82 Colours and their Relations. [January, 



intrinsic colour and fluorescence, this tincture exhibits the 

 transition between fluorescence and dichroism, another 

 example of that tendency to gradation so conspicuous in 

 many natural phenomena. In some varieties of fluor-spar 

 there is dichroism combined with fluorescence, the green and 

 blue fluor imparting to the light transmitted through it a 

 green colour, while the superficial tint is deep blue. In the 

 solution of the disulphide of quinine, again, there is 

 fluorescence without dichroism, the transmitted light being 

 colourless, and only the superficial light exhibiting the blue 

 tint due to its fluorescent property. In uranium glass we 

 have again dichroism combined with fluorescence, the 

 transmitted light being yellow, the superficial fluorescent 

 tint blue. But in the tincture of the bark of the horse- 

 chesnut, when dropped in small quantity into water, there 

 is a curious combination and succession of effects. The 

 transmitted light is at first colourless, as in the case of the 

 quinine solution, while the superficial tint is blue, but 

 deeper than that proceeding from quinine. In a short time, 

 however, the transmitted light in the case of the horse- 

 chesnut bark acquires a straw-colour, which gradually 

 deepens, the blue fluorescence still continuing without much 

 diminution, so that we have again dichroism and fluorescence 

 combined. Ultimately, however, the solution, though ex- 

 ceedingly weak, acquires the tint of brown sherry as respects 

 both the transmitted and the superficial light, the fluorescent 

 blue having gradually died away. 



Fluorescence itself forms the transition between in- 

 trinsic and adventitious colour. There can be no doubt 

 that this phenomenon is caused by the vibration of the 

 molecules of the fluorescent body. The peculiarity is 

 that this motion may be established by ethereal waves 

 lying beyond the limits of the visible spectrum. The most 

 remarkable case is that presented by the extremely minute 

 ethereal waves proceeding from aluminium electrodes, which, 

 as has been shown in a previous essay already referred to, 

 are very far removed from the visible spectrum beyond its 

 violet extremity — beyond even the limits of actinic action. 

 Yet these minute waves can excite in the phosphate of 

 uranium vibrations which, in their turn, originate fresh 

 ethereal vibrations lying within the limits of visibility. It 

 is thus rendered evident that the vibrations excited in the 

 uranium salt are very much slower in their rate than are 

 the ethereal vibrations by which they are established, and 

 that these uranium vibrations in their turn give rise to fresh 

 ethereal vibrations synchronous with their own slower rate, 



