FL UOEESCENCE. 7 1 



much increase the effect by plunging the test tube in water 

 and looking at it downwards nearly parallel to the test- 

 tube. The reason of that is that the incident rays fall 

 upon the fluid and cause it to give out light in all direc- 

 tions, blue light, or whatever else it may be, according to 

 the nature of the fluid ; but when the water is not there, a 

 portion "of the light so given out does not enter the eye at 

 all, but suffers total internal reflection at the outer surface 

 of the tube. On the other hand, if you look inside the tube 

 the light suffers absorption on the part of the fluid itself. 

 It does not much signify in the case of sulphate of quinine, 

 which is sensibly transparent, but if you examine any 

 coloured fluorescent fluid looking down the tube inside, 

 you lose light from defect of transparency, the light being 

 absorbed by the fluid ; whilst if you look outside you lose a 

 great deal by the total internal reflection at the surface of 

 the glass. But if you plunge the test-tube into water, and 

 look down from the outside, any of the emitted light which 

 gets into the glass of the tube is able to get out again, so 

 that you can look down from above in a very slanting 

 direction and still get all the light, and as the stratum 

 which emits the light is seen very much foreshortened, the 

 brightness of the light is thereby increased. 



Now as to the cause of this phenomenon. From the first 

 I believed the cause to be this : that the incident rays so 

 act on the ultimate molecules of the body as to throw them 

 into a state of agitation, which agitation they in their turn 

 are capable of communicating to the ether. Everybody 

 now, I believe, considers that light is produced by the 

 vibration of a certain subtle medium, which we call the 

 luniiniferous ether, and I will take a dynamical illustration 

 of the phenomenon according to this view. Suppose you 

 had a number of ships at rest on an ocean perfectly calm. 

 Supposing now a series of waves, without any wind, were 

 propagated from a storm at a distance along the ocean, 

 they would agitate the ships, which would move backwards 

 and forwards ; but the time of swing of the ship would 

 depend on the time of its natural oscillation, and would 

 not necessarily synchronise with the periodic time of the 

 waves which agitated the ship in the first instance. The 

 ship, being thus thrown into a state of agitation, would itself 

 become a centre of agitation, and would produce waves 



