86 ELEMENTARY CHEMICAL MICROSCOPY 



to the condenser system. Fastened to S is an arm G which 

 carries the radiant R. In the diagram the radiant is an acety- 

 lene 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 condenser. 

 This is, however, an incorrect position for use, the proper posi- 

 tion 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 tungsten 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. 48, page 99). 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 objec- 

 tive 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. 



A very simple and efficient device is shown in Fig. 40, in which 

 the radiant moves with the illuminator in focusing, thus avoid- 

 ing the necessity of realignment for different- sized specimens or 

 changes of objective. It consists of a thin, bent aluminum plate 

 S inserted between the body tube T of the microscope and the 

 vertical illuminator I. Being provided with a hole a trifle larger 



