570 Prof. C. V. Raman on the Scattering of 



3000 c.p. silica mercury-vapour lamp with glass dome fur- 

 nishes a suitable source. The lamp is suspended immediately 

 behind a small aperture in a dark screen and is viewed from 

 a distance of two or three yards. A green-ray filter may 

 be put in front of the aperture, but is not essential. An 

 alternative arrangement for obtaining a small and powerful 

 source of homogeneous light is to load the carbon rods of an 

 electric arc with plenty of common salt, and to condense the 

 light from the luminous yellow mantle of the arc upon the 

 pin-hole in the screen. With either arrangement, the 

 appearance of radial streamers issuing from the source as 

 seen in white light is not obtained. We see, instead, a 

 circular area round the source filled with granular patches 

 of light, and outside this, a relatively dark ring, followed by 

 a well-defined circular halo, and some faint outer rings of 

 luminosity. 



A remarkable feature which is worthy of mention is the 

 peculiar circulatory or irregular movement which is best 

 seen in the radial streamers surrounding a white source of 

 light, and less clearly in the granular patches surrounding a 

 monochromatic source of light. These movements disappear 

 gradually when the eye is held with a fixed gaze towards the 

 source, but start again immediately whenever any movement 

 of the eyeball or of the eyelids occurs. 



3. Discussion of Observations. 

 The effects described above obviously present a striking 

 resemblance to the phenomena observed when light is diffracted 

 by a large number of irregularly placed apertures or particles 

 of uniform size, e. g. lvcopodium duststrevvn on a glass plate. 

 Recently, De Haas* has published an elaborate study of such 

 diffraction phenomena, and has shown that the formation of 

 the radial streamers in the coronas surrounding a source of 

 white light, and their replacement by a granular field in 

 monochromatic light, may be explained by considering the 

 interference of light diffracted by individual particles vmich 

 are assumed to be irregularly distributed over the aperture. 

 The resemblance is not merely qualitative, as some careful 

 visual estimates made by me seem to show that the relative 

 intensity of the streamers in the area immediate]}^ sur- 

 rounding the source and in the first circular halo is about 

 the same as that observed when a source is viewed through a 

 glass plate dusted with lycopodium. There thus seems little 



* Proc. Roy. Soc. Amsterdam, 1918, p. 1278. It may be remarked 

 here that the phenomenon discussed in the present paper is different 

 from the coloured haloes seen in certain pathological states of the eye 

 and described by Tyndall in his lectures on Light, and by other writers. 



