ILLUMINATION OF OBJECTS; REFLECTED LIGHT 29 
appear to be white by reflected light, and black by transmitted 
hght, the result of the scattering of light rays. 
Sheaves and bundles of very fine, long, acicular crystals, of 
white or colorless compounds, usually appear to be yellowish 
or brownish by transmitted light. 
Whenever the problem arises of deciding upon the color of 
an object always: (i) tip and move the mirror; and (2) hold 
a piece of pure white card or paper at a slight angle between 
substage condenser and mirror. Note well the effects of these 
experiments upon the colors seen in the image. If time is taken 
to follow this procedure the worker will rarely be at fault. 
c d. Reflected Light, Axial or Oblique, must be employed 
for the study of the surfaces of opaque objects or for the purpose 
of ascertaining the surface configuration of objects of any nature. 
In investigations of this sort the preparation may be illu¬ 
minated either by rays of light whose paths are oblique to the 
surface of the object and also to the optic axis of the microscope 
or by rays whose paths are parallel (or approximately so) to the 
optic axis and normal to the surface of the preparation. 
Oblique Hght rays are obtained either by means of small reflec¬ 
tors attached to the objective or by directing upon the object 
the rays from a radiant Ijdng above the plane of the surface of 
the object. When a radiant is employed, as, for example, an 
arc lamp or a concentrated filament Mazda lamp, a lens should 
be interposed between light and mirror in order to obtain parallel 
rays and facilitate the proper placing of the illuminating beam. 
Illumination by a reflecting mirror may be obtained either by 
means of the mirror of the microscope, provided its swinging 
arm is long enough to allow raising the mirror above the plane 
of the stage, or by attaching to the objective a reflecting surface. 
This type of illuminator was very popular at one time but has 
been almost entirely superseded by devices known as vertical 
illuminators (see Figs. 32, 33) in which the reflecting surface is 
mounted in a cell attached to the microscope just above the 
objective. In these de\dces the reflector, which may be either 
a mirror or a disk of clear glass, sends the illuminating beam of 
Hght through the objective which acts as the condenser, con- 
