338 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



a higher rate may, though the molecules of the body 

 thrown into vibration by more rapid movements 

 than their own, will still vibrate in their own period. 

 Thus a fluorescent body will permit vibrations of a 

 longer period than that to which its molecules tend to 

 oscillate to pass through it unaffected. Vibrations of 

 its own period it will, however, absorb, and its own 

 molecules being thrown into activity, it will itself 

 produce the vibrations. That is to say, light of the 

 same colour as the body itself emits, it will absorb 

 and emit, but light of a less refrangibility, of a less 

 speed of vibrations, it will permit to pass unaffected. 

 The molecules of the fluorescent body will, however, 

 be thrown into vibration by vibrations faster than 

 their own, but thus excited, thev will vibrate with 



' \> 



their own rapidity. In other words, light of a dif- 

 ferent colour than that which the fluorescent body 

 emits, but due to vibrations whose period is less than 

 that of the molecules, will excite the fluorescent body, 

 and it will emit a light of its own period of vibration. 

 Thus the sulphate of quinine solution is excited by 

 vibrations of the ultra-violet region, and emits a blue 

 light ; light, that is, whose period of vibration is less 

 than that of the ultra-violet rays. By this means 

 vibrations whose rapidity is too great to produce the 

 sensation of vision are transformed into vibrations 

 whose rapidity is less, and can excite the retina, and 

 they are thus rendered visible. Sulphate of quinine, 

 therefore, increases the length of the visible spectrum 

 by diminishing the rate, by diminishing the refrangi- 

 bility, of the ultra-violet rays. But sulphate of 

 quinine is not fluorescent in the yellow because the 

 period of vibration of the yellow rays is less than that 

 in which the molecules of sulphate of quinine os- 

 cillate. 



PHOSPHORESCENCE is the property which some 

 bodies possess of being luminous in the dark after 



