Volltext: Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine

[Ch. 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 
(From the Catalogue of Viogtländer 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 are 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 (§ 380a). 
§ 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 Zeiss 
the upper carbon was 19 mm. in diameter and soft-cored. The lower omega-


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