The Microscope. 19 



microscopicul objects, even the most advanced-, is likely to meet 

 with the trouble. At the beginning of his c.ireer he will meet 

 it at the start and suffer severely from its ravages. But care 

 and intelligence will reduce the abominable trait of all micro- 

 scopical cements to subjection. Still, there are some that have 

 the undesirable quality more fully developed than some others. 

 To fasten down the cover over a preparaiion mounted in a 

 fluid is an important function of the cement, but to build up a 

 cell to hold the object and the surrounding medium is as impor- 

 tant, and one with which the beginner should early become 

 familiar. The weight of the cover, thin and light as it is, would 

 often -be heavy enough to crush the delicate object and make it 

 unfit for microscopical study. The microscopist aims to have 

 his specimens, so far as the structure and the arrangement of 

 the parts are concerned, exactly as nature made them ; a crushed 

 and distorted object may not tell the truth as to the structure 

 and arrangement of the parts The object must be protected from 

 the weight of the cover while the cover itself protects it from 

 outside influences. A cell must therefore be made deep enough 

 to hold the object and the preservative medium, and high enough 

 to lift the cover an infinitesimal distance above the object. This 

 distance must be microscopic or it may make the cell so deep 

 that the object will float about in the mounting medium, if that 

 is a fluid, and may, when the slide is pla(;ed on the s.age of the 

 microscope, slowly glide down to the lower edge and so inter- 

 fere with the examination. A cell that is too deep may also 

 interfere with the action of the objective by removing the object 

 so far from it as to prevent the proper focussing. Some object- 

 ives have a very short focus, and the cell must be made accord- 

 • ingly. For some of these very high power and short focus 

 objectives, the object is often mounted on the lower surface of 

 the cover, without a cell, so as to bring it within optical reach 

 of the lens. This is always done with the test diatoms that are 

 used to show the good qualities of certain objectives, and the 

 mounting medium is usually Canada balsam, or something sim- 

 ilar that will become hard when dry, and so lift the cover a 

 minute distance above the slide to protect the object from any 

 destructive pressure from the weight of the protecting glass, 

 and thus take the place of the cell. This mounting on the cover 

 is not often necessary with ordinary objects for study with even 



