ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 37 



card at an angle of about 45 degrees between condenser and 

 mirror, or place a disk of ground glass in the ring attached to 

 the lower part of the condenser, thus obtaining, in part, reflected 

 light and a gray or white background. 



The ring attached to the lower part of the condenser and 

 arranged to swing aside serves to carry disks of blue glass to be 

 employed when working with artificial light. By this means a 

 much less fatiguing illumination is obtained, and providing the 

 proper intensity of cobalt glass is at hand, white light giving 

 proper color values is secured. Blue glass should always be 

 placed below the condenser when working with yellow artificial 

 lights. Most manufacturers supply blue glass disks with all 

 their Abbe condensers. When the apparatus is to be employed 

 in photography, yellow-green glass disks are furnished to be used 

 as ray filters. 



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 

 reflectors 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 Nernst lamp, a condensing lens is usually inter- 

 posed between light and object in order to concentrate the light 

 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 silvered metal 

 paraboloid. The paraboloid illuminator was very popular at 

 one time but has been almost entirely superseded by devices 

 known as vertical illuminators (see page 76) in which the re- 



