Energy Distribution of Diffraction Gratings. 897 



Grating No. 8. Constant, *0123 mm. 

 Angle of Planes, 8°. 



In this case the angle o£ the reflecting planes is so small 

 that the maxima and minima of the oblique image meet the 

 grating spectra to the left of the central image. These 

 grating spectra and the central image are strong, and the 

 colour distribution is most remarkable. The spectrum of 

 the 3rd order, for example, may contain only green light, 

 while another contains only red and blue, as shown by a 

 coloured plate in the forthcoming edition of 'Physical Optics 7 

 (Wood's). 



The intensity distribution with the heat-rays is as follows: — 



Quartz Rays. 





CO, Rays, 





Central image . . . 



m 



Central image ... 



22 



1st order, left . . . 



U 



1st order, left ... 



70 







„ „ right... 



8 



In this case we find, for the C0 2 rays, the second order 

 absent, and a very strong first order, Avhich lies very near 

 the point towards which the energy is thrown by the 8° 

 reflecting planes. As in the previous case, the C0 2 band 

 appears wide and distinctly resolved into a double band at 

 the centre. The oblique image lies nearer to the central 

 image than the first order spectrum for the quartz rays, 

 consequently it receives the larger portion of the energy. 



This grating has been already discussed in connexion with 

 the theory. 



Grating No. 9. Constant '0265 mm. 



The angle of the reflecting planes was very small, about 

 6°, and most of the energy appears in the first order spectrum 

 from the C0 2 rays, which were the only ones used in this 

 case. 



The distribution of intensity was as follows : — 



First order, left 40 



Second ,, „ 9 



Third „ „ 10 



Central image 32 



First order, right 9 



Here again we have concentration of energy in the first 

 order spectrum as a result of the small angle of the reflecting 

 planes. 



