MOUNTING OF OBJECTS. GLASS SLIDES. 201 



them peculiarly liable to decay, may often be kept, by complete 

 seclusion from the air and by immersion in a preservative fluid, in 

 a state so nearly resembling that in which they were at first pre- 

 pared, that they will continue, during an indefinite length of time, 

 to exhibit their original characters with scarcely any deterioration. 

 The method of ' mounting ' Objects to be thus preserved will 

 differ, of course, both according to their respective natures and 

 also according to the mode in which they are to be viewed, whether 

 as transparent or as opaque objects. Thus they may be setup 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 trans- 

 mitted light, they must always have glass below them ; but if they 

 are to be seen by the light reflected from their surfaces, they may 

 often be preferably mounted on wood, card, or some other material 

 which itself affords a black back ground. In almost all cases in 

 which Transparent objects are to be mounted, use will have to be 

 made of the slips of Glass technically called slides or sliders, and 

 covers of thin glass ; and it will therefore be desirable to treat of 

 these in the first instance. 



148. Glass 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 Micro- 

 scopists in this country, into slips measuring 3 in. by 1 in. : for 

 objects too large to be mounted on these, the size of 3 in. by 1^ 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' (§ 170), that a facility of cutting glass 

 with a glazier's diamond becomes useful. The glass slides 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 ordinary 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 consider- 

 ably in thickness, it will be advantageous to separate the thick from 

 the thin, and both from those of medium substance : the last may 

 be employed for mounting ordinary objects ; the second for mount- 

 ing delicate objects to be viewed by the high powers with which 

 the Achromatic Condenser is to be used, so as to avoid any un- 

 necessary deflection 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 



