38 
ELEMENTARY CHEMICAL MICROSCOPY 
reflecting condenser and the preparation on the stage, a drop 
or two of homogenous immersion fluid is placed between con¬ 
denser and object slide. If, however, an object lies in the path 
of the rays, refraction, reflection and diffraction take place 
and the object becomes brightly illuminated, or if submicro- 
scopic particles are in suspension in the medium between object 
slide and cover glass diffraction patterns result and appear to 
the eye as brilliant points of light surrounded by more or less 
distinct alternate bright and dark rings. These points of light 
exhibit rapid vibratory motions (Brownian movement). To 
prevent axial light from passing through the illuminator an 
opaque stop is placed in the optic axis of the device. The field 
is therefore black or nearly so, save 
for a slight halo at its edges, while 
the objects appear bright or bril¬ 
liantly colored upon a dark back¬ 
ground. 
In Fig, 13 a simple paraboloid 
reflecting illuminator is Shown 
diagrammatically in section, with 
the directions of the light rays 
so exaggerated as to make clearer 
the reason the field of view is 
dark. 
Sections of typical illuminators are shown in Fig. 14, A, 
B, C, D. It will be seen that although the construction 
may be different in different types, the rays emerge at 
approximately similar angles. In illuminators of these types 
(B, C, D) the curvatures of the reflecting surfaces are ground 
after mathematically calculated curves which will bring the 
light rays approximately to a focus at a point just at the 
upper surface of the slide or slightly above this plane. In 
the diagrams for simplicity, cover glasses and preparations 
have been omitted. 
An exception to the above statement, relative to the construc¬ 
tion of reflecting condensers, is found in the Beck^ dark-field 
1 Made by R. & J. Beck, London. 
Fig. 13. Dark-field Illumination. 
