156 PRACTICAL PHOTO-MICROGRAPHY. 
to a poor negative, for it is easy to make a good slide 
from a good negative. But sometimes it is impossible 
to obtain a really plucky negative, the nature of some 
objects precluding this; in such a case, where the 
negative is wanting in contrast, the best plan is to keep 
the exposure as short as is compatible with getting the 
details developed, to develop very deeply and after 
fixation, if necessary, to reduce with weak ferricyanide, 
carefully watching the progress all the time, and stopping 
it when the lights are clear and the image as little 
affected by the reducer as the process will allow. When 
we have a very dense black-and-white negative, the 
plan is to give an exposure which appears even in excess 
of what would be required to give the details, and to 
develop no further than is necessary to. yield the 
required density. 
Whatever developer is used it will always be found 
advantageous to use the clearing solution of acid alum; 
the effect is not invariably patent to the eye, but we 
believe the step is a wise one. And varnishing the slide 
has always had a beneficial effect, not only in tending 
to preserve the slide from damp and mechanical injury, 
but also in clearing it up. When the plate has been 
dried, a colourless varnish, usually cold, is flowed over 
it; and a varnish lately introduced by Messrs. Sanger- 
Shepherd and Co. appears to us as good and as con¬ 
venient as any. This varnish is applied after the 
manner of collodion, being poured in a small pool near 
one corner of the plate, and caused to run evenly all 
the film and off at the opposite corner back 
over 
