10 REPORT — 1867. 



spectrum of the small circular image S, and AB the spectrum of the radiation, the 

 centre of which is beyond the violet, and nearly in the place where the intensity 

 of the chemical or invisible rays is a maximum. 



In order to analyze this compoimd radiation, let the image of the sun S, fig. 1, 

 be taken from homogeneous red light R, fig. 3, and refracted by the prism, and we 

 shall have its radiation nb at a little distance from the bright portion R, as in fig. 3. 

 In homogeneous yellow light (Y, fig. 4) the radiation ab will be at a greater distance 

 from y than in the red light. In homogeneous riolet light (V, fig. 5) the radiation 

 ah will be at a greater distance from V than in the yellow light. 



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 consists of rays more refrangible than the portion RY 

 and V from which it is derived, and that the difference between the refractive in- 

 dices of these portions increases with the refrangibility of the r.ays .at RY and V. 



The compound spectrum MN, AB, fig. 2, is therefore composed of all these sepa- 

 rate 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 ^^■hicll it is derived. 



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

 w^as 1'596, the index of the extreme violet was 1-ClO, .and that of the centre of the 

 radiant image l'()40. 



In the preceding experiments the radiation is produced by the action, on the 

 retina, of the small and bright image of the sim ; 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 difiicult 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 

 experiments have been repeated, but no explanation of them has been suggested, 

 excepting the untenable one that the separation of the radiant image from the 

 ordinai-y spectrum might be the result of parallax. 



A better theoiy, and one of great interest, if true, may be sought in the pheno- 

 mena of fluorescence, discovered in sulphate of quinine by Sir John Ilerschel, and 

 in fluor spar and other substances by myself, and in the beautiful explanation of 

 them by Professor Stokes. In this theory the ijivisible radiation of the chemical 

 rays is rendered visible by being scattered by gTanular surfaces, just as the invisible 

 chemical rays in the ordinary specti'um are rendered visible by being reflected and 

 scattered by the particles of fluorescent bodies. 



On the Laivs of Symmetry of Crystalline Forms. By A. K. Catton. 



A contribution towards the exjjression of the anyle between the Optic axes of a 

 Crystal in terms of the angles between the faces. By A. R. Catton. 



On the Theory of Double Befraction, ivith speciul reference to the influence of 

 the Material Molecules on the propagation of Light in Crystals. By A. E. 



Catton. 



On a Mechanical Means of producing the differenticd^ motion required to equa- 

 lize the focus for the different planes of a solid. By A. Clavuet, F.Il.S.* 



A New Fact of Binocular Visio)). By A. Claudet, F.B.S. 



Photograpliic Portraits obtained by Single Lenses of Rod- Crystal and Topaz 



By A. Claiqdet, F.R.S. 



* See Proceedings of the Eoyal Society, 180". 



