338 The Microscope. 



through the weight of the cover- glass, or to protect it from the 

 force due to the subsequent contraction of the mounting-medium, 

 brought about through the evaporation of its generally volatile solvent 

 whilst drying. In working with animal tissues, however, the em- 

 ployment of a cell for the purposes mentioned is seldom required. 

 The writer is of the opinion that if the specimen be sufficiently thin 

 and properly mounted in a balsam which contains not too much 

 solvent, no disturbances of the above character will ever occur to 

 effect any appreciable change in the arrangement or appearance of 

 the cellular elements. When it is desir ed to mount sections of con- 

 siderable thickness, a cell will be found very useful if not necessary. 

 The disadvantage of the cell is that it precludes the use of high 

 powers in the study of the specimen. It not only protects it from 

 violence but from a too close scrutiny as well. To overcome this 

 disadvantage as much as possible a cell should never be any deeper 

 than is absolutely necessary; in fact a theoretically perfect cell 

 should be exactly as thick as the specimen to be mounted. This, of 

 course, is practically impossible, thoixgh an attempt should always 

 be made to attain this perfection. 



Cell-Building. — One of the most simple and oldest ways to 

 make a cell is to take a piece of tissue-paper, or of thicker quality 

 if desired, cut it to the size and shape of the cover-glass, punch a 

 circular hole in the center and glue it to the slip. The section is 

 then arranged within the circular space, the mounting-medium is 

 added and the cover adjusted to fit the paper. When well done, 

 such an arrangement is as good as anything ever devised, the 

 advantage being that very thin cells of known thickness can thus be 

 procured. As, however, the method is rather tedious when com- 

 pared with later devices, the use of the paper cell has now become 

 nearly obsolete. The materials now employed for cell building are 

 the cements mentioned in the last lesson, and, for their better 

 application, an instrument — the turntable — has been invented. This 

 instrument consists of a circular, horizontal, revolving disc, at one 

 side of which is a platform for a hand-rest, the plane of which is 

 slightly above and parallel with that of the disc. On the center of 

 the disc are a number of concentric circles corresponding with the 

 diameters of the various round cover glasses, and which act as a 

 guide to the brush when applying the cement. Clips are also 

 provided for holding the slip. The choice of a turntable is not of 

 very great importance, as they are all based on the same idea, and 

 are all about equally servicable. In the Griffith's self-centering 



