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 
Tungsten 
^Filament 
Fig. II. Clamp for holding Silverman 
Illuminator below objectives when 
Fig. 12 . Lamp used in the Silver- 
man Illuminator. 
used with Greenough type Binocular 
Microscopes. 
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 “ daylite ” 
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 
