OF MICROSCOPIC OBJECTS. 13 



these a small knob is firmly affixed as a handle, or where practi- 

 cable the whole may be made out of a solid piece. In cleaning 

 thin glass, it should be placed betwixt the covered sides of the 

 discs, and may then be safely rubbed with a sufficient pressure, 

 and so cleaned on both sides by the leather. If, however, the 

 glass be greasy, as is sometimes the case, it must be first washed 

 with a strong solution of potash, infusion of nut-galls, or any of 

 the commonly used grease-removing liquids : and with some im- 

 purities water, with the addition of a few drops of strong acid, 

 will be found very useful, but this last is not often required. 



The above method of cleaning thin glass should always be 

 used by beginners ; but after some experience the hand becomes 

 so sensitive that the above apparatus is often dispensed with, 

 and the glasses, however thin, may be safely cleaned betwixt the 

 finger and thumb with a cambric handkerchief, having first 

 slightly damped the ends of the fingers employed to obtain firm 

 hold. When the dirt is very obstinate, breathing upon the glass 

 greatly facilitates its removal, and the sense of touch becomes so 

 delicate that the breakage is inconsiderable ; but this method can- 

 not be recommended to novices, as nothing but time spent in 

 delicate manipulation can give the sensitiveness required. 



It was before mentioned that the ordinary glass slides were 

 sometimes worthless, especially for fine objects, from having a 

 rough surface, which presented a dusty appearance under the 

 microscope. This imperfection exists in some thin glass also, and 

 it is irremediable ; so that it is useless to attempt to cleanse it ; 

 nevertheless, care should be taken not to mistake dirty glass for 

 this roughness, lest good glass be laid aside for a fault which does 

 not belong to it. 



When any object which it is desired to mount is of consider- 

 able thickness, or will not bear pressure, it is evident that a wall 

 must be raised around it to support the thin glass this is usually 

 termed a " cell." There are various descriptions of these, accord- 

 ing to the class of objects they are required to protect; and 

 here may be given a description of those which are most gene- 

 rally used in mounting " dry " objects, leaving those required for 

 the preservation of liquids until we come to the consideration of 

 that mode of mounting. Many have made use of the following 



