102 
PRACTICAL PHOTO-MICROGRAPHY. 
be calculated from that one. But when we come to 
alter the aperture of the condenser, for example, to suit 
our objective and our object, and specially when we 
come to deal with thick and thin sections, and still 
more with different colours, we find ourselves entirely 
at a loss for any useful rule. 
We are in the same uncertain state as to the sensi¬ 
tiveness of our plates, which is, of course, an important 
factor in the determination of exposure ; for none of the 
standards of plate-sensitiveness allow for colour in the 
object. Consequently we do not propose to give any 
rule for exposure, preferring to advise the beginner to 
make experiments for himself, and to observe carefully 
the results of his various exposures. We shall also 
advise that the developing solution, once chosen, shall 
not be materially altered, and we shall give such details 
in a later chapter (XI.) as may serve to indicate in what 
direction the worker may have erred in exposure, by 
pointing out the appearance and character of negatives 
when mistakes have been made in this respect. 
Reds and yellows in our objects may entail an 
enormous increase of exposure ; blues and violets are 
highly actinic, and require little exposure ; if the latter 
colours are pale it will often be found very difficult to 
get a good strong background without over-exposing 
the object itself. The yellow screens used with ortho- 
chromatic plates may increase the necessary exposure 
for an ordinary plate twenty or more times, while an 
orthochromatic plate may require, with the same screen, 
only two or three times the normal exposure. 
