EXPOSURE OF PLATE. 17 
lost or obscured. If we look through an over- 
exposed negative towards a strong light, as in front 
of an open window, we may see the image quite 
distinctly ; whereas the positive picture obtained 
by placing a piece of black cloth or pasteboard 
behind the negative will scarcely be seen at all, or 
will appear weal and indistinct. 
Under-exposure is recognized in developing by 
the slowness with which the image appears, and 
the absence of any photographic imprint from the 
less transparent or more deeply colored parts of 
the field. No amount of developing or intensifi- 
cation will give the necessary strength, even to the 
background, when the plate has been considerably 
under-exposed, and after fixing, the shadows in the 
picture, viewed as a positive, will be entirely trans- 
parent, whereas in the over-exposed plate they 
were grayish or foggy. 
To get the best general result, it will often be 
necessary to give an exposure which is longer or 
shorter than would be desirable for some parts 
of the picture. Thus, if the object be a very thin 
diatom having delicate markings, it will be neces- 
sary to give a brief exposure, for the purpose of 
preserving these, and the background will conse- 
quently be weaker than would be desirable. The 
result will be that in the positive, the outlines of 
the object will not be in strong contrast with the 
ground of the field in which it appears. 
In single objects having regular outlines, —e. g. 
diatoms, — this may be readily remedied by fill- 
ing in the background by hand with an opaque 
