512 Prof. C. V. Raman and Mr. V. S. Tamma on a 



normals to which generate the surface of a cone, and hence 

 the intensity of the disturbance at the singular point must be 

 very great in comparison with that at other points on the 

 wave. On emergence from the crysial the singularity per- 

 sists, and, since in its immediate neighbourhood the wave- 

 front is approximately symmetrical in shape about the axis 

 of the cone of normals, it can easily be seen that the ad- 

 vancing front would retain the same general configuration 

 exhibiting a singularity or concentration of luminosity along 

 its course, as indicated in fio-. 1. 



Fig-. 1. 



We have thus in effect a continuous image of the source 

 along a line. When the source instead of being placed on 

 the surface is removed to a distance from the crystal, the 

 waves which diverge from it are in the first instance 

 spherical, but on entry into the crystal these divide at once 

 into two sheets, the points of intersections. of which must be 

 in the nature of foci or concentrations of luminosity in the 

 wave-front. On emergence of the waves from the crystal 

 the same effect is propagated outwards, giving a continuous 

 focus or image of the source in much the same way as in the 

 ca<e illustrated in fig. 1. When the luminous source is of 

 finite dimensions we have an image corresponding to each 

 point of it, and it is easy to see that we should have a com- 

 plete picture built up which would be of the same dimensions 

 as the source and similarly oriented. 



The spectral dispersion of the image is also easily under- 

 stood. For, the direction in which the singularity travels 

 within the crystal being inclined to the normal to the plate, 

 the direction in which the singularity travels on emergence 



