201 Sir David Brewster on the Radiant Spectrum. 



If we now refract laterally these homogeneous radiant spectra, 

 fig. 3 will be changed into fig. 6, fig. 4 into fig. 7, and fig. 5 into 

 fig. 8, thus proving that the radiant portion of the spectra con- 

 sists of rays more refrangible than the portion R, Y, and V from 

 which it is derived, and that the difference between the refractive 

 indices of these portions increases with the refrangibility of the 

 rays at R, Y, and V. 



The compound • spectrum M N, AB, fig. 2, is therefore com- 

 posed of all these separate spectra ; and if we refract it laterally, 

 as shown in fig. 9, we produce the oblique radiant spectrum 

 M' N', A' B', thus proving that the radiant image consists of rays 

 more refrangible than the homogeneous light from which it is 

 derived. 



In a rude experiment with a prism of flint glass, whose mean 

 index of refraction was 1*596, the index of the extreme violet was 

 1*610, and that of the centre of the radiant image 1*640. 



In the preceding experiments the radiation is produced by the 

 action, on the retina, of the small and bright image of the sun ; 

 but the same results are obtained, and more distinctly exhibited, 

 by placing a surface of finely ground glass either on the front of 

 the prism, or behind it, and near the eye. 



The existence of a radiant image beyond the violet end of the 

 spectrum, as in fig. 2, is a fact difficult to explain. I have had 

 an opportunity of describing or showing it to several distinguished 

 philosophers — to the Marquis Laplace and M.Biot in the autumn 

 of 1814, and more recently to others, by some of whom the ex- 

 periments have been repeated; but no explanation of them has 

 been suggested, excepting the untenable one that the separation 

 of the radiant image from the ordinary spectrum might be the 

 result of parallax. 



A better theory, and one of great interest, if true, may be sought 

 in the phenomena of fluorescence, discovered in sulphate of qui- 

 nine by Sir John Herschel, and in fluor-spar and other sub- 

 stances by myself, and in the beautiful explanation of them by 

 Professor Stokes. In this theory the invisible radiation of the 

 chemical rays is rendered visible by being scattered by granular 

 surfaces, just as the invisible chemical rays in the ordinary spec- 

 trum are rendered visible by being reflected and scattered by the 

 particles of fluorescent bodies. 



