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/lit39438/657/
Ch. XV] DARK GROUND ILLUMINATION 649 
1 
1 
'ù 
x 
< 
be quite sharp and some light will escape the edge 
of the diaphragm D«, and reach the screen. 
The result is very striking as even a slight inhomo¬ 
geneity of the medium in the glass cell will deviate 
light sufficiently to pass the second diaphragm and 
thus be seen. 
Suppose a slight cord of; a substance of different 
refractive index from its surroundings to exist at a1 
in fig. 376. This cord will scatter the light. A ray 
which would normally strike D2, and be intercep¬ 
ted, will spread out in all directions. The part of 
this light which strikes the objective will go to form 
a screen image of the cord at a1 (fig. 376). 
The sensitiveness of this method depends upon the 
sharpness of the image of the diaphragm Di, and the 
closeness of adjustment of D2, so as to encroach as 
little as possible upon it. With a very sharp im¬ 
age, it is possible to detect the minutest striae and 
inhomogeneities in the specimen. 
The image sharpness may be disturbed as much by 
imperfections in the condensing lens C2, as by an in¬ 
homogeneity of the specimen, hence these imperfec¬ 
tions, if present, will show distinctly on the screen. 
In fact, the method is as well designed to show the 
Fig. 376. Dark Ground Illumination; Toepler 
Method of Striae. 
L Arc. 
C, First part of the magic lantern condenser. 
/), Diaphragm. 
C2 Condenser of long focus. It must l>e as perfect a lens 
as can be found. 
S Specimen with slight inhomogeneity. 
a An inhomogeneity in the specimen which deviates the 
light. 
Dt Diaphragm intercepting direct light from the lantern. 
0 Objective. ‘ 
a' Image of a. Note that a small objective above the axis 
is used. The dotted lines show the course of the rays which 
are slightly deviated from their original path by the inhomo¬ 
geneity of the specimen. I
        

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