CHAPTER III. 



MOUNTING AND PREPARATION OF OBJECTS FOR 

 THE MICROSCOPE. 



I. The Effect of Mounting Media. — ^The clearness 

 with which we see an object depends in great part upon 

 the contrast between its density or, colo.r and the back- 

 ground furnished by its surroundings. Under the mi- 

 croscope Httle 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 



38 



