THE WAVE THEORY OF LIGHT. 319 







corresponds to 28 one hundred thousandths of a 

 centimentre for wave-length, 28 as compared with 

 red light, which is 7*3 ; or nearly four-fold. Thus 

 wave-lengths of four times the amplitude, or one- 

 fourth the frequency per second of red light 

 have been experimented on by Professor Langley 

 and recognised as radiant heat. 



Everybody knows the "photographer's light," 

 and has heard of invisible light producing visible 

 effects upon the chemically prepared plate in the 

 camera. Speaking in round numbers, I may say 

 that, in going up to about twice the frequency I 

 have mentioned for violet light you have gone 

 to the extreme end of the range of known light 

 of the highest rates of vibration ; I mean to say 

 that you have reached the greatest frequency that 

 has yet been observed. Photographic, or actinic 

 light, as far as our knowledge extends at present, 

 takes us to a little less than one-half the wave 

 length of violet light. 



You will thus see that while our acquaintance 

 with \vave motion below the red extends down 

 to one quarter of the slowest rate which affects 



