MOUNTING AND PREPARATION OF OBJECTS. 39 
of the mounting medium; and it is also desirable by 
choosing a medium of proper density to minimize as far 
as possible the errors due to excessive refraction. Thus, 
when starch is mounted in air, the difference in refrac- 
tive index is so great that the heavy black edges produced 
are most deceptive, while in clove-oil the difference is too 
slight for clear definition. 
2. Temporary Mounting Media.—When an object is 
of such nature that it can be well examined in air, it may 
be placed upon an ordinary glass slide (usually 3 in. by 
1 in., best with ground-glass edges) and examined directly 
under the microscope. Even in this case, however, if 
the high objectives are used, the specimen should be 
covered with a cover-slip in order to protect the front lens. 
Of the cover-glasses commonly sold the square ones are 
most convenient for ordinary handling, and the round 
ones best suited for making permanent mounts, as will 
be seen later. The effect of the thickness of the cover- 
slip upon microscopic vision has already been consid- 
ered in Chapter II. In fine work it is often desirable to 
measure the thickness of the covers used in order to select 
those of a standard size or to adjust the objective or 
draw-tube to those which deviate from it. This may 
be effected conveniently by means of micrometer calipers 
or with certain special forms of apparatus placed upon 
the market for this purpose (Fig. 22). 
Most objects are more clearly seen in some denser 
medium than air, and, when mounted in any liquid, a 
cover-slip is always necessary even for low powers. The 
choice of a medium must be determined by two consider- 
