go NATURALISTS’ ASSISTANT. 
under side of the stage should be bevelled around the central ~ 
opening to admit of oblique illumination, and it is often con- 
venient to have a thread cut in the opening itself to admit of 
o] 
using objectives as “condensers” in using high powers. ‘The 
stage should also be provided with clips to hold the slide in 
any desired position. Stage forceps are more bother than 
they are worth. 
The illumination of the object is accomplished by a mirror 
and by a “bulls eye’; the mirror is supported beneath the 
stage and should be so arranged as to be readily placed at 
different distances from the object and also so that the light 
can be thrown at various angles upon the slide. Two mirrors, 
one plain and the other concave, are usually furnished so 
that varying intensities of light may beemployed. With high 
powers a lens is frequently employed to add to the illumina- 
tion and is interposed between the murror and the stage. 
This is called a condenser. Some microscopes have the 
mirror so arranged as to swing above the stage and_ thus 
illuminate opaque objects; in others this illumination is ef- 
fected by the “bulls eye”’ a large lens of common glass 
mounted on a separate standard. 
It is usual to have some method of cutting off undesired 
rays of light coming from the mirror. This is accomplished 
by having apertures of various sizes so arranged that they 
may be brought beneath the object. Various methods are 
adopted to accomplish this but it is difficult to say which is 
best. The microscope as described with its base, its supports, 
and its means of connection of the various parts form what 1s 
known as the ‘‘stand,’’ and this will now be considered. 
