ILLUMINATION OF OBJECTS; REFLECTED LIGHT 29 



appear to be white by reflected light, and black by transmitted 

 light, 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 light 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 lying 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 been almost entirely superseded by devices known as vertical 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, $3) in which the reflecting surface is 

 mounted in a cell attached to the microscope just above the 

 objective. In these devices the reflector, which may be either 

 a mirror or a disk of clear glass, sends the illuminating beam of 

 light through the objective which acts as the condenser, con- 



