EXPOSURE OF PLATE. 75 
The time of exposure depends upon the nature, and 
especially upon the color, of the object, as well as upon 
the source of light and the optical apparatus used. 
Transparent objects, in which the details are not 
sharply marked, require comparatively little 
time, and if overtimed, all delicate markings or 
details of structure are lost in such objects. They 
should therefore receive a short exposure and 
long development, and may require intensifi- 
cation. 
On the other hand, opaque objects, or such as 
are practically opaque for the actinic rays, namely, 
those of a deep red, yellow, orange, or brown color, 
will entirely arrest the chemical rays, and no de- 
tails of structure can be obtained, no matter how 
long the exposure. 
For this reason, many objects which appear very 
beautiful under the microscope, such as the brown 
septate spores of some of the microscopic fungi 
( Conomycetes), pollen grains of large size and of a 
deep yellow color, deeply stained (carmine) sec- 
tions of animal tissues, etc., are quite unsuitable 
for photography. 
These colors, however, are used in feeble degree 
of intensity, to give photographic definition or 
contrast to transparent objects, and to bring out 
structural details, by virtue of the special affinity 
which certain staining agents have for certain his- 
tological elements of animal and vegetable tissues. 
Violet is theoretically transparent for the actinic 
rays. The colors from the violet end of the spec- 
trum, however, are better adapted for the staining 
