148 



MICROSCOPIC OBJECTS. 



These must be so inclosed that the air is completely 

 excluded from them ; otherwise a change will soon take 

 place, and the objects be spoilt. They will keep un- 

 changed in the mounting (§ 6) for many years. When 

 it can conveniently be done, it is as well to mount them 

 in Canada balsam, which renders them less liable to 

 injuiy from exposure to the air than any other plan, and 

 reduces the refraction at the edges. Sir David Brewster 

 recommends mixing cold- drawn castor oil with the Canada 

 balsam. In this case the edges of the slide must be 

 cemented, as the oil prevents the balsam becoming hard. 



(9.) Writing -Diamonds. — Every slide containing an 

 obj ect should have its name legibly written upon it. When 

 the mounting is entirely of glass, this must be wTitten 

 upon it with a diamond. Writing- diamonds are of two 

 kinds ; namely, a splinter — ^that is, a diamond broken into 

 pointed pieces — and the second a turned point : the latter is 

 the most preferable, but is more expensive, the diamond 

 requiring much time and labom- to be turned conical. 



(10.) Papering Slides. — By far the greater number of 

 slides containing microscopic objects require to be covered 

 with paper, to give a neatness and uniformity of appear- 

 ance to them. The best plan for this purpose is, first, 

 to cover the edges with a slip of very thin paper pasted 

 over them ; secondly, take a piece of dark-coloured paper 

 cut rather smaller than the slide, and having a hole 

 punched in the middle to allow the object to be seen 

 through it, and paste it on the under side of the slider; 

 (always select the thickest glass for the under slide ;) 



