366 The Microscope. 



arranged, and the cover-glass is then placed exactly as heretofore 

 described. Much depends on a correct estimate of the quantity of 

 mounting-medium required. In mounting without a cell this is not 

 of so much importance, for if too much is used it can be squeezed 

 out and cleaned off. But with a cell, although the squeezing i^rocess 

 is simple, the cleaning off is much more difficult. Especially is this 

 true as regards balsam. To clean off the outer wall of a cell 

 streaked with exuded balsam is no easy matter. It is better, how- 

 ever, to use too much rather than too little, for, in the latter case, it 

 will usually l)e found necessary to I'emove the cover and supply the 

 deficiency. Care should be taken also to avoid the inclusion of bub- 

 bles. If bubbles form in a glycerin or dammar mount, no time need 

 be wasted in an attempt to get them out. It is better to do the 

 mount over again. With balsam, especially when quite thin, a few 

 bubbles at the top will probably work out, but if the medium be 

 thick the bubbles may remain suspended in the depth, and not work 

 to the surface until the edge has hardened. The avoidance of the 

 bubbles depends on the care with which the cover is lowered. (If 

 bubbles are present before the cover is lowered, they should be 

 removed by piercing them with a hot needle). In the second lesson, 

 page lOS, will be found full instructions for doing this. It is only 

 necessary to substitute the top of the cell for the slip. 



Sealing. —After the cover-glass has been properly set, it should 

 be held in place by means of a spring-clip. If a balsam mount, the 

 slide should be laid away to harden; if glycerin has been employed 

 it should be cleaned at once and cemented. The cleaning should be 

 done with water, applied with a camel's hair brush, or as a stream 

 from a wash-bottle. Great care must be taken that the water does 

 not get under the cover. After a thorough washing, it shauld be 

 dried and inspected to see if any water or glycerin linger on the 

 surface. If all is dry, it is ready for the seal. With balsam mounts 

 it is only necessary to wait until the exuded medium has hardened 

 when it can be scraped away. Do not attempt to cleanse it with 

 turpentine as it may eat under the cover. All being ready, the clip 

 is to be removed and the slide centered on the turn-table. With the 

 disc rapidly revolving, a light ring is applied by means of a soft 

 camel's hair brush to the juncture of the cover and cell. If a gly- 

 cerin mount, the beginner will probably have trouble, as the cover- 

 crlass, through the pressure of the brush, is liable to slide away from 

 its position. If, however, the pressure is very light, and the force 

 applied from the right direction (which must be learned by experi- 



