40 ELEMENTS OF APPLIED MICROSCOPY. 
ations. First, as suggested above, its refractive index 
should be far enough, and yet not too far, removed from 
that of the object to be examined. Second, it should be 
of such a nature that it will not set up destructive, os- 
motic, chemical, or other changes in the substance of the 
specimen. For aquatic plants and animals and a great 
many other objects water fulfils these requirements ; and 
for specimens of animal tissue the normal fluids of the 
body furnish ideal mounting media. With denser ob- 
jects, where it is necessary to minimize contrast effects, 
glycerin or some oil is preferable. 
3. Principles of Permanent Mounting.—In making 
permanent mounts of microscopic objects it is necessary 
to guard against changes due to (a) physical displace- 
ment of the object, mounting medium, or cover; (b) chem- 
ical changes in the substance or the mounting medium or 
loss of the latter by evaporation; and (c) bacterial decom- 
position of the specimen or its substratum. First, then, 
the cover-glass must be firmly attached to the slide either 
by an adhesive mounting medium or by the application 
of a special cement. Second, the medium must be pro- 
tected from evaporation and made of such a nature 
that it will not undergo chemical change. Third, the 
mounting medium must be antiseptic. 
These requirements are fulfilled in three quite differ- 
ent ways: by mounting “dry” or in air, in which case 
the absence of moisture serves as an antiseptic, by mount- 
ing in aqueous media mixed with glycerin, and by mount- 
ing in balsam, a resin which sets and forms a solid mass 
impervious to all external agencies. 
