-894 Profs. Trowbridge and Wood on Groove-Form and 



In the case of grating No. 4, the heat-ray spectra were 

 found between the oblique image and the true central image, 

 as will be seen from the chart. The explanation of this is not 

 clear and it seems probable that some grave mistake was 

 made in its examination. 



It will be necessary to repeat the observations with this 

 grating, for no spectra were found to the right of the central 

 image, with the exception of a very weak one of the first 

 order, with the C0 2 rays, and we should expect strong ones 

 in the position of the oblique image at 27° # 5. 



The distribution of intensity was as follows: — 



Total area of central image and spectra... 100 



Quartz rays, first order, left 36 



Central image 64 



C0 2 rays, first order, left 32 



third ,. „ 12 



Central image 46 



The absence of the second order with the C0 2 rays probably 

 results from the circumstance that the widths of the reflecting 

 edges are equal to the inoperative surface between them. 

 There are therefore gaps between the reflected wavelets 

 as in the case of a wire grating for which a = L, and the 

 spectrum of two orders higher or lower than the one in the 

 direction in which the waves travel will fall out, i. e. 

 the waves are self-destructive in these directions. The 

 oblique image lies in the direction of the 4th order spectrum 

 for the C0 2 rays, consequently the 2nd order should be 

 absent as was found to be the case. It is interesting to see 

 thai with quartz rays we have a strong first order spectrum 

 at this point. Our failure to find a strong 2nd order 

 spectrum for quartz rays, and 4th order for C0 2 rays, in 

 the direction of the oblique image probably resulted from 

 our failure to rotate the grating sufficiently to bring this 

 region upon the bolometer. 



Grating No. 5. Constant, 0*0123 mm. 



Angle of reflecting planes, 22° and 30°. Microscopical 

 examination showed that the 30° planes were very poor re- 

 flectors ; they appeared covered with dark patches, showing 

 that the surface was very ragged and there \*»as no oblique 

 image to the right of the central image, which was wholly 

 absent with visible light. With the beat rays the intensity 

 distribution was as follows : — 



