CHAPTER III. 
MOUNTING AND PREPARATION OF OBJECTS FOR 
THE MICROSCOPE. 
1. The Effect of Mounting Media.—The clearness 
with which we see an object depends in great part upon 
the contrast between its density or color and the back- 
ground furnished by its surroundings. Under the mi- 
croscope little remains but difference in density to empha- 
size outlines, and the distinctness of a given object will 
thus vary widely according to the medium in which it 
lies. The student may obtain an idea of the importance 
of this factor and of the necessity for taking it into ac- 
count in interpreting microscopic appearances, by the 
examination of potato-starch grains in air, water, gly- 
cerin, and clove-oil. In air the grains will show heavy 
black edges and little internal structure. In water the 
edges are less pronounced and the hilum and oyster- 
shell markings appear. In glycerin the same charac-  — 
teristics are still more pronounced, and in clove-oil the 
edges are almost invisible, so that the grains have a spec- 
tral appearance and the hila are very strongly marked. 
Obviously, it is necessary in making deductions as to 
the real structure of an object to consider the influence 
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