250 THE AMERICAN MONTHLY [Aug 



ing the rock in air and then dropping it into a hot, satu- 

 rated solution of Glauber's salts. On cooling, the pro- 

 cesses of crystallization break up the rock mass. 



Mounts. — Balsam mounts are apt to deteriorate, unless 

 some means are adopted to prevent the access of air or 

 moisture. They may be protected by running a ring of 

 melted paraffin wax around the edge of the cover-glass, 

 by means of the turn-table, and afterwards, when set, 

 protecting this in its turn with a ring of finishing cement. 

 A very beautiful and instructive preparation may be 

 made by heating xylene balsam on a slide until the xylene 

 has almost evaporated, and then adding a few crystal of 

 sulphonal. The preparation should be gently warmed 

 until the sulphonal melts and mixes with the balsam. 

 The cover-glass is then put on. If perfect crystals are 

 obtained this mount will show, with the aid of the polar- 

 iscope, not only the most gorgeous colorings, but also per- 

 fect examples of the black cross. 



Pointer. — An occular pointer is a useful accessory for 

 class demonstrations when it is desired to indicate any 

 particular structure or object in the field of view. To 

 make one, cut a circle of cardboard of a size sufficient to 

 fit easilv in the ocular tube. Out of the centre of the 

 circle punch a second circle, having a diameter slightly 

 larger than that of the ocular diaphragm. Blacken this 

 cardboard ring, and, with a little gum, fasten an eye lash 

 to edge of the smaller circle so that the ciliiim shall pro- 

 ject halfway across the opening. Bemove the eye-lens of 

 the ocular, and drop the cardboard ring into the tube so 

 that it rests on the diaphragm. The cilium is now at the 

 level of the real image, and the specimen on the stage of 

 the microscope can be so placed that the pointer will in- 

 dicate exactly any particular portion to which it may be 

 desired to draw attention. 



Alcohol is used by many microscopists both for killing 

 their specimens and for preparing them for the labora- 



