The ^Microscope. 363 



exceptions to this, as we shall see, but it is the rule, and the 

 cover glass brings into play the important law of Nature called 

 capillary attraction. The narrow space between it and the slip 

 below is filled with the mounting fluid and is held in place, in 

 part at least, by capillary attraction. If it was not for this use- 

 ful law of Nature we should not be able to immerse our micro- 

 scopical objects in any preservative media except those that 

 are solid or capable of becoming solid. Such media are used 

 and aro exceedingly valuable. Canada balsam is one of them. 

 It will be referred to extensively hereafter. 



A more important function of the cover glass, yet not the 

 most valuable, is to protect the front lens of the objective. 

 Some objectives have so great a magnifying power that to focus 

 them they must be brought uncomfortably close to the object 

 to be examined, uncomfortably close, that is, for the microscop- 

 ist that is then always in great anxiety lest the front lens of his 

 objective should come in contact with the cover, and be scratched 

 or perhaps broken. The microscopist is always careful to avoid 

 this, for an objective with a scratch on the front lens may be a 

 ruined objective ; if the front lens is broken it will certainly be 

 ruined beyond repair. If there was no thin film of glass above 

 the object, the difficulty in using high power objectives, or indeed 

 any kind of microscope lenses, would be greatly increased. It 

 would be exceedingly awkward to have the object sticking to 

 the front lens when it was racked upward ; and if it happened 

 to be hard or gritty it might do great harm. Without the cover 

 glass certain kinds of objectives could not be used, and the 

 microscopist would thus be forced to do without some of the 

 most important aids to investigation that the optician has given 

 him. These are immersion objectives, with which a drop of 

 water or of oil or of a liquid formed of a combination of various 

 substances, is placed between the front of the objective and the 

 top of the cover glass. These objectives are the finest and most 

 valuable that the working microscopist has ; without them he 

 would be limited to the use of what are called dry objectives, or 

 those that are used with air only between them and the cover 

 above the object. All low power objectives are dry, as are many 

 high powers, but the best of the latter kind are made on a 

 principle rather different from the dry lenses and are worthless 

 without the water or other liquid between their front and the 



