12 PREPARATION AND MOUNTING 



As before mentioned, the thin glass is made of various thick- 

 nesses, and the beginner will wish to know which to use. For 

 objects requiring no higher power than the one-inch object-glass, 

 the thicker kinds serve well enough ; for the half-inch the medium 

 thickness will be required ; while, for higher powers, the thinnest 

 covers must be used. The "test-objects" for the highest powers 

 require to be brought so near to the object-glass that they admit 

 of the very thinnest covering only, and are usually mounted be- 

 twixt glasses which a beginner would not be able to use without 

 frequent breakage ; but if these objects were mounted with the 

 common covers, they would be really worthless with the powers 

 which they require to show them satisfactorily. 



It may be desirable to know how such small differences as 

 those betwixt the various thin glass covers can be measured. 

 For this purpose there are two or three sorts of apparatus, all, 

 however, depending upon the same principle. The description 

 of one, therefore, will be sufficient. Upon a small stand is a 

 short metal lever (as it may be termed) which returns by a spring 

 to one certain position, where it is in contact with a fixed piece 

 of metal. At the other end this lever is connected with a " fin- 

 ger," which moves round 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 propor- 

 tion 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 accu- 

 rate degrees. This kind of apparatus, however, is expensive, and 

 when not at our command, the thin glass may be placed edge- 

 wise in the stage forceps, and measured very accurately with the 

 micrometer. 



Cleanliness with the 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 clean- 

 ing 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 



