272 KELVIN 



numbers) about four times the wave-length, or one- fourth 

 the period, of visible or red light. Let us take red light at 

 400 million million vibrations per second, then the lowest 

 radiant heat, as yet investigated, is about 100 million million 

 per second of frequency of vibration. 



I had hoped to be able to give you a lower figure. Pro- 

 fessor Langley has made splendid experiments on the top of 

 Mount Whitney, at the height of 15,000 feet above the sea- 

 level, with his "Bolometer," and has made actual measure- 

 ments of the wave-length of radiant heat down to exceedingly 

 low figures. I will read you one of the figures; I have not 

 got it by heart yet, because I am expecting more from him.* 

 I learned a year and a half ago that the lowest radiant heat 

 observed by the diffraction method of Professor Langley 

 corresponds to 28 one hundred thousandths of a centimetre of 

 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 fre- 

 quency 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. Photo- 

 graphic, 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 wave 

 motion below the red extends down to one quarter of the 

 slowest rate which affects the eye, our knowledge of vibra- 

 tions at the other end of the scale only comprehends those 



* Since my lecture I have heard from Professor Langley that he has 

 measured the refrangibility by a rock salt prism, and inferred the wave- 

 length of heat rays from a "Leslie cube" (a metal vessel filled with hot 

 water and radiating heat from a blackened side). The greatest wave- 

 length he has thus found is one-thousandth of a centimetre, which is sev- 

 enteen times that of sodium light the corresponding period being about 

 thirty million million oer second. 



November, 1884. W. T. 



