138 PREPARATION AND MOUNTING 



course, vary according to the relative sizes of the negative and 

 desired picture. With a one-inch object glass, which is a very 

 convenient focus, it will have to be changed usually betwixt one 

 and four feet. The negative must be lighted by an argand gas- 

 burner or camphene-lamp, and the rays rendered as parallel as 

 possible by the use of a large lens placed betwixt the light and the 

 negative. It is not easy to arrange the apparatus so as to get the 

 light uniform ; but a little practice will soon do away with this 

 difficulty. Ordinary ground-glass is too coarsely grained to focus 

 upon, as the magnifying power used to examine the minute re- 

 flection must be considerable. One of the slides must therefore 

 be coated with the collodion, submitted to the silver-bath, and 

 after washing with water be 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 required to remove the plate a little dis- 

 tance from the object-glass, as object-glasses for the microscope 

 are slightly " over corrected," and the chemical rays which ac- 

 complish the photography are beyond the visual ones. The exact 

 distance required to give a picture to bear the greatest enlarge- 

 ment 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 photography, 

 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 various collodions, 

 lamps, and powers being 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 micro- 

 scope stage and so, to say the least, disfigure it. When the ex- 

 posure has been continued sufficiently long, the pkture may be 

 developed by any of the ordinary methods, but some of the best 

 productions have been brought out by the aid of pyrogallic and 

 citric acid solution, with the addition of a little alcohol. The 

 " fixing " may be effected by a strong solution of hyposulphite of 



