Cutting, Grinding, and Mounting Hard Tissues 187 



bake until the balsam is quite hard. Grind down to the 

 required thinness on a hone. Wash well with water, dry, 

 add a few drops of fluid balsam in benzole, and apply a 

 cover-glass. 



Metal Specimens. 



The preparation of specimens of metal for the micro- 

 scope involves the greatest care, the principal object being 

 to obtain a perfectly level surface, free from all scratches 

 and marks, with the highest degree of polish. This will 

 be better illustrated by an example. 



The student having obtained a sample of metal, the first 

 thing to do is to carefully file or grind the surfaces he 

 wishes to examine. The marks thus made must be taken 

 out with a very smooth file or emery-cloth, gradually 

 diminishing the coarseness of the cloth until he reaches 

 the finest grade of all. 



From this stage the polishing must be done on parch- 

 ment or chamois leather stretched very tightly on wood, 

 the leather being covered with fine crocus-powder or rouge 

 moistened with a little water. 



This is the most important stage of the specimen, es- 

 pecially if the metal be very soft, and the student should 

 frequently examine the metal through the microscope — a 

 matter of a few moments only — by clamping it in the new 

 metal-holder recently introduced by Messrs. Watson, as 

 shown on page 38. 



It will then be seen that parts stand in very high relief. 

 The object of the leather polishing being to gradually grind 

 away the soft and leave the hard parts, great care must be 

 exercised iu doing this. 



The specimen is now ready for further treatment — viz., 

 etching. The object of this is to further develop the 

 structure, as will be seen from below. 



Etching. — This is done by various reagents, the choice of 

 which is mainly a matter of personal opinion, but perhaps 

 the most generally used, and the best for beginners, are 



