'204 PREPARATION 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 

 Ta^s 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 prepared 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 



