20 SOME HINTS ON THE PREPARATION AND 



in the same manner as previously described for a balsam mount, in 

 which no cell was employed; care being taken to let it fall gently 

 so as to drive out the excess of balsam over the edge of the cell, and 

 so do"wn on to the surface of the slide, without moving the shells out 

 of their positions or leaving any vacuvle within the cell. Should the 

 latter mishap occur, the only remedy is to push the cover off to one 

 side and apply sufficient fresh balsam to fill the vacancy; replacing 

 the cover in position by a gentle sliding motion. This necessity, 

 however, is to be avoided by all means, if possible, since it makes a 

 " smeary" job, very difficult to clean off nicely afterward. Should 

 any small air bubbles yet remain, after all our precautions, they need 

 cause no annoyance or thought, as they will disappear of themselves, 

 as do those remaining in mounts made without cells. The excess of 

 balsam that flows over on the slide during the placing of the cover 

 and closing of the cell, should be roughly wiped off with a wisp of 

 tissue paper, so as to leave but a thin layer for the final cleaning, 

 when the mount is hard and complete. Also at this time the cover 

 should be carefully adjusted with the needles or forceps, so as to 

 rest concentrically upon the surface of the cell, equally distant all 

 around its circumference from the outer edge of the latter; and 

 should be firmly pressed down upon the cell; and over the same. 

 The cover, however, must never be pressed in its centre, or over the 

 balsam. Being elastic, it readily bends downward under such pres- 

 sure, driving out a portion of the balsam, over the edge of the cell: 

 the pressure being removed, the glass returns to its former level, 

 drawing in air to replace the expelled balsam, and thus compelling 

 us to do our work all over again. The slide should now be set aside 

 in a warm but not hot place, and allowed to thoroughly harden, when 

 the superfluous balsam may be scraped off with a dull knife blade, 

 and the final cleaning made with a rag moistened with benzole, or 

 better chloroform; after which soap and water will restore the 

 pristine polish of the glass. ^^ finish must be left to the taste of 

 the worker. If all the proceedings, thus far, have been neatly and 

 carefully executed, the slide will present an exceedingly neat appear- 

 ance, without any ring of cement being run upon the edges of the 

 cell, but it will look still better if finished with white zinc cement, 

 and a narrow ring of color, as described in my first paper, care being 

 taken to coat the edge of the cover, first, with shellac, or some other 

 cement, which is insoluble in balsam or benzole. 



