196 PRACTICAL MICROSCOPY. 



its edge is charged with diamond dust in the following 

 manner: Reduce a small splinter of diamond to a fine 

 powder on a hard steel plate, then run a small quantity of 

 tallow on to it, and mix thoroughly with the steel crusher. 

 Press the tip of the longest finger into the mixture, bringing 

 away a thin coat. The slitter should then be moistened 

 with petroline, and a bloodstone or agate pressed gently 

 against it ; at the same time the finger should be brought 

 over the revolving disc in such a position that the edge may 

 just scrape the tallow off. This should be evenly distributed 

 round the edge of the disc, which is readily done by rapidly 

 touching it while revolving. 



The rock, fossil wood, or other mineral, is now to be 

 ground flat on one side, and firmly cemented by old balsam 

 or marine glue (solid) to a glass slide, say 3x2 inches and 

 inch thick. This can readily be slid along the guide- 

 plate B, insuring perfect parallelism, the requisite thickness 

 of each section being regulated by the screw C, which 

 raises and depresses the guide-plate. The sections may be 

 about Tg- of an inch in thickness, more or less, according to 

 the subject, but the thinner they are cut, so much more 

 labour is saved in the subsequent operations. The sound 

 indicates when the slitter is cutting properly. 



One side must be ground down on the leaden lap, which 

 may be substituted for the slitting disc A in the bench 

 shown in Fig. 182. This lap is about 10 inches in 

 diameter and f of an inch thick ; it is used with fine 

 emery and water for the first grinding, the final being 

 performed with still finer emery upon a ground brass 

 lap, preferably made to run in the same direction as the 

 face-plate of a lathe in fact, one of the cheap lathes 

 shown in Fig. 183 will do exceedingly well for this. work, 

 and no doubt the ingenious student will be able to rig 

 up a slitter and leaden lap to fit this form of lathe, 



