ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 
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parts arranged one above the other. The upper part consists of a fixed 
ring, supporting the removable iris diaphragm. This diaphragm is 
operated by a lever, easily accessible from the front of the substage, 
and is so arranged as to come directly in contact with the object-slide if 
desired, thus being in the most effective position for use without the 
condenser. When the condenser is in use, this iris can be used to limit 
the volume of light entering the objective without limiting the angle of 
the illuminating cone. This method of controlling the light is of the 
Fig. 53. 
utmost importance in the examination of highly refractive transparent 
bodies, such as living bacteria, &c. The middle section of the substage 
is movable vertically on the main substage axis, and consists of a ring, 
with centering screws, carrying an Abbe condenser 1*20 N.A. The con- 
denser ring swings laterally to the left of the instrument in such a 
manner that the condenser is entirely out of the path of rays from the 
mirror, and is also perfectly free for changing accessories. The con- 
denser ring, the arm on which it is carried, and the sliding support, are 
