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 

 I 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- 

 sHp 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- 



