VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 87 
by sliding the entire ocular in the sleeve or by sliding the lens C 
or both. The clamping joint E permits tilting the condenser 
so as to obtain the correct angle of incidence upon the disk or 
prism. To exclude all other light from the illuminator, a screen 
S is attached to the condenser system. Fastened to S is an arm 
G which carries the radiant R. In the diagram the radiant is 
an acetylene light, adjustable both up and down and forward 
and back in the mounting H. To make the nature of the burner 
clearer the flame is shown with its broad side toward the con¬ 
denser. This is, however, an incorrect position for use, the 
proper position being always with the edge of the flame toward the 
illuminator in order that the full intensity of the radiant may be 
obtained. When, instead of the acetylene burner, a tiny tung¬ 
sten lamp is supplied for use with this device a parabolic cover 
and reflector is placed back of the bulb and holds it in proper 
place against the screen (see Fig. 45, page 101). The light rays 
from the radiant pass through the condenser system, strike the 
reflecting device of the illuminator and are totally reflected 
down through the objective 0 upon the specimen M. The light 
rays reflected from M pass through the objective and strike the 
disk F at an angle other than that of total reflection and thus 
pass through to form the image in the ocular of the microscope. 
Owing to the relatively great distance between the reflecting 
disk F and the objective it is essential that the inner surfaces of 
I, A and O be kept a dull black in order to prevent internal 
reflections. 
The disadvantage of employing ordinary objectives instead of 
those in special short mounts will be apparent at once from the 
diagram, for, as just pointed out, the danger of internal reflec¬ 
tions is very great; moreover, the length of I and A prevent low 
powers from being employed unless the microscope is provided 
with a substage upon which the specimen can be supported. 
With specimens placed upon the stage any attempt to focus the 
upper surface will entail raising the body tube of the microscope 
until the rack and pinion are out of mesh. 
Maintaining the Alignment of Radiant and Illuminator may 
readily be accomplished in microscopes provided with an adjust- 
