FLUORESCENCE. 63 



of the green colouring matter of leaves may, to a certain 

 extent, be regarded as extreme cases of the same general 

 phenomenon. 



I will refer now to this diagram. It consists of two 

 parts ; the upper part is diagrammatic for the same reasons 

 as before ; that is, you do not see the upper half and the 

 under half simultaneously, but you may see first one and 

 then the other. When the green colouring matter is purified 

 by a particular process which would take too long to describe, 

 and you take rather a dilute solution of it, and analyse 

 the transmitted light, you get these bands of absorption. 

 When you allow the upper spectrum. to enter a very dilute 

 alcoholic solution in this manner, then the red light which 

 is* given out in the regions of the spectrum near which the 

 fluid is comparatively transparent, is given out there 

 comparatively slowly, and accordingly it is less brilliant 

 in the neighbourhood of the surface than is that corre- 

 sponding to regions of great absorption. When the fluid is 

 extremely dilute, then the intensity of the light in the 

 former part of the spectrum becomes so small that you 

 hardly see it, but its intensity where the fluid uses up the 

 incident light more quickly in producing this phenomenon 

 may still be visible, so that you may get corresponding to 

 the position of these bands of absorption in the transmitted 

 light these red bands where the light is given out very 

 copiously, the solution being so d'lute that the red light 

 given out in the intervals is hardly perceptible at all. 



These phenomena are not very easily shown at a distance 

 to a large audience, but they are very striking when you 

 look at them, as two or three at a time may. in a room 

 where the sunlight is introduced or where an electric lamp 

 is used. I will, however, endeavour by and by to show 

 some of them by the aid of the electric light, but I am 

 afraid they will be seen very feebly compared to what I 

 have described, which are the effects seen when the light is 

 concentrated into a small space. 



Now allow me to go back for a moment to explain one 

 particular matter I forgot at the time. I said that the 

 light transmitted through a solution of sulphate of quinine 

 was ordinary light, and that if you examined it, and 

 scrutinised especially the blue part of the spectrum, you 

 would see nothing to account for the blue colour. If you 



