ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 35 



of the tungsten filament; the source of light therefore occupies 

 approximately two-thirds of a circle. The lamp is shown in Fig. 

 12, natural size. With low powers and the illuminator therefore 

 some distance above the object, almost axial rays are projected 



FIG. ii. Clamp for holding Silverman 

 Illuminator below objectives when 

 used with Greenough type Binocular 

 Microscopes. 



FIG. 12. Lamp used in the Silver- 

 man Illuminator. 



from the side, but with higher powers or with the illuminator 

 well lowered the illumination becomes more and more oblique. 



The student must always remember that a change from one 

 magnification to another in order to better resolve an object 

 is also accompanied by a corresponding change in the character 

 of the illumination which of necessity must produce a change 

 in the appearance of the structural details being studied. This 

 type of illuminator is in no manner a substitute for a vertical 

 illuminator but has a field of usefulness distinctly its own. The 

 lamps are made in colorless (clear) glass or blue " day lite " 

 glass, the latter approximating north sky illumination. 



Illumination by Combined Reflected and Transmitted Light. 

 This system is commonly resorted to in the photomicrography 

 of opaque objects in order that in the finished photograph they 

 may be made to stand out more prominently and that their 



