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 O 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- 



