1921-22.] On the Phenomenon of the “ Pacliant Spectrum.” 25 
also Phil. Mag., September 1867), which has not up to now been satis- 
factorily accounted for, and to which my attention has been recently 
drawn by Dr C. G. Knott while I was on a visit to Edinburgh. 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 diffraction-halo, 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. 
{Issued se^xtrately December 27 , 1921 .) 
