Tue Microscope. 83 
to know more about any particular tube cast, to use a higher 
power,— that is to say, a higher capacity, as indicated by the 
(corrections and) numeric aperture of the objective. In such 
cases, it may be well to use a (so-called) homogeneous immer- 
sion objective having an ‘‘ extra limital aperture.” (That sounds 
big, but only means an aperture of “ more than 82° in balsam.”’) 
It is obvious that the use of animmersion objective with a loose 
cover-glass will soon cause trouble, therefore when such an ex- 
amination is to be be made, the cover may be held in place by 
using three small pieces of wax, instead of the hair or paper, 
and pressing the cover down upon these, until the desired dis- 
tance between the slide and the cover has been attained. The 
urine may then be run under the cover by placing a drop at the 
edge. In such cases it is well to use very large covers to reduce 
the danger of moving the slide so far that the immersion 
medium may run over. I have frequently used covers 2 x 24 
inches; the slide being 24 x6inches. Very few stages are fitted 
to take a slide of this size, however. 
And right here, let me condemn such mechanical stages as 
have the milled heads above the stage. They are all well enough 
for amateur work—looking at mounted slides—but the room is 
not there, and the usual form of stage is to be preferred, even 
though the projection of the milled heads may be such as to 
prevent the complete rotation of the stage (and this is a yery 
nice point—to talk about). 
Ordinarily, however, a much smaller slide will answer and 
do good work. It is well to make a practice of uzing very thin 
covers, instead of thick ones, as some particular structure may 
be seen in the field which it is necessary to examine with a very 
high power. In such cases the thicker covers would prevent 
the use of objectives with very short working distance. 
The indications of the various kinds of casts are perhaps 
too well known to need description. 
TURN-TABLE IMPROVEMENT. 
E. H. GRIFFITH. 
URN a disk of zinc white cement on the center of the turn- 
table and when hard, ring with pen and ink. For centering 
purposes the white center is of great value. The cement can 
easily be removed with benzole at any time if desired. 
