FLUORESCENCE. 73 



of the luminiferous ether amongst the ultimate molecules 

 of bodies. It seems to me quite contrary to dynamical 

 principles to suppose that any such causes should be 

 adequate to account for the production of vibrations of one 

 period from vibrations of another." Having written that, 

 I am not responsible for the view which has been so 

 wrongly attributed to me. I can only account for it in 

 this way. I suppose it was from the title I gave my paper, 

 " On the change of refrangibility of light," which I chose 

 because undoubtedly the most striking part of the 

 phenomenon, which had not been hitherto suspected, was 

 that the light given out was of a different refrangibility 

 from the light going in. With regard to the duration of 

 the phenomenon, I thought it possible that, though very 

 large compared with the time of a single luminous vibration, 

 it might elude our means of observation ; but subsequently 

 Mons. E. Becquerel showed, by a very ingenious instrument, 

 which is here on the table, which he calls a phosphoroscope, 

 that the phenomenon is really of appreciable duration. 

 The light enters at one side, and falls on -a cell containing 

 the substance to be examined, in this case a salt of 

 uranium enclosed between a pair of discs constituting a 

 double fly with holes of a somewhat sectorial shape, so 

 arranged that the holes in the one exactly correspond with 

 the spaces between the holes in the other. When you turn 

 the wheel round, supposing there is no substance there, 

 you see no light, because you are always looking across a 

 plate of metal. At one time there is a hole in the front 

 disc, which is covered by the second plate, and when there 

 is a hole in the second plate it is covered by the front disc ; 

 a series of holes coming alternately in the two discs. But 

 supposing a substance is interposed, and the light is let in, 

 and you turn the wheel, the illumination is let on and cut 

 off alternately with great rapidity ; but you see the light, 

 not at the moment when the body is illuminated, but a 

 small fraction of a second afterwards, through the hole 

 which is opposite your eye ; in that way you see it a very 

 short space of time after it has been lit, so to speak, by the 

 incident light ; it gets a number of doses of light in one 

 rotation, and you get a number of glimpses of it immediately 

 afterwards. In that way many substances which show this 

 phenomenon appear luminous, and you see them by the light 



