OP MICROSCOPIC OBJECTS. 43 



underneath, and worked with a small handle like a common 

 syringe. By turning a small milled head the air may be 

 allowed to re-enter when it is required to remove the bell- 

 glass and examine or perform any operation upon the object. 

 The mode of using this instrument will be described here- 

 after, but it may be here stated that substitutes have beer* 

 devised for this useful apparatus; but as it is now to be 

 obtained at a low cost, it is hardly worth while to consider 

 them. Much time is, in many instances, certainly saved by 

 its use, as a very long immersion in the liquids would be 

 required to expel the bubbles, where the air-pump would 

 remove them in an hour. 



The next thing to be considered is what may be termed 

 CEMENTS, some of which are necessary in every method of 

 mounting objects for the microscope. Of these will be given 

 the composition where it is probable the young student can 

 use it ; but many of them are so universally kept as to be 

 obtainable almost anywhere ; and when small quantities 

 only are required, economy suffers more from home manu- 

 facture than from paying the maker's profit. 



Amongst these, CANADA. BALSAM may, perhaps, be termed 

 the most necessary, as it is generally used for the preserva- 

 tion of many transparent objects. It is a thick liquid resin 

 of a light amber-colour, which on exposure to the atmosphere 

 becomes dry and hard even to brittleness. For this reason 

 it is seldom used as a cement alone where the surface of 

 contact is small, as it would be apt to be displaced by any 

 sudden shock, especially when old. In the ordinary method 

 of using, however, it serves the double purpose of preserving 

 the object and fixing the thin glass cover ; whilst the com- 

 paratively large space upon which it lies lessens the risk of 

 displacement. By keeping, this substance becomes thicker - r 

 but a very little warmth will render it liquid enough to use, 

 even when to some extent this change has taken place. 

 When heated, however, for some time and allowed to cool, it 

 becomes hardened to any degree, which may be readily 

 regulated by the length of time it has been exposed and 



