28 PREPARATION AND MOUNTING 



a dial like that of a watch, whereupon are figures at fixed 

 distances. When the lever is separated from the metal, 

 which is stationary, the other end being connected with the 

 " finger " of the dial, that " finger " is moved in proportion 

 to the distance of the separation. The thin glass is, there- 

 fore, thrust betwixt the end of the lever and fixed metal, and 

 each piece is measured by the figures on the dial in stated 

 and accurate degrees. This kind of apparatus, however, is 

 expensive, and when not at our command, thin glass may 

 be placed edgewise in the stage forceps, and measured very 

 accurately with the micrometer, or by the calliper eye- 

 piece described by Dr. Matthews in No. 8, for October, 1869, 

 of the Journal of the Quekett Microscopical Club. 



Cleanliness with thin glass is, perhaps, more necessary 

 than with the slides, especially when covering objects which 

 are to be used with a high power ; but it is far more difficult 

 to attain, on account of the liability to breakage. The 

 usual method of cleaning these covers is as follows : Two 

 discs of wood, about two inches in diameter, are procured, 

 one side of each being perfectly flat and covered with clean 

 wash-leather. To the other side of these a small knob is 

 firmly fixed as a handle, or, where practicable, 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 

 nutgalls, or any of the commonly used grease-removing 

 liquids ; and with some impurities water, with the addition 

 of a few drops of strong acid, will be found very useful, but 

 this last is not often required. 



This method of cleaning thin glass should al ways 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 fingers and thumb with a cambric handkerchief, 



