64 



Prof. G. a. Stokes. 



[May 25, 



original excitement, not having been touched by the rays which came 

 through the lens. 



2. The halo is due to the action of the less refrangible rays, which, 

 as is well known, discharge the phosphorescence. Their first effect, 

 as is also known, is however to cause the phosphorus to give out 

 light ; and if the exposure were very brief, or else the intensity of the 

 discharging rays were sufficiently reduced, the part where they acted 

 was seen to glow with a greenish light, which faded much more 

 rapidly than the deep blue, so that after a short time it became 

 relatively dark. 



3. This change of colour of the phosphorescent light can hardly fail 

 to have been noticed, but I have not seen mention of it. In this 

 respect the effect of the admission of the discharging rays is quite 

 different from that of warming the phosphorus, which as is known 

 causes the phosphorus to be brighter for a time, and then to cease 

 phosphorescing till it is excited afresh. The difference is one which 

 it seems important to bear in mind in relation to theory. Warming the 

 phosphorus seems to set the molecules more free to execute vibrations of 

 the same character as those produced by the action of the rays of high 

 refrangibility. But the action of the discharging rays changes the 

 character of the molecular vibrations, converting them into others 

 having on the whole a lower refrangibility, and being much less 

 lasting. 



4. Accordingly when the phosphorus is acted on simultaneously by 

 light containing rays of various refrangibilities, the tint of the result- 

 ing phosphorescence, and its more or less lasting character, depend 

 materially on the proportion between the exciting and discharging 

 rays emanating from the source of light. Thus daylight gives a bluer 

 and more lasting phosphorescence than gaslight or lamplight. I took 

 a tablet which had been exposed to the evening light, and had got 

 rather faint, and, covering half of it with a book, I exposed the other 

 half to gaslight. On carrying it into the dark, the freshly exposed 

 half was seen to be much the brighter, the light being, however, 

 whitish, but after some considerable time the unexposed half was the 

 brighter of the two. 



Again, on exposing a tablet, in one part covered with a glass vessel 

 containing a solution of ammonio-sulphate of copper, to the radiation 

 from a gas flame, the covered part was seen to be decidedly bluer than 

 the rest, the phosphorescence of which was whitish. The former part, 

 usually brighter at first than the rest, was sure to be so after a very 

 little time. The reason of this is plain after what precedes. 



A solution of chromate of potash is particularly well suited for a 

 ray filter when the object is to discharge the phosphorescence of 

 sulphide of calcium. "While it stops the exciting rays it is transparent 

 for nearly the whole of the discharging rays. The phosphorescence is 



