OP THE MICROSCOPE. 207 



handled. For purposes of mere examination and study, 

 mounting is unnecessary, but when the objects are to be kept 

 for future reference it is indispensable. It is true that where 

 the specimens are large they might be kept in bottles in a pre- 

 servative fluid, and taken out when wanted. This would be 

 very inconvenient, however, and with very minute or delicate 

 objects it would be almost impracticable. 



There are three modes in which objects are mounted: 1. Dry, 

 the object being simply attached to the slide and suitably pro- 

 tected. 2. In balsam, the object being immersed in Canada 

 balsam, damar medium, copal varnish, or some similar mate- 

 rial. 3. In fluid, the object being mounted in some of the pre- 

 servative liquids previously described. Specimens may be 

 mounted in any of these ways, so as to be viewed either as 

 transparent or opaque objects, and the instruments and mate- 

 rials required are neither numerous nor expensive. With those 

 named in the following list almost any ordinary object may be 

 neatly put up, though it is of course to be expected that occa- 

 sions will frequently arise when special instruments and 

 methods, which are not described by any author, will be 

 needed. Experience alone can enable the microscopist to treat 

 such cases successfully. 



SLIDES. Most objects are mounted between two pieces of 

 glass, one of which is called the slide and the other the cover. 

 As it is convenient to have these slides all the same size, so that 

 they may be easily arranged in cabinets, the Microscopical 

 Society of London has adopted a slide three inches long by one 

 inch wide as the standard size for use amongst their members, 

 and this size has been generally adopted by microscopists 

 throughout the world. All the best slides that are found in 

 market are of this size, and the microscopist who fails to adopt 

 it will be subject to great inconvenience when he desires to ex- 

 change objects with others who are pursuing similar studies. 

 Several other sizes are employed by the French, most of them 

 being quite small (2j by i and 2f by ), but as these small slides 

 are the only ones that can be used with some French micro- 

 scopes the stages of which are too small to take a slide 3 by 1 

 they are usually kept in stock by dealers in microscopic appar- 



