250
USE OF PROJECTION MICROSCOPE
[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
Objectives.
(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-