




146 THE NORTHERN MICROSCOPIST. 

2. The specific intensity of the electric light being much greater 
than that of other artificial lights, a sufficient illumination can be 
got by means of a much smaller pencil than it is necessary to em- 
ploy to secure the same luminous intensity with the illumination of 
gas or diffused daylight. Rays much more oblique can be utilised. 
To employ the electric light the lamp is placed in a little case, 
the covering of which is pierced with an aperture. The micro- 
scope is set on the case, the mirror having previously been removed 
from the axis or taken off altogether. The light of the lamp is then 
concentrated by means of a plano-convex lens, and directed to the 
condenser of the microscope. It is by the management of this. 
latter that the illumination is modified. 
V. PHOTOMICROGRAPHY. 
Another advantage of electric illumination is that it allows the 
micrographer to photograph instantly any object which shows itself 
in the field of the microscope. The operation is simple, and it 
will be as well to devote a few lines to speaking of the modus 
operandt. The gelatino-bromide process is the most simple, as the 
plates can be bought ready prepared. We use those of Marion, 22, 
Soho Square, London, which are excellent, and may be preserved 
for any length of time; and for photographic apparatus, the small 
photographic chamber of Ross and Co., which fits into the micro- 
scope in the place of the eyepiece. 
The plate is exposed in the ordinary way for a period varying 
with the light and the magnification employed. We have obtained 
excellent results with the one-tenth of Tolles in ten minutes. 
The plate is allowed to soak in rain water and then put into a 
basin containing 20 c.c. of the developing bath, which is a sufficient 
quantity for quarter plate negative. 
When the image has come out with all its details, the plate is 
taken from the bath, washed carefully and fixed by the aid of a 
rather strong solution of hypo-sulphite of soda. It is then washed 
again with the greatest care, sheltered from the dust, and allowed 
to dry. 
On account of the sensibility of gelatino-bromide plates all the 
preceding operations must be performed in a very weak yellow 
light, and this, moreover, ought to pass through ruby-red glass. 
before being admitted into the room. When the negative is obtained, 
positive proofs are taken in the ordinary manner, or again positive 
enlarged proofs may be obtained on gelatino-bromide paper. For 
this it is only necessary to project the negative by the aid of an 
apparatus for projection—a single sciopticon will do—upon a sheet 
of the prepared paper which has then to pass through the same 
series of operations as the plate. 
In conclusion is given the formula of the developing bath : 

