204 FREPARATION AND MOUNTING 
object-glass, which is of a very convenient focus, it will vary 
usually betwixt one and four feet. The negative must be 
lighted by an argand gas-burner or camphine-lamp, and the 
_ rays rendered as parallel as possible by a large plano-convex 
lens placed betwixt the light and the negative. It is not 
easy to arrange the apparatus so as to get the light uni- 
form; but a little practice will soon obviate this difficulty. 
Ordinary ground-glass is too coarsely grained to focus upon, 
as the magnifying power used to examine the minute reflec- 
tion must be considerable. One of the slides must therefore 
be coated with collodion, submitted to the silver-bath, and 
after washing with water, allowed to dry. Upon this may 
be focussed the reflected image, and its minuteness examined 
with a powerful hand-magnifier, or another microscope 
placed behind in a horizontal position. When the utmost 
sharpness of definition is obtained, it is usually necessary to 
remove the plate a little distance from the object-glass, as 
those for the microscope are slightly over-corrected, so that 
the chemical rays which accomplish the photography are 
beyond the visual ones. The exact distance required to 
give a picture to show the greatest distinctness cannot be 
given by rule; but experiments must be made at first, and 
it will always be the same with the object-glass which we 
have tested. 
The plate may now be propane as in ordinary photo- 
graphy, and placed upon the stage whilst the light is 
shaded. When all is ready, the shade is removed and the 
process allowed to go on, usually for thirty or forty seconds ; 
but no certain rule can be given as to the required time, on 
account of the variety of collodions, lamps, and powers used. 
It may be here mentioned, that it is well to contrive some 
little frame to receive the prepared plate, as the silver-bath 
solution is liable to get upon the microscope-stage, and so, 
to say the least, disfigure it. When the exposure has been 
continued sufficiently long, the picture may be developed by 
any of the ordinary methods, but some of the best produc- 
tions have been brought out by the aid of pyrogallic and 
