116 PHOTO-MICROGRAPHS. 
These diatoms should be resolved with ease by 
central light. For the more difficult test-diatoms, 
and especially for Amphipleura pellucida, oblique 
illumination will be required. 
When a considerable number of diatoms are 
arranged upon a single slide, it is impossible to 
make satisfactory photographs of all at one ex- 
posure, as the focal adjustment and time of expos- 
ure which would be best for one is not the best 
for others. In this case, as already stated in 
Section IX., the aim should be to get the best 
average result. 
The photographic method is well adapted for 
the illustration of a work upon the Diatomacee ; 
but as a matter of economy it would be necessary 
to arrange several species upon a single plate. 
The best results would be obtained by making a 
separate negative for each diatom. These might 
be made with an amplification considerably above 
that admissible in the published work, and after- 
wards reduced to the proper size. For this pur- 
pose silver-prints of uniform tone should be made, 
and the diatoms should be cut out and pasted 
on a large sheet of white cardboard. A reduced 
negative should then be made from this, from 
which the gelatine plate used in heliotype print- 
ing could be prepared. 
Another method would be for an expert to 
mount selected diatoms for each plate with special 
reference to uniformity as to amplification and 
exposure required. 
When a number of negatives are used to make 
