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. 



i 



FIG. 374. DEMONSTRATION OF THE ABBE DIFFRACTION THEORY TO A SINGLE 



OBSERVER. 



L Arc. 



C, Condenser. 



S Pinhole or Slit. 



C 2 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. 



