MOUNTING AND PREPARATION OF OBJECTS. 57 



brush charged with asphaltum round their edges. 

 This operation is facilitated if the thin glass is held 

 in the clip 4, fig. 23. The cement should be as 

 thick as possible to prevent its running in between 

 the glasses. Preparations liable to injury from pres- 

 sure, will if thin, require a ring of cement painted 

 round one or both glasses and allowed to dry before 

 the preparation is placed between them, and if thick 

 a ring of paper or zinc answers the same purpose. 



Whole insects or opaque parts of the same are 

 rendered transparent by soaking in turpentine for 

 two or three days, and afterwards mounted in 

 balsam. 



The balsam must be first evaporated by gentle 

 heat, and when it has attained the consistency of 

 resin, it is dissolved in an equal quantity of bensol 

 and kept in a bottle with an air-tight glass cap for 

 use. 



Balsam should only be used when nothing else 

 will answer, as its high refractive index renders the 

 finer details of certain structures invisible, besides 

 the yellow colour of this medium causes as much 

 diminution of the actinic power of the light, in the 

 illuminated parts of the field, as the shaded portions 

 of the object itself; thus markings, plainly visible 

 in the microscope, fail to make any impression on 

 the sensitised surface of the plate. 



Damar, being whiter in colour and its refractive 

 index less, gives better results. As a cement it is 

 much superior, not becoming brittle like balsam after 

 some time. 



