CHOICE AND USE OF APPARATUS 



people imagine and a scratched lens is an inefi&cient 

 lens. When using high magnifications a cover glass 

 should always cover our object and the same remark 

 applies to objects examined in Canada Balsam. This 

 substance is likely to cause serious trouble if it finds 

 its way on to an objective. It must be removed, 

 that is obvious, but it sets hard, it must not be 

 scraped away for fear of damaging the optical glass 

 and, as it is used to cement the lenses together, 

 there is the great danger that any solvent used to 

 remove the Balsam from the face of the objectives, 

 may also dissolve their setting. Our digression may 

 seem somewhat unnecessary, but the very great im- 

 portance of keeping all chemicals and even water, 

 from coming into contact with the lenses of our 

 instrument cannot be insisted upon too strongly. 



In whatever substance we examine our object, 

 water, glycerine or Balsam, there is a right and a 

 wrong way of applying the cover slip. It must not 

 be dropped or laid down flat upon the object, if we 

 do this we shall certainly imprison a number of air 

 bubbles and that must be avoided. One edge of 

 the cover slip must be laid against the edge of the 

 mountant, as the liquid used for mounting our 

 object is called, then placing a needle beneath the 

 cover slip it must be gently lowered into position. 

 We shall now find that all air bubbles are driven 

 out as the cover slip is lowered. 



When we have cut an exceptionally good section 

 or when we have some specially interesting object 

 we may wish to make a permanent slide. The exact 



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