336 Prof. Challis on the Undulatory Theory of Light, 



propagation. This effect I have called a 'Transmutation of 

 Rays; 5 and I beg it may be understood that in making use of 

 these terms I mean only to express a result deduced from 

 the mathematical theory.'" In writing the last sentence I 

 had in mind the statement made by Professor Stokes, that 

 change of refrangibility always took place from a greater to a 

 less refrangibility. As there was nothing in my theory of 

 Transmutation which pointed to such a limitation, and as the 

 experimental evidence for it appeared to be only negative, I 

 preferred stating the theoretical results in all their generality, 

 without citing any experiments bearing upon them. But now 

 that the experiments of Dr. Tyndall have shown that this law 

 of transmutation applies to the less refrangible as well as to the 

 more refrangible rays, and that there may be change from less 

 to greater refrangibility, I feel at liberty to say that the theory 

 is in complete accordance with these experimental results. I 

 take this occasion to remark that the term " Transmutation of 

 Rays/* which has acquired special interest since Dr. TyndalPs 

 experiments have shown that it expresses a law of nature, was 

 originated by me, on purely theoretical grounds, in the commu- 

 nication here referred to, published more than eight years ago, 

 and has since been adopted without any reference to its oc- 

 currence in that communication. 



(3) In my Theory of the Composition of Colours, contained in 

 the Philosophical Magazine for November 1856, I have endea- 

 voured, under section (5), to give reasons for a distinction be- 

 tween "terrestrial light/ 5 that is, light which has been reflected, 

 refracted, or generated by terrestrial substances, and direct solar 

 light. I was induced to do this by the persistent assertion of 

 experimenters that a composition of yellow and blue solar rays 

 does not produce a green colour, whereas the composition of 

 such rays emanating from yellow and blue terrestrial sub- 

 stances undoubtedly produces green. More recently, an ex- 

 periment by Sir J. Herschel, described in the 'Proceedings of 

 the Royal Society 5 (vol. x. No. 35, p. 82), has led me to infer 

 that the distinction I sought to account for does not really exist. 

 This experiment renders it very probable that in cases in which 

 green is not perceived to result from a mixture of yellow and 

 blue solar rays, the rays are of too great intensity for the eye to 

 distinguish the colour. At least Sir J. Herschel found, after 

 concentrating a solar spectrum by an achromatic lens, so as to 

 bring the yellow, green, and blue spaces pretty close together, 

 that, on diminishing the intensity of the tight, the green appeared 

 to be so diffused as to encroach greatly on the yellow and blue 

 spaces. When making the experiment of covering white paper 

 with alternating parallel spaces, not inconsiderable in breadth, 



