80 Colours and their Relations. [January, 



and scattered by reflection in all directions ; but it is an 

 entirely new set of waves owing their origin to the vibrations 

 of the molecules or ultimates established by the motive 

 energy of the incident light — these vibrations being of the 

 same rate as those producing certain colours when they sub- 

 sist in the ether. 



Among other phenomena which favour this latter view is 

 that presented by the scarlet geranium. It has long been 

 observed that the colour of that flower continues to glow 

 w r ith apparently deeper intensity in the twilight. Now were 

 the colour produced by the scattering in every direction of 

 a portion of the incident light deprived of all its con- 

 stituent waves, save those which combine to produce scarlet, 

 the colour ought to become sensibly weaker as the incident 

 light diminishes. But its appearing more intense, after the 

 incident light has been greatly weakened, tends to prove 

 that the scarlet colour is really produced by the vibrations 

 of the colouring-matter of the petal — these vibrations sub- 

 sisting for a considerable time after the stimulus of the in- 

 cident light is lessened, and generating by their reaction 

 vibrations in the ether synchronous with themselves. The 

 apparent increase of intensity in the twilight is due to the 

 circumstance that the scarlet colour is then less diluted with 

 that portion of the incident light which is actually scattered 

 in all directions from the surface of the petal during sunshine. 



In the majority of cases the nature of the action is masked 

 by the circumstance that the molecules or ultimates cease 

 to vibrate almost immediately after the stimulus of the in- 

 cident light ceases, though some wall-papers show their 

 colours for a few seconds after the extinction of a candle, 

 which has been placed near them. Nor is this owing to the 

 mere persistence of the image on the retina ; for it continues 

 after the brighter image of the candle itself has disappeared. 

 The same view is also strengthened by the phenomenon of 

 lustre. For lustre is simply a portion of the incident light 

 scattered from the coloured surface in every direction ; but 

 it is quite distinguishable from the coloured light of the 

 surface itself, generated by the vibrations of the particles 

 of which the coloured body is composed. 



In all inorganic bodies the intrinsic colour is for the most 

 part equably distributed over the surface or throughout the 

 mass. In some chemical compounds the different ultimates 

 constituting the compound molecule vibrate at different 

 rates; but these become so blended as to produce compound 

 tints ; and they cannot be separated by submitting the sub- 

 stance to microscopical examination. 



