.and Energy distribution of Diffraction- Gratings. 887 



It occurred to one of us that a promising method of attack 

 would be to stamp or rule gratings with such wide grooves 

 that their form could be determined with certainty, and then 

 investigate the energy distribution among the spectra with 

 very long heat-waves, i. e. with " residual rays " of various 

 wave-lengths. 



The manufacture of these echelette gratings, and their 

 behaviour with visible light, have been described in a pre- 

 ceding paper *. 



The investigation was made with the large vacuum spectro- 

 bolometer described in a previous paper by one of the 

 present writers |. The gratings were mounted on the table 

 of the instrument, and the slit illuminated with the radiant 

 energy in question. Two groups of rays were used in the 

 investigation, the residual rays from quartz with a mean wave- 

 length of 8*6 fM and the C0 2 radiation from a Bunsen flame, 

 with a wave-length of 4*3 //, or about half as great as that of 

 the quartz rays. The smallest grating constant used was 

 0*0123 mm., or seven times the width of the grooves on the 

 gratings ruled on Rowland's first machine, the largest, '05 mm. 

 It is evident that when these gratings are used with the long 

 heat-waves above referred to, the ratio of the grating constant 

 to the wave-length is about the same as that which obtains 

 in the case of visible light and the optical gratings in common 

 use. The nature of the ruled surface of the echelette 

 gratings used in the present investigation, and the method 

 by which it was studied, have been described in the previous 

 paper. 



In order to make a thoroughly satisfactory study of the 

 distribution of the energy it would be advisable to keep the 

 angle of incidence fixed (for example normal) and swing 

 the bolometer or thermopile through the spectra. With the 

 instrument at our disposal at the present time this was 

 impossible, and it was necessary to make the spectra pass 

 across the bolometer by rotating the grating. This compli- 

 cates the discussion of. the results in no small degree, for the 

 energy distribution varies with the angle of incidence, as 

 can be seen easily with an ordinary grating. We have, 

 however, already obtained results which are in qualitative 

 agreement with theory, and which show that the method is 

 admirably adapted to the experimental investigation of the 

 problem. We shall, in the present treatment, discuss the 

 results by the Fresnel method, considering the interference 

 between secondary wavelets originating on the surface of the 



* Supra, p. 770. f Supra, p. 768. 



