24 
Proceedings of the Royal Society of Edinburgh. [Sess. 
II. — On the Phenomenon of the “ Radiant Spectrum ” observed by 
Sir David Brewster. By 0. V. Raman, M.A., Palit Professor 
of Physics in the Calcutta University. Communicated hy Dr C. G. 
Knott, F.R.S., General Secretary. 
(MS. received July 27, 1921. Head November 7, 1921.) 
In a paper on “ The Scattering of Light in the Refractive Media of the 
Eye” published in the Philosophical Magazine for November 1919 (p. 568), 
I discussed the explanation of the luminous effects observed when a' 
small brilliant source of light is viewed directly by the eye against a 
dark background, and especially of the marked difference between the 
cases in which the source emits white light and highly monochromatic 
light respectively. In both cases the source appears to be surrounded by 
a diffraction-halo: but the structure of the halo is markedly different in 
appearance. In the former case, the source appears to shoot out streamers 
of light radiating from it in all directions, these streamers showing marked 
colour, and in fact appearing as elongated spectra in the outer parts of the 
halo. With the monochromatic light-source, on the other hand, the radiant 
structure of the halo is not observed, and we have instead surrounding the 
light-source a halo showing dark and bright rings and exhibiting a finely 
mottled or granular appearance. It was pointed out in the paper that 
these effects are precisely what might be expected on the hypothesis that 
the halo seen surrounding the source is due to the diffraction of light by a 
large number of particles of constant size — presumably the corneal cor- 
puscles — present in the refractive media of the eye. The radiant structure 
of the halo in white light and its granular structure in highly mono- 
chromatic light is, on this view, due to the field of light diffracted by 
individual particles varying arbitrarily in intensity from point to point 
as the result of the mutual interference of the effects of the large number 
of such particles. A closely analogous structure of the luminous field may 
be observed in diffraction-haloes obtained in other ways, e.g. with the aid 
of a glass plate dusted with lycopodium powder through which a small 
distant source of light is viewed. 
The facts mentioned above provide a very simple explanation of a 
remarkable observation made long ago by Sir David Brewster, and com- 
municated to the Royal Society of Edinburgh {Proceedings, vi, p. 147 ; see 
* See also a brief note by Tait, Proc. R.S.K, vi, p. 167. 
