chip, xxvi.] FLUORESCENCE. 337 



the length of the visible spectrum is increased, the 

 increase taking place beyond the violet, and the light 

 being of the colour stated. Rays in the ultra-violet 

 part become by this means visible. 



Canary glass, that is, glass coloured with uranium, 

 emits a faint nebulous light under similarcircumstances. 

 Many substances become self-luminous when light 

 falls upon them, the kind of light emitted being 

 dependent on the substance. Certain forms of fluor- 

 spar have this property, which, on this account, is 

 called FLUORESCENCE. A solution of chlorophyll emits 

 red light, a decoction of madder in alum emits 

 yellow and violet light. An aqueous solution of 

 asculiiie (extracted from horse-chestnut), and alco- 

 holic solutions of stramonium are also fluorescent. 

 All these substances exhibit the property when ordi- 

 nary white light falls upon them ; but they do not 

 necessarily exhibit it with all the separate colours of 

 the spectrum. Thus, as we have seen, sulphate of 

 quinine gives a blue colour when placed in the ultra- 

 violet rays ; but if placed in the green or yellow 

 region of the spectrum, no fluorescence is visible ; 

 while chlorophyll will emit the red in whatever part 

 of the visible sp3ctrum it may be placed. It thus 

 appears that the rays which are emitted by the 

 fluorescent body are never of a greater refrangibility 

 than those which fall upon them, and are generally of 

 a less refrangibility. The phenomena are explained 

 by supposing that the molecules of a particular body 

 tend to vibrate at a particular rate. Vibrations of 

 a longer period cannot excite the molecules of the 

 body, but vibrations of the same period will excite 

 vibrations in the body, just as one tuning-fork, tuned 

 to vibrate with a certain rapidity, will throw a neigh- 

 bouring tuning-fork, tuned to the same rapidity, into 

 activity. While, however, vibrations of a slower rate 

 cannot excite the molecules of the body, vibrations of 

 w 7 



