From George Gabriel Stokes   17th Decr 1864

Lensfield Cottage Cambridge | 17^th Dec[embe]^r 1864

My dear Tyndall,

I feel confident I never expressed an opinion as the impossibility of a raising of refrangibility as a general physical law. I expressed a strong opinion as to the generality of the law as applied to the phenomenon of internal dispersion (fluorescence) (See my paper¹ in the Phil. Trans. for 1862 Part II arts. 80, 229, 240) from the uniform result of my experience.

As to whether the rendering wholly luminous by a concentration of rays of non-luminous heat should or should not be included under the term fluorescence, is of course mainly a question of definition of terms. It seems to me better not to include it. For though there is this feature in common—that luminous rays are emitted in consequence of the action of incident non-luminous rays, of course of a different refrangibility—there is this striking difference, that in the one case is body is raised, by non-luminous heat, to a temperature at which bodies <hereafter LT Typescript> in general become luminous, in the other body not so raised in temperature, a cold liquid for example, gives out it may be blue rays, an effect to be obtained by temperature only by going much beyond a r<adiant> heat.

Yours very truly | G. G. Stokes

RI MS JT/1/S/238

RI MS JT/1/TYP/4/1397