The Microscope. 239 



ing it through suitable filter-paper, so as to prevent all chance dirt 

 from marring the mount. When ready to begin a group, fix the 

 cover-glass centrally over the area of squares by means of three 

 little touches of wax, and then also adjust, close to the same cover- 

 glass, one of the cover- glasses containing the diatoms previously 

 selected for the grouping; or, if necessary, two or more, according 

 to the complexity of the proposed design. With the selecting bristle 

 in the right hand, and the eye adjusted to the lens, bring the glass 

 containing the selected diatoms into the field of view, then carefully 

 select as a center, a perfect disc, say, a coscinodiscus ; now shift the 

 gelatined cover-glass into view and deposit the disc at its center, and 

 carefully adjust it so that its center shall seem to cover the intersec- 

 tion of a group of the small squares ; around the disc, as a center, 

 adjust a series of small circular forms, space them at equal distances 

 from each other. Should it next be desired to introduce a series of 

 navicular or slender forms they may be adjusted into position by 

 lining them over the guide lines radiating from centre of disc, or 

 through the diagonals of the squares; in this manner proceed until 

 the design is completed. 



When the grouping is finally inspected, it is permanently fixed 

 to the gelatine layer by holding the slide on a level, under the 

 mouth, and breathing on it very carefully a few times. This is 

 perfectly reliable and more expeditious than breathing through 

 rubber or glass tubes for the same purpose. 



For the purpose of mounting it is well to have a quantity of 

 cells finished on slides and kept on hand. The slides are centered 

 on turn-table and shallow cells of black shellac; are built up to suit 

 the diameter of the cover-glass to be mounted thereon. This cell is 

 filled with a drop of Canada balsam pressed out of a metal tube. 

 The cover-glass containing the arranged diatoms is now freely 

 immersed in filtered spirits of turpentine and also flushed with it, so 

 as to expel all air from the diatoms and to clean off all motes or 

 particles that may have lodged upon it during or after preparation 

 of the same. The cover- glass is then set upon its edge to drain 

 off superfluous turpentine, and while it drains, gently soften the 

 the shellac cell over a spirit lamp, pick up the cover-glass and gently 

 lay it centrally over cell, and press firmly into contact with the cell; 

 the slide is then set aside with the cell side down, and supported on 

 a level, to obviate as much as possible the floating out of place of 

 any of the forms, which are sometimes displaced while drying. 



The procedure described above is essentially that followed by 



