Bauhaus-Universität Weimar

Titel:
Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Person:
Gage, Henry Simon and Henry Phelps Gage
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39438/658/
650 
DARK GROUND ILLUMINATION 
imperfections of the second condenser Q, as to show 
the specimen. The difficulty can, of course, be les¬ 
sened by drawing the condenser face C*, out of the focus 
of the objective O. Dust, fingermarks, etc., will then 
produce a general blur, rather than a distinct image. 
§ 915. Foucault’s method.—A slight modification 
of the old method of Foucault for testing telescope ob" 
jectives also gives good results. This method (fig. 377) 
dispenses with the use of the first diaphragm Di, the 
crater of the arc being the first diaphragm in this case. 
Instead of the ordinary condenser, is substituted a lens 
or set of lenses which are to form a very sharp image of 
the crater. Just in front or behind the objective 
(wherever the sharp image of the crater is formed) is a 
diaphragm which just covers up the crater image. Such 
a diaphragm may be made by fastening a round 
piece of black paper to a piece of plate glass. The ob¬ 
jective brings the specimen to a focus upon the screen 
in the usual way. Any inhomogeneities in the speci¬ 
men scatter the light so that some of it gets by the 
central stop at D2. It is this scattered light which 
serves to form the image on the screen at a1. ' 
If the inhomogeneities of the specimen are great 
enough, the specimen may be projected by the method 
just described, (see fig. 377), except that instead of 
using a central stop, the lens is provided with an iris 
diaphragm, and the image of the arc is focused on 
Fig. 377- Dark Ground Illumination, 
Foucault’s Method. 
L Arc. 
C Condenser of long focus, as perfect as can be found. 
5 Specimen. 
a Inhomogeneity in the specimen which deviates the light. 
0 Objective. 
Dj Central stop to intercept the direct light from the arc. 
a' Image of a, projected on the screen. 
The dotted lines show the course of the rays deviated by an 
inhomogeneity in the specimen which pass to one side of the cen¬ 
tral stop and reach the screen.
        

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