22 PHOTO-MICROGRAPHS. 
(Plate XIII). This diatom is slightly convex, 
and when the centre is in focus, the margins are 
not. This difficulty is best met by the use of 
objectives having the greatest possible penetrating 
power, — focal range, — and by focusing so as to 
obtain the best possible general result, viewing the 
field as a whole. 
When a number of objects which differ in thick- 
ness, degree of transparency, or in color,are grouped 
together in the same field, we have to contend with 
a photographic difficulty arising from the fact that 
the time of exposure required to bring out the 
details is greater for certain of these objects than 
it is for others. Under these circumstances, the 
best we can do is to give such an exposure as will 
produce the best average result. Thus in my 
photograph of the fourth square of Moller’s Typen- 
platte (Plate XII.), the thicker diatoms, such as 
Lupodiscus Argus, would be improved by a longer 
exposure, but this would destroy all the details 
in some of those which are thinner and more 
transparent. 
Photo-micrographs are especially useful for the 
illustration of scientific papers and works requir- 
ing illustrations of this kind, of which only small 
editions are wanted. 
By means of the heliotype process, prints may 
be obtained at short notice, which, being in print- 
er’s ink, are more durable than silver-prints, at a 
cost high | is simply that of printing, for the gela- 
tine plate from which the printing is done is inex- 
pensive, and quickly made from any good negative, 
