214 MOUNTING OF OBJECTS. 



whether as transparent or as opaque objects. Thus they may be 

 set up dry, or in Canada balsam, or in some preservative liquid ; 

 they may need to be simply covered with thin glass, or they may 

 require to be surrounded by a "cell;" if they are to be viewed 

 by transmitted light, they must always have glass below them ; 

 but if they are to be seen by the light reflected from their sur- 

 faces, they may often be preferably mounted on wood, card, or 

 some other material which itself affords a black background. 

 In almost all cases in which transparent objects are to be 

 mounted, use will have to be made of the slips of glass techni- 

 cally called slides or sliders, and of covers of thin glass, and it will 

 therefore be desirable to treat of these in the first instance. 



116. G-lass Slides. The kind of glass usually employed for 

 mounting objects, is that which is known as " flatted crown;" 

 and it is now almost invariably cut, by the common consent of 

 Microscopists in this country, into slips measuring 3 in. by 1 in. ; 

 for objects too large to be mounted on these, the size of 8 in. by 

 1 J in. may be adopted. Such slips may be purchased, accurately 

 cut to size, and ground at the edges, for so little more than the 

 cost of the glass, that few persons to whom time is an object 

 would trouble themselves to prepare them ; it being only when 

 glass slides of some unusual dimensions are required, or when it 

 is desired to construct "built up cells" ( 136, 137), that a facility 

 of cutting glass with a glazier's diamond becomes useful. The 

 glass slide prepared for use, should be free from veins, air bubbles, 

 or other flaws, at least in the central part on which the object is 

 placed ; and any whose defects render them unsuitable for ordi- 

 nary purposes, should be selected and laid aside for uses to which 

 the working Microscopist will find no difficulty in putting them. 

 As the slips vary considerably in thickness, it will be advanta- 

 geous to separate the thick from the thin,&nd both from those of 

 medium substance : the last may be employed for mounting or- 

 dinary objects ; the second for mounting delicate objects to be 

 viewed by the high powers, with which the achromatic condenser 

 is to be used, so as to avoid any unnecessary refraction of the 

 illuminating pencil by the thickness of the plate which it has to 

 traverse beneath the object ; whilst the first should be set aside 

 for the attachment of objects which are to be ground down, and 

 for which, therefore, a stronger mounting than usual is desirable. 

 Where very hard substances have to be thus operated on, it is 

 advantageous to attach them in the first instance to pieces of 

 very thick plate-glass; only transferring them to the ordinary 

 slides, when they have been reduced to nearly the requisite thin- 

 ness ( 129). 



117. Thin G-lass. The older Microscopists were obliged to 

 employ thin laminae of talc, for covering objects to be viewed 

 with lenses of short foci ; but this material, which was in many 

 respects objectionable, is now entirely superseded by the thin 

 glass manufactured for this express purpose by Messrs. Chance 



