128 Radiation of Light from Diffracting Apertures. 



light. The shapes o£ these lines o£ flow of energy would be 

 specially interesting in the neighbourhood of the focus. As 

 a first step towards a detailed study of these problems, a few 

 experimental observations have been made by placing a 

 narrow screen (such as a needle or a plate of very small 

 dimensions) or else a wide screen containing a narrow 

 aperture in some selected part of the field and tracing 

 the phenomena observed in its rear. Some very striking 

 results have been obtained, especially with a circular 

 aperture illuminated by a point source of light. If a 

 small screen is placed in the focal plane so as to cut off 

 the entire geometrical cone of rays, a bright image of the 

 source may be traced along the axis behind the screen and 

 for a considerable distance beyond. This effect is entirely 

 due to the light diffracted by the boundary of the circular 

 aperture "*. With a narrow screen placed in the diffraction 

 field due to a rectangular aperture illuminated by a linear 

 source of light, two independent shadows each bordered by 

 diffraction fringes and differing in intensity have been 

 observed behind the screen, these being formed respec- 

 tively by the two luminous edges of the aperture. If the 

 narrow screen be placed in the region of shadow close 

 to the geometrical pencil of rays, the two shadows differ 

 considerably in their intensity, and the difference becomes 

 less and less as the screen is moved more and more into the 

 region of shadow. The writer hopes to take up the fuller 

 study of these problems at an early opportunity. 



The investigation described in this paper was carried out 

 in the Laboratory of the Indian Association for the Culti- 

 vation of Science. The writer has much pleasure in 

 gratefully acknowledging the helpful interest taken by 

 Prof. C. V. Raman during the progress of the work. 



Calcutta, 

 February 8th, 1918. 



* Porter and Hufford have about the same time observed that the rays 

 diffracted by a circular disk can form an optical image of the source along 

 the axis of symmetry (see Phil. Mag. and Phys. Rev. April 1913). The 

 phenomenon observed by me is somewhat analogous, but differs from 

 that observed by these writers, as the image in this case is formed by the 

 rays diffracted by the boundary of a circular aperture. 



