THE GERMAN CHOLERA COMMISSION. 103 
one-fourth inch up, by placing a diaphragm in the back of the 
objective employed.* The diaphragm is, however, to be taken out 
again in every case when the objective is used for transmitted light. 
Objects not transparent cannot be viewed by this illumination, as 
the working rays of light have to pass through. 
The polariscope can be used in connection with this apparatus. 
For this purpose the condenser must have room enough underneath 
the stage to have an attachment for holding the polarizer. Polarized 
light can be used then for central as well as oblique illumination. 
In using the condenser, the plane mirror is generally used. 
Only when viewing with very low powers, when the plane mirror 
does not completely illuminate the whole field of view, the concave 
_mirror is used. In every instance where the mirror is once adjusted 
for full illumination, the changing of the diaphragms does not 
affect it. 
When using lamp-light, it is recommended to use as large a con- 
densing lens as possible, or perhaps a large glass ball filled with 
water, in order to secure an evenly illuminated field of view with- 
out moving the flame too near the microscope. The condensing 
lens or the glass ball is placed in such a position between the lamp 
and the microscope that an image of the flame is projected on the 
plane mirror. 
When, in using immersion lenses, very oblique illumination is 
desired, or when dark field illumination under high amplification 
is used, it is advantageous to place a drop of water on the upper 
surface of the condensing lens of the apparatus, so as to fill up the 
space between it and the under side of the object-slide with a 
medium denser than air. 
The usefulness of this apparatus has been recognised by all who 
have beccme familiar with its use, and it is not only employed as 
an ordinary accessory, occasionally, but as a constant auxiliary in 
daily application.— American Monthly Microscopical Journal. 
THE GERMAN CHOLERA COMMISSION. 
SECOND report from Dr. Koch, as head of the German 
Cholera Commission, has been published in the German 
papers. Dr. Koch states that, subsequent to the issue of his last 
report, several post-mortem examinations of cholera subjects were 
* The aperture shutter would be useful here.—Eb. 
