Ch. XV]
DARK GROUND ILLUMINATION
647
will appear changed as in the case of projection. The sharpness of
the pattern, and the intricacy of design are however much finer
than it is possible to project.
Fig. 374. Demonstration of the Abbe Diffraction Theory to a Single
Observer.
L Arc.
C, Condenser.
S Pinhole or Slit.
C2 Condenser, preferably an achromatic combination.
G Grating.
T Telescope with high power eyepiece.
The telescope is focused on the grating and the diffraction pattern is focused
just in front of the telescope objective. By placing suitably shaped dia¬
phragms at this point, the image as seen in the eyepiece will be modified.
Dark Ground Illumination : Method of Striae
§ 912. Many beautiful experiments in Physics and Chemistry
can be shown by what is best known as the Schliren-methode of
Toepler. This method will yield results almost as striking as those
obtained by polarized light.
See Wiedemann Annallen, CXXXI, p. 33.
The use of this method enables one to demonstrate any slight
lack in homogeneity of a medium which is sufficient to deviate a
beam of light.
To adapt this method to projection the following apparatus can
be used:
§ 913. Apparatus for the experiments with striae.—
(1) Magic lantern with the usual equipment of arc lamp,
projection objective, and the first element of the large condenser.
(2) A special condensing lens or combination. This need not
be of especially large diameter or short focus (5 cm. diameter, 20
cm. focus will answer), but it should be as free as possible from
spherical and chromatic aberration, and must have no scratches
and be kept perfectly clean.