. An improvement was 
suggested by Dr. Goring, which consisted in 
shortening the tube below the lens, and re- 
moving the stop from that end of it to the 
Other, so that it should be just beneath the 
object; in this manner the illuminating rays 
may be brought to a focus on the object, the 
ale ones being cut off by the stop. 
Ve have derived much advantage from the 
use of this condenser in the small doublet mi- 
croscope already noticed, and by a slight mo- 
dification of it we can obtain a variety of 
illumination, which is often very useful. ‘Our 
_ condenser consists of three tubes, one sliding 
within the other; the outer one is fixed to the 
under side of the stage; the second carries at 
its upper end the stop, the distance of which 
from the object may thus be changed, and the 
inner one carries the condensing lens. A stop 
_ may be screwed into the bottom of the latter, 
if it is desired to adopt Dr. Wollaston’s plan. 
ie have derived the greatest advantage from 
the use of this condenser, however, by using 
it with direct light from a radiant point at 
_ some distance,—a bright cloud, for instance, 
in the day-time,—and a lamp or candle on the 
_ Opposite end of the table at night. In either 
of these cases, the focus of the illuminating 
: may be made coincident with the plane 
of the object, without that glare which will 
_ almost certainly be produced if the source of 
_ light is nearer and more intense. When thus 
a 
" used, Dr. Goring’s condenser approaches in its 
VOL. III. 
character to that of Sir D. Brewster,* which 
we shall now describe, adding some ideas of 
our own in reference to its construction. 
The principle of illumination on which Sir 
D. Brewster lays great, and we think fully- 
deserved stress, is, that the focus of the illu- 
minating rays shall be coincident with the 
object, so that there shall not be two sets of 
rays at different angles, one proceeding from 
the luminous object and the other from the 
object to be magnified. This can only be 
attained in any degree by making the image 
of the illuminating body coincident with the 
object; and it will be perfectly accomplished, 
in proportion as the rays forming that image 
are themselves free from aberration. If, for 
example, the white rays have been separated 
into their component colours by the condenser, 
these colours will be imparted to the object, 
the appearance of which will also be rendered 
less distinct by the spherical aberration of the 
condensing lens. Hence an achromatic lens, 
or, if this be objected to on account of its 
expense, a Herschel’s aplanatic doublet, should 
be used as the condenser; the latter we have 
found to answer very well, as the centre of 
the circle illuminated by it is very nearly free 
from false colour. Further, if a mirror be 
employed to change the course of the rays, 
it should be of metal, in order to avoid the 
false rays reflected from the first surface of the 
glass. If day-light be employed, no other 
recaution is necessary; but if the illumination 
é obtained from a lamp or candle, it will be 
necessary to limit the amount of light ad- 
mitted. This must not be by a stop placed 
beneath the lens, for the reason already speci- 
fied ; but it should be accomplished by a stop 
or shade placed as near as possible to the flame, 
so that the image of that and of the flame 
may be brought—virtually if not exactly—to 
the same focus. We have found that the same 
end may be attained by removing the lamp or 
candle to a greater distance, so as to diminish 
the intensity of the light to the required 
amount, and only a very low flame will then 
be required, as the condensation of the rays 
is much more perfect than with the ordinary 
lens. The achromatic condenser is strongly 
recommended also by M. Dujardin, an eminent 
microscopist of France; and we know that it 
is now considered an indispensable addition to 
microscropes of the highest class, affording, 
as it does, the means of resolving objects, 
previously considered too difficult to admit of 
a clear view of their nature. We are disposed 
to think, however, that some improvement is 
necessary in order to develope the highest 
powers of this instrument. The rays of light 
proceeding from the radiant Ba or object, 
being brought to a focus by the condenser on 
its surface, cross each other there, and should 
proceed to the object-glass of the microscope, 
as if they came from the object itself. Now, 
unless they are made to converge upon the 
object at the same angle at which they diverge 
* Treatise on the Microscope, from the Ency- 
clopedia Britannica. ‘ 
A 
