10 PREPARATION AND MOUNTING 



kinds of slides, because they are usually " sheet " glass ; but is 

 seldom found in those of the quality known amongst dealers by 

 the name of " patent plate." This latter is more expensive at 

 first, but in the end there is little difference in the cost, as so many 

 of the cheaper slides cannot be used for delicate work if the 

 mounted object is to be seen in perfection. These slides vary 

 considerably in thickness ; care should, therefore, be taken to 

 sort them, so that the more delicate objects with which the higher 

 powers are to be used may be mounted upon the thinnest, as the 

 light employed in the illumination is then less interfered with. 

 To aid the microscopist in this work, a metal circle may be pro- 

 cured, having a number of different sized openings on the outer 

 edge, by which the glass slides can be measured. These openings 

 are numbered, and the slides may be separated according to these 

 numbers ; so that when mounting any object there will be no need 

 of a long search for that glass which is best suited to it. 



When fresh from the dealer's hands, these slides are generally 

 covered with dust, &c., which may be removed by well washing 

 in clean rain-water ; but if the impurity is obstinate, a little wash- 

 ing soda may be added, care being taken, however, that every 

 trace of this is removed by subsequent waters, otherwise the crys- 

 tals will afterwards form upon the surface. A clean linen cloth 

 should be used to dry the slides, after which they may be laid by 

 for use. Immediately, however, before being used for the recep- 

 tion of objects by any of the following processes, all dust must be 

 removed by rubbing the surface with clean wash-leather or a 

 piece of cambric, and, if needful, breathing upon it, and then 

 using the leather or cambric until perfectly dry. Any small par- 

 ticles left upon the surface may generally be removed by blowing 

 gently upon it, taking care to allow no damp to remain. 



We have before said, that any object to be viewed in the 

 microscope must have its support; but if this object is to be pre- 

 served, care must be taken that it is defended from the dust and 

 other impurities. For this purpose it is necessary to use some 

 transparent cover, the most usual at one time being a plate of mica, 

 on account of its thinness ; this substance is now, however, never 

 used, thin glass being substituted, which answers admirably. 

 Sometimes it is required to " take up " as little space as possible, 



