THE MICROSCOPI8T. 



MOUNTING CRYSTALS FOR POLARIZED LIGHT. 



These must be so enclosed that the air is completely ex- 

 cluded, otherwise a change will take place, and the objects be 

 spoiled. When it can conveniently be done, it is well to 

 mount them in Canada balsam. Sir David Brewster recom- 

 mends mixing cold-drawn castor oil with the Canada balsam. 

 In this case the edges of the thin glass cover should be ce- 

 mented, as the castor oil prevents the balsam from becoming 

 hard. 



Each preparation should be properly labelled, either with a 

 writing diamond on the glass slide, or on the paper cover of the 

 slide ; and it may save trouble if this be invariably performed 

 as soon as mounted. 



