FLUORESCENCE. 59 



predominance of blue, could only have come from light of 

 the same ref rangibility in the incident beam; but in following 

 out the consequences of that, I was led of necessity into 

 utter extravagances as regards the cause of the phenomena, 

 extravagances which did not appear to have any resemblance 

 to truth ; and on further reflection it occurred to me that 

 perhaps after all this blue colour was not produced by the 

 blue rays of the spectrum at all, but by other rays. We 

 know that the spectrum contains rays which are invisible, 

 but. in all other respects behave exactly like light. But 

 the invisibility is a mere accident, so to speak, depending 

 on the organization of the human eye : and the eyes of 

 animals in general are probably very much like the human 

 eye in this respect, although it is quite possible that certain 

 animals may see rays which we do not ; but that we cannot 

 well make out. We know that light contains besides the 

 visible rays others which are invisible ; some less refrangible 

 than the red, and others more refrangible than the violet. 

 We know that the latter show themselves especially by 

 their chemical effect, for example, on a properly prepared 

 photographic plate, and abound in sunlight and daylight, 

 which show strongly the blue light given out by quinine 

 solutions, while lamp-light, which we know to be poor in 

 those rays, shows it but feebly. 



Then it occurred to me that perhaps this blue colour which 

 the solution gives out is after all the work of the invisible 

 rays which we know to accompany the visible ones. If we 

 suppose this fluid, which looks colourless like water, to be 

 excessively opaque, inky black as it were, with regard to 

 the invisible rays of high ref rangibility, and if we further 

 suppose that these invisible rays are capable of so working 

 on the fluid as to cause it to give out visible light, then 

 the explanation of epipolic dispersion and the nature of 

 epipolised light will be perfectly plain. 



Now as we have been going on for some way without any 

 experiment, I will have the room darkened, and will en- 

 deavour to show you one or two. I will take the original 

 fluid in which the first phenomenon was discovered Here 

 is this green solution of leaves, and you will be able to see 

 a little of the red light. Here is the yellow glass I spoke 

 of, and in this the green band will be seen very copiously ; 

 here, again, is a solution of quinine, and with this I will 



