90 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 

 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 be employed. With high 

 powers a lens is frequently employed to add to the illumina- 

 tion and is interposed between the mirror 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 is 

 known as the "stand, " and this will now be considered. 



