250 USE OF PROJECTION MICROSCOPE [Cn. IX 



The three objectives selected should be in a revolving nose-piece 

 (fig. 142) so that one can pass quickly from one power to another. 

 The lecturer and operator must always keep in mind that for an 

 audience giving their entire attention, a delay of even a quarter of 

 a minute seems a very long time, hence every precaution should be 

 taken to avoid delays. 



380. Preparation of the carbons for an exhibition. The 



carbons supplied for projection are soft-cored, and sharpened 

 somewhat like a lead pencil. This end form is unlike that assumed 



FIG. 142. TRIPLE NOSE-PIECE OR REVOLVER FOR QUICKLY CHANGING 



OBJECTIVES. 



(From the Catalogue of Viogtldnder und Sohn). 



in the actual use of the carbons (fig. 141), and until the carbons 

 have burned for some time, one will not get the best light from 

 them. Hence it is wise to get the carbons formed by burning them 

 in the lamp for five minutes or so before using them for a lecture or 

 an exhibition. 



Soft-cored carbons arc a necessity for micro-projection, for the 

 crater remains more uniform and it does not wander around the 

 end of the carbons and thus get out of line of the general axis so 

 frequently as would be the case with solid carbons ( 38oa). 



380a. Cored and solid carbons. Some workers with the projection 

 microscope use a large, cored carbon above (i.e., for the positive) and a solid 

 carbon for the negative one. For example, in a projection outfit from Zciss 

 the upper carbon was 19 mm. in diameter and soft-cored. The lower omega- 



