78 
ELEMENTARY CHEMICAL MICROSCOPY 
These reflectors are mounted upon small metal rods passing 
through the adapters at right angles to the optic axis; a milled 
head at the end of the rod permits changing the angle of incli¬ 
nation of the reflecting surface. 
In several types the lateral opening for the incident light is 
made variable in diameter either by means of an iris diaphragm 
or a rotating collar provided with openings of different sizes. 
A typical prism illuminator is shown diagrammatically in Fig. 
32. The reflecting device consists of a totally reflecting prism 
P so mounted as to permit tipping slightly and thus changing 
r 
the direction of the reflected ray R. Incident light I is projected 
upon the prism through the horizontal opening O. A diaphragm 
D extending not quite halfway across the aperture of the adapter 
serves to screen the prism and to prevent interfering reflections 
from blurring the image formed in the microscope. 
The construction of a disk illuminator is shown in Fig. 33. The 
incident rays I, I strike a glass or mica disk G and are reflected 
by it through the objective attached below. The rays I, I enter 
through a circular opening O. The size of this opening may be 
changed by turning the collar C which is provided with circular 
openings of three different diameters. 
Adjustment of Vertical Illuminators. — WFen the object to 
be examined is small and is supported upon a glass object slide 
