358 On the Phenomenon of the "Radiant Spectrum" 



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 o£ 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 ly co- 

 podium powder through which a small distant source of 

 light is viewed. 



The facts mentioned above provide a very simple explan- 

 ation of a remarkable observation made long ago by Sir 

 David Brewster, and communicated to the Royal Society of 

 Edinburgh (Proceedings, vi. p. 147 * ; see also Phil. Mag. 

 September 1867), which has not up to now been satisfactorily 

 accounted for, and to which my attention has been recently 

 drawn by Dr. 0. G. Knott while I was on a visit to Edin- 

 burgh. Brewster noticed that when a spectrum of a small 

 brilliant source of white light is formed, either by a prism 

 or by diffraction, and viewed directly by the eye, a patch of 

 light is seen lying in the continuation of the spectrum well 

 beyond its violet end and exhibiting streamers radiating 

 from its centre. That this is a diffraction-effect is shown by 

 the fact that a similar and even more striking effect may be 

 observed in the diffraction -halo due to a glass plate dusted 

 with lycopodium held together with a 60° glass prism before 

 the eye, when a small distant source of white light is viewed 

 through the combination. The prism disperses the image of 

 the source into a spectrum. It also disperses the diff'raction- 

 haio, and since the diffraction-rings are of different size for 

 the different wave-lengths and are shifted to different extents 

 owing to the dispersive power of the prism, the achromatic 

 centre of the halo is shifted laterally to a considerable 

 extent, its new position generally lying at a point much 

 removed beyond the violet end of the spectrum of the source 

 itself. The elongated spectra which form the radiating 

 streamers are rotated through various angles by the dispersion 

 of the prism, being drawn out laterally on one side and shut 

 up or drawn together on the other side, and they then 

 appear to diverge from the shifted position of the achromatic 

 centre of the halo, which, as remarked above, now lies well 

 beyond the violet end of the spectrum of the source. The 

 analogy between this effect and Brewster's phenomenon is 

 so striking that there can be no doubt that the latter is 

 essentially of the same nature, the diffraction in this case 

 being due to the structures within the eye itself. 



* See also a brief note by Tait, Proc. E. S. E. vi. p. 167. 



