PRACTICAL PHOTO-MICROGRAPHY. 
I 24 
and this can only be accomplished by frequent or 
constant changes of water. A rose tap will wash a 
plate more quickly than any “washing machine” ever 
invented. But if water is allowed to run for a few hours 
into a dish or box containing the plates, they may be 
considered free from hypo. Ten minutes under a good 
rose will effect the same. If we propose to intensify 
later, extra care must be taken in the washing. 
After pyro development, the plates are often stained 
to a yellow colour, which is sometimes useful. But as a 
rule it is well to remove this stain as follows: To a pint 
of saturated solution of common alum add two drams 
of ordinary hydrochloric or sulphuric acid, and immerse 
the stained plate in this till the stain is removed—a few 
minutes. Then wash well. 
Plates are dried in “drying racks,” which are well 
known in photography. We prefer these racks to have 
very wide grooves, so that we can let the plates lean 
slightly over, face downwards; any dust falling then 
lights on the back rather than on front of the plates. 
If a negative is valuable, or if we expect to take many 
prints from it, we should varnish it with one of the 
varnishes used by photographers. The coat of varnish 
should not be thicker than necessary, as a thick coat 
would prevent good contact between plate and paper 
or lantern-slide plate in “contact-printing.” 
