318 POPULAR LECTURES AND ADDRESSES. 



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. 

 Professor Langley has made splendid experiments 

 on the top of Mount Whitney, at the height of 

 1 5,000 feet above the sea-level, with his " Bolometer," 

 and has made actual measurements 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 ex- 

 pecting more from him. 1 I learned a year and 

 a half ago that the lowest radiant heat observed 

 by the diffraction method of Professor Langley 



1 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 seventeen times that of sodium light the 

 corresponding period being about thirty million million per second. 



November, 1884. W. T. 



