78 PHOTO-MICROGRAPHS. 
paint, or by the use of a paper diaphragm in 
printing. 
Again, a marked difference in the transparency 
or color of different portions of the field makes it 
impossible to give the time required to bring out 
the details in certain places without over-timing 
the background, and the more transparent parts 
of the picture. Photographers who are not fa- 
miliar with the difficulties peculiar to photo-mi- 
crography are not fair critics of pictures of this 
kind, which are not to be judged as photographs, 
but as photo-micrographs; and it will not do to 
sacrifice microscopical details for the purpose of 
making an ideal photograph. 
X. DEVELOPMENT OF PLATE. 
BEGINNERS are advised to use the ferrous-oxalate 
developer, and there seems to be no good reason 
why they should change to any other, after they 
have learned the use of this. The formula is 
simple, the developer works in a satisfactory man- 
ner, and, so far as the writer can judge, the results 
are as good as those obtained by any other 
method. 
Formula. 
Oxalate of Potash, a saturated solution. 
Protosulphate of Iron, a saturated solution. 
Filter each solution, and keep in separate bottles. 
Just before using, add one part of the iron solu- 
tion to six or seven parts of the oxalate. The 
