CHAPTER II 



LUMINESCENCE AND INCANDESCENCE 



Modern physical theory supposes that light is a suc- 

 cession of wave pulses in the ether caused by vibrating 

 electrons. The light to which we are most accustomed — 

 sunlight, electric light, gaslight, etc., — is due to electrical 

 phenomena connected more or less directly with the high 

 temperature of the source of the light. Every solid body 

 above the temperature of absolute zero is giving otf waves 

 of different wave-length (A) and frequency (v) but of the 

 same velocity {v),m vacuo, 180,000 miles, or 300,000 kilo- 

 metres a second. In fact, v (a coiistant)= A^;, so that it is 

 only necessary to designate the wave-length in order 

 to characterize the waves. This is radiant energy or 

 radiant flux. 



As everyone knows, the long waves given off in largest 

 amount from objects at comparatively low temperatures 

 give the sensation of warmth. As we raise the tempera- 

 ture, in addition to these longer heat waves, those of 

 shorter and shorter wave-length are given off in sufficient 

 quantity to be detected. At 525° C, rays of about A = .76a 

 in length are just visible as a faint red glow to the eye. 

 As the temperature increases still shorter wave-lengths 

 become apparent, and the light changes to dark red (700° ), 

 cherry red (900°), dark yellow (1100°), bright yellow 

 (1200°), white-hot (1300°) and blue-white (1400° and 

 above). Above A=: .4/^ the waves again fail to affect our 

 eye, and, although they are very active in producing 

 chemical changes, we have no sense organs for perceiving 



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