On the Phenomenon of the "Radiant Spectrum." 357 

 The amended equation of Poiseuille thus becomes 



_ 7r>y*(H-F) I _ * v V iH + F/J I 

 '" SYalog* ! " *< H + *> 



F v ; 



v 



The apparent mean velocity v being equal to -— g- it 



becomes more convenient for purposes of calculation to 

 write it thus : 



_ irr yt(H- F) V<Z(H 3 -F 8 ) 



8 Ya log, ? 37raf (H + F) 3 log, ^ 



XXXVI. 0>i £/i£ Phenomenon of the " Radiant Spectrum " 

 observed by Sir David Brewster. By C. V. Raman, M.A., 

 Palit Professor of Physics in the Calcutta University*. 



IN a paper on (t 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 souice appears to shootout 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 mono- 

 chromatic 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 pre- 

 cisely 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 — pre- 

 sumably the corneal corpuscles — present in the refractive 

 media of the eye. The radiant structure of the lialo in 



* Communicated by the Author. Read before the Royal Society of 

 Edinburgh, Nov. 7, 1921. 



