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of Edinburgh, Session 1866 - 67 . 
it is derived, and that the difference between the refractive indices 
of these portions increases with the refrangibility of the rays at 
RY and Y. 
The compound spectrum MN, AB, fig. 2, is therefore composed 
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 quinine 
by Sir John Herschel, and in fluor spar and other substances 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 spectrum are 
rendered visible by being reflected and scattered by the particles of 
fluorescent bodies. 
