ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
245 
the object coincide with the colourless part of tho image of the source. 
The achromatism of the condenser is therefore so far useful that it 
augments the extent of the image which can be utilized. The illumi- 
nation will also be so much more perfect as the image of the source is 
better defined, so that the result will be more satisfactory if the con- 
denser is aplanatic, if it is well centered, and if it functions in conditions 
for which the curvature of the lenses are calculated. 
The author gives the following practical rules for the regulation of 
the light : — 
(1) Furnish tho instrument with a low objective, and, after having 
centered the condenser, illuminate with the plane mirror and focus the 
object. 
(2) Raise or lower the condenser until the image of the luminous 
source is seen somewhere in the field. 
(3) Make the image coincide with the object by displacing the 
source of light, or, where this is impracticable, by displacing the mirror. 
(4) Exchange the low- for the high-power objective which is to be 
used, and again focus. Centre if necessary, and displace the image of 
the source as before. 
(5) Remove the eye-piece and look into the body-tube at the image 
of the aperture ; then by means of a diaphragm cause the image of the 
luminous pencil to occupy about a third of the total aperture. 
When a lamp is used, the flame should be turned sideways to the 
Microscope. When the sky is the source of light, the condenser is 
focused by bringing into the field the image of some distant object, and 
then slightly turning the mirror to make it disappear. 
If the object is too large to be superposed on the image of the flame, 
or if it is desired to have a large field uniformly illuminated, the con- 
denser must be either raised or lowered. But in many cases, as e. g. 
when the condenser is united to the preparation by an immersion-liquid, 
it is impossible to move the condenser. In these cases a lens known as 
the illuminating lens is interposed between the lamp and mirror. The 
adjustment of this lens usually offers some difficulty. The mode of 
operation is as follows: — 
(1) With the lamp about 0*25 to 0*30 m. from the mirror, the flame 
is centered and focused. 
(2) The illuminating lens is then placed nearly in the focus of the 
flame, and centered by making use of the image furnished by its 
mounting. 
(3) A uniform illumination of the field is then effected by slight 
movements of the lens in its mounting. 
These conditions are very difficult to fulfil when the lens is inde- 
pendent of the lamp ; but the regulation of the light becomes compara- 
tively easy when the lens is united to the lamp by a special mounting 
provided with articulations, by which the lens can be moved in two 
rectangular directions. The most convenient lamp in this connection 
is that of Beck. 
Glasses coloured more or less blue are the best means to employ in 
order to modify the intensity of the illumination. 
For oblique illumination the above rules are no longer applicable. 
It is necessary to proceed by trial. After having correctly regulated the 
