126 Prof. S. Banerji on the Radiation of Light 



practically zero-value on the boundaries, but much less 

 suddenly than in the case previously discussed. The experi- 

 mental data, the calculated intensities of the illumination, 

 and the theoretical and the experimentally observed positions 

 o£ maxima and minima are shown in Table IV. The agree- 

 ment is fairly satisfactory. For comparison with the 

 preceding case, the illumination curve has been plotted 

 in fig. 4. As in the case of the circular boundary, the 



R*. 4. 



minima of illumination are absolute zeros. It will be 

 noticed also that ^/ 1 changes sign as it passes through 

 its value at the boundary (<£/#= + 1), showing that the 

 radiations emitted by the edge on the two sides of the 

 boundary differ in phase by ir. 



4. Other Forms of Boundary. 



The cases in which the surface is bounded b}^ forms of 

 apertures other than those considered previously are of 

 interest from the point of view of the general theory 

 of diffraction. Figs. 19 and 22 (PI. IV.) represent photo- 

 graphs of the effect observed when the surface is bounded 

 by quadrilateral and triangular apertures respectively. 

 These photographs were obtained using a point source of 

 light and an annular aperture placed symmetrically in 

 the focal plane. It will be noticed that the boundaries 

 appear as black lines with luminous fringes on either 

 side, thus showing a complete analogy with the case of the 

 circular and rectangular boundaries previously considered. 



In order more fully to study the luminosity at the 

 boundaries of the triangular, quadrilateral, and other forms 

 of aperture, an arrangement was devised in which a screen 

 containing a small circular hole could be placed excentrically 

 in the focal plane and rotated in this plane. This hole 



