144 MICROSCOPIC OBJECTS. 



glass a larger diamond is necessary, while for very thin 

 glass Mr. Pritchard recommends a writing- diamond with 

 a well-tm-ned point. 



(3.) To select Glass for Slides. — If you determine to 

 employ plate-glass, the principal point to be considered 

 is the colom* and surface. The former should be of a 

 liglit blue tint, and the latter free from holes and flares : 

 its thickness not more than |^th of an inch. Window 

 or table sheet-glass is, however, the most generally used, 

 and requires to be free from veins and minute bubbles. 

 The colour is best observed by looking along the edge of 

 pieces several inches wide. 



(4.) On Edging Glass Slides. — The edges when cut by 

 the diamond are rough, and unless intended to be covered 

 with paper should be ground smooth. This may be 

 effected by rubbing them on a flat cast-iron plate with 

 emery and water. It is well, for this purpose, to have 

 three of these flat plates, or tools as they are technically 

 termed. They should be about 7 inches in diameter and 

 -|- an inch thick. These tools are sometimes made of 

 bell-metal, brass or pewter : but for common use cast-iron 

 wiU answer the purpose ; it is less costly, and any num- 

 ber may be readily cast from one wood pattern. By having 

 three flat tools, and occasionally grinding one alternately 

 against the other two, and these two against each other, 

 all errors in their surface may be destroyed. Thus, if 

 two tools both hoUow in the centre be ground together, 

 they wiU be brought flatter, and thus by grinding them 

 with the third tool other errors are corrected. When the 



