Energy Distribution of Diffraction Gratings. 895 



Quartz Rays. 





C0 2 Rays. 





First order, left .. 



. 63 



Second order, left 



46 



„ ,, right.. 



. 7 



Central image . . . 



54 



Central image 



. 30 







In this case the slit width was 0*5 mm., and the width of 

 the bolometer strip was 0*5 mm., so that the resolving power 

 was high. The deflexions of the galvanometer could be 

 repeated with an error of less than 1 mm. (with total 

 deflexions of 35 mm.). 



The first order spectrum for the quartz rays falls exactly 

 in the direction of the oblique image, and contains 63 per cent, 

 of the energy. With visible light there is scarcely a trace 

 of any central image, practically all of the light going into 

 the oblique image, yet with the heat-rays we find the central 

 image quite strong. This is what we should expect, for 

 the heat-rays are diffracted by the edges of the grooves to 

 the same extent as are light-waves by an ordinary optical 

 grating, and the concentration is not complete even when the 

 direction of a spectrum coincides with that of the oblique 

 image. 



With the C0 2 rays the first order spectra are absent. This 

 is in agreement with theory, for if we draw the reflected 

 wave-fronts moving off at an angle of 44°, and make their 

 width such as would obtain with a grating constant of '0123 

 and angles such as specified, we find that if we reduce their 

 width a trifle the path-difference between the disturbances 

 coming from their edges, in the direction of the first order 

 spectrum, is exactly X. 



We should, however, expect the central image to very 

 nearly disappear from the same circumstance, for in this case 

 the path-difference will be 2\. Its appearance is probably 

 due to the circumstance that the ideal condition of fig. 1 is 

 not fulfilled, i. e. there is a narrow dark region between the 

 reflected wave-fronts. We have moreover disturbances from 

 the other edges of the groove which have not been taken 

 into account. 



Grating Xo. 6. Constant, '0123 mm. 



Angles of Reflecting Planes, 18° and 29°. 



The 18° edge was bad, showing little reflexion with visible 

 light. The oblique image had a blue central maximum, 

 bordered by red and yellow maxima. This peculiarity ha3 

 been discussed in the previous paper, and was found to result 



