ABSORPTION OF LIGHT, ETC. 49 



coloured juice of the plant. It is the same thing with a green 

 leaf. The structure is irregular, optically considered ; there are 

 constantly reflections, backwards and forwards, of light, which 

 penetrates a little depth and is reflected, and has to pass 

 through a certain stratum of this colouring matter, to which 

 the name chlorophyll has been given, but which is really a 

 mixture. That is what takes place generally as regards the 

 coloration of bodies ; it is a phenomenon not of reflection, 

 not of selection of one kind of light for more copious reflec- 

 tion than anqther, but of absorption, or the swallowing up of 

 certain kinds of light. Keflection comes in, in order to enable 

 us to see the light which otherwise would not enter the eye 

 at all, but would go off in another direction. 



The spectrum of this green fluid, which is a , substance to 

 which I have paid a great deal of attention, is very peculiar. 

 It is a mixture of several substances with closely-allied 

 chemical properties. The peculiar spectrum may be seen in a 

 green leaf itself, if you place it behind a slit and analyse it by 

 transmitted light, or allow a strong light, such as that of 

 the sun, to fall upon it and analyse the reflected light. 



Now you will say, Are there no colours in any case produced 

 by reflection 1 ? is there no case in which this preferential 

 selection is made 1 How is it, for instance, if you take a plate 

 of gold ; that reflects light regularly, but the light is coloured 

 yellow 1 I said the cause of the coloration of bodies in the 

 great bulk of cases was what I have just described, but I did 

 not say that was the sole cause of coloration. The light 

 reflected from gold is in fact coloured ; in the case of gold or 

 of copper there is a preferential selection in the act of reflec- 

 tion of one kind of light rather than another, and that 

 preferential selection is not confined to the metals, although 

 it is chiefly in gold and copper that we ordinarily perceive it. 

 There are many cases in which substances which absorb light 

 with intense avidity present a similar reflection of coloured 

 light, and in these substances the connection between the 

 intense opacity of the substance and the coloured reflection 

 can be better studied than in the case of metals, because a 

 metal is, under ordinary circumstances, opaque. Certain of the 

 aniline colours, for instance, show the coloured reflection in a 

 notable manner. These specimens on the table (referring to 

 plates of glass on which solutions of certain aniline colouring 

 matters had been evaporated) were given to me by the late 







