The Radiant Spectrum

Abstract

PROF. RAMAN has recently directed attention (NATURE, September 1, p. 12) to some observations by Brewster on what the latter called the “radiant spectrum ” (Phil. Mag., vol. 2, p. 202, 1867). Brewster advanced the hypothesis that the phenomenon was due to the granular surfaces of the eye rendering the ultra-violet rays visible by fluorescence. Prof. Raman proposes the alternative hypothesis that diffraction by the corneal corpuscles of the eye accounts for the phenomenon. Brewster's view is at variance with the fact that when a colour screen, opaque to ultraviolet rays, is placed in the optical path between the source, the prism, and the eye, it does not render the “radiant spectrum” appreciably less visible. Fluorescence set up by ultra-violet rays can therefore be safely excluded as a possible cause of the phenomenon. Prof. Raman's view that diffraction effects by the corneal corpuscles of the eye are responsible is at variance with the following facts:—