Measurements in the Extreme Infra-red Spectrum. 767 



this nature are advantageous in the isolation of long-wave 

 radiation was originally shown by Lord Rayleigh *. 



Observation of Interference Curves. 



The method of procedure in a series of observations was 

 in general as follows : — At the outset the quartz plates were 

 adjusted in the manner described above ; when the sodium 

 interference bands were perfectly clear, as viewed by re- 

 flected light, the plates were brought into contact and the 

 micrometer drum of the dividing engine turned back an 

 amount equal to the backlash of the thread. A further 

 rotation of the drum in the same sense then caused a sepa- 

 ration of the quartz plates. A series of measurements of 

 the intensity of radiation at this point was then made by 

 radiomicrometer deflexions and the drum-head turned one 

 division further, observations taken, aud this procedure 

 repeated until the entire interference curve was obtained ; 

 in other words, observations were made as long as maxima 

 and minima of a sufficiently distinct character were apparent. 



The results are shown graphically in PI. XIII. figs. 3-6, in 

 which the drum-head divisions are plotted as abscissae and 

 the intensities of radiation as ordinates. Pigs. 3, 4, 5, 6 

 refer to the Reststrahlen of rock-salt, sylvine, potassium 

 bromide, and potassium iodide. A difference of the abscissas 

 of any two points of a given curve therefore gives the thick- 

 ness of the air-film expressed in drum divisions (1 division 

 = 5*23 /x). The point a in all curves was obtained by ex- 

 trapolation, for the reason that the observations were not 

 reliable until the air-layer attained a thickness of 5 to 10 /*. 



Discussion of the Interference Curves. 



All curves show distinctly a wave character ; furthermore, 

 the maxima and minima are practically as required by the 

 Airy formula. In figs. 3, 4, and 5 the intensities of the 

 maxima and minima do not, as might be expected, decrease 

 continuously with increasing thickness of air-film, but rather 

 show a very systematic and periodic series of fluctuations or 

 " beats." In fact one is forcibly impressed by the similarity 

 these curves bear to the graphs obtained by mechanically 

 recording sound beats. That such an analogy is more than 

 an apparent one is evident, for it is clear that in the 

 Reststrahlen of rock-salt, sylvine, and potassium bromide 



* Lord Ravleigh, Weekly Evening Meeting of the Royal Institution 

 of Great Britain, 29th March, 1901. " 



