OF MICROSCOPIC OBJECTS. 27 



"turned" through them, and a small raised handle placed 

 at one end. The diamond must he passed round the inner 

 edge, and so managed as to meet again in the same line, in 

 order that the circle may be true, after which it may be 

 readily disengaged. The sizes usually kept in stock by the 

 dealers are one-half, five-eighths, and three-quarters inch 

 diameter ; but other sizes may be had to order. 



For the information of the beginner it may be mentioned 

 here that the price of the circles is a little more than that 

 of the squares ; but this is modified in some degree by the 

 circles being rather lighter. If appearance, however, is 

 cared for at all, the circles look much neater upon the slides 

 when not covered with the ornamental papers ; but if these 

 last are used (as will shortly be described) the squares are 

 equally serviceable. 



As before mentioned, the thin glass is made of various 

 thicknesses, 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 

 to near to the object-glass that they admit of the very 

 shinnest covering only, and are usually mounted betwixt 

 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 4< finger," which moves round 



