the Doctrine of Radiant Energy. 267 



Sir John Herschel's presentation of the matter * is not very 

 expHcit. " The solar rays, then, possess at least three dis- 

 tinct powers : those of heating, illuminating, and effecting 

 chemical combinations or decompositions ; and these pov/ers 

 are distributed among the differently refrangible rays in 

 such a manner as to show their complete independence on 

 each other. Later experiments have gone a certain way to 

 add another power to the list — that ot exciting magnetism." 

 Although the marginal index runs " Calorific, luminous, and 

 chemical rays," the choice of words in the text, as well as the 

 reference to magnetism (for surely no one believed in a special 

 magnetizing entity), points to the conclusion that Herschel 

 held the modern view. 



For the decade between 1850 and 1860, the citation upon 

 which I most rely as indicative of the view held by the 

 highest authorities, and by those capable of judging where the 

 highest authority was to be found, is from Prof. Stokes's cele- 

 brated memoir upon Fluorescence f. On p. 465 we read : — 

 " Now according to the Undulatory Theory, the nature of light 

 is defined by two things, its period of vibration, and its state 

 of polarization. To the former corresponds its refrangibility, 

 and, so far as the eye is a judge of colour, its colour," And 

 in a footnote here appended: — 



" It has been maintained by some philosophers of the first eminence 

 that light of definite refrangibility might still be compound ; and though 

 no longer decomposable by prismatic refraction may still be so by other 

 means. I am not now speaking of compositions and resolutions depen- 

 dent upon polarization. It has been suggested by advocates of the undu- 

 latory theory, that possibly a difference of properties in lights of the same 

 refrangibility might correspond to a difference in the law of vibration, 

 and that lights of given refrangibility may differ in tint, just as musical 

 notes of given pitch differ in quality. Were it not for the strong convic- 

 tion I felt that light of definite refrangibility is in the strict sense of the 

 word homogeneous, I should probably have been led to look in this direc- 

 tion for an explanation of the remarkable phenomena presented by a 

 solution of sulphate of quinine. It would lead me too far from the subject 

 of the present paper to explain the grounds of this conviction. I will 

 only observe that I have not overlooked the remarkable eftect of absorbing 

 media in causing apparent changes of colour in a pure spectrum ; but this 

 I beheve to be a subjective phenomenon depending upon contrast." 



It can scarcely be necessary to insist that " light " is used 

 here in the wider sense, a large part of the memoir dealing 

 with the transformation of invisible into visible light. 



The allusion in the note is, of course, to Brewster. This 

 distinguished discoverer never accepted the wave theory, 

 and was thus insensible to the repugnance with which his 



* Art. Light, Enc. Met. 1830, § 1147. 



t " On a Change of Refrangibility of Light." Phil. Trans. 1852. 



