76 PIHOTO-MICROGRAPHS. 
of objects which are to be photographed by trans- | 
mitted light than are those from the red end of 
the spectrum. 
In making photo-micrographs, the effort of the 
operator should be to produce negatives having 
good printing qualities, from which well-defined 
silver-prints can be made, and in which the back- 
ground shall be as light as possible. The object 
should be sharply drawn, showing all details of 
structure possible. 
When a good field has been found and carefully 
focused upon the screen, it will generally be neces- 
sary to make a number of exposures, in order to 
obtain the best possible photographic result. Each 
plate is given a little more or less time than the 
preceding one, as seems most desirable. If the 
operator is convinced by a careful comparison of 
the negatives that one of them is as good, photo- 
graphically, as can be made, it is a good plan to 
make another exposure of the same duration, so 
that he may have a duplicate in case of injury 
to one of the negatives. All the inferior nega- 
tives may as well be rejected at once, for it is 
hardly worth while to retain poor negatives when 
we have better ones of the same object ; and when 
we have a field upon the screen from which a 
desirable photo-micrograph may be made, it is poor 
economy to spare plates until the best possible 
result has been attained. 
Over-exposure produces a feeble image, which 
appears quickly when the plate is placed in the 
developing solution, and in which the details are 
