OF MICROSCOPIC OBJECTS. 27 
“turned” through them, and a small raised handle placed 
at one end. The diamond must be 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 
so near to the object-glass that they admit of the very 
thinnest 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. Jor 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 “ finger,” which moves round 
