Ch. XV] DARK GROUND ILLUMINATION 649
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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