THE WINDSOR AND ETON SCIENTIFIC SOCIETY. 151 
images of all the colours of the spectrum, arranged at different 
distances. If a screen is placed anywhere in the series of images, 
it can only be in the right position for one colour, and every other 
colour will give a blurred image, and this source of confusion is 
called chromatic aberration. So much for the eye-piece, and I 
pass on to the objective, and I must confine myself mainly to the 
subject of immersion objectives, and the testing of objectives. The 
fact has been for a long time known that the quality of the image 
of an object depends upon the angular aperture of the objective, 
from the observation that certain points of detail in an object which 
are invisible with an objective of a certain magnifying power can 
be brought out by an objective which has the same magnifying 
power, but greater angular aperture. As a matter of fact, no 
amount of increase of illumination can make a dry lens equal to a 
wide-angled immersion lens. The advantage of wide-angled lenses 
consists in their greater capacity than dry lenses for picking up 
diffraction spectra. 
Let me pass on from the somewhat complex subject of aperture 
to the testing of objectives, as to which the most vague ideas pre- 
vail. The popular idea is that even the highest powers can be 
tested without a condenser, and regardless of the kind of illumina- 
tion which is employed. A great point in the testing of objectives 
is that the person who is testing should be in good health. You 
cannot properly test an objective if you have those unpleasant 
specks known as musce volitantes floating about in your vitreous 
humour. Anyone who has attempted to count the strice of a finely- 
marked diatom with a wire micrometer when troubled with muscz 
will be fully alive to the annoyance which may arise from them. 
Micro organisms (bacteria) stained and mounted in Canada balsam 
afford a very good test for objectives from a half-inch up to the 
highest powers. ‘They should, even with full aperture of the con- 
denser, appear sharply defined, with no appearance of milkiness, 
and surrounded by no spurious nebulous zone. Bacteria can be 
easily obtained from putrid. meat or vegetable infusion, and to 
mount a specimen the following steps are carried out: A small 
portion of the fluid (the size of a pin’s head) is placed in the centre 
of each of two cover-glasses by means of a needle, and one cover- 
glass superimposed on the other, so that the two surfaces of fluid 
are in contact with each other. The cover-glasses are then separated 
by pressure between the fingers, and in this way a thin layer of the 
fluid is obtained on each cover-glass. This is allowed to dry, and 
is then passed several times rapidly through the flame of a spirit 
lamp. A drep of staining fluid, methyl blue, methyl violet, Gentian 
violet, Bismarck brown, fuchsia, or other, is then dropped into the 
cover-glass, and after a few minutes washed off with distilled water. 
The specimen can then be dried by pressure between folds of 
