in 



[ 534 ] 



L. Groove-Form and Energy Distribution of Diffraction 

 Gratings. By Augustus Trowbridge, Ph.D., Professoi 

 of Physics, and Irving B. Crandall, A.M., Scholar 

 Physics, Princeton University*. Su^ u/ls» eL/i-0^ 



[Plates VIIL-Xfi A 



Contents. 

 § 1. Introduction. 



2. Method and arrangement of apparatus. 



3. The concentration of energy in a single order. 



4. Corrections to the curves. 



5. The relation between the shape of the groove and the direction 



of concentration. 



6. Weak orders ; the retardation over a single element. 



7. Conclusion. 



§i. 



N a series of papers which have appeared in this Journal 

 (vol. xx.), by R. W. Wood and one of the present 

 authors, it has been shown that it is possible to construct 

 diffraction-gratings, suitable for use in the infra-red spectrum 

 which concentrate the greater part of the energy into a 

 single order, and that it is possible to predict where this 

 concentration will occur from the shape of the groove which 

 is ruled on the grating surface. 



In the investigation described in one of the papers referred 

 to abovej, a study was made of four gratings with constants 

 0*0123 mm., and two with constants 0*0265 mm., all six 

 ruled on gold with a carborundum crystal placed in various 

 azimuths. The distribution of the energy among the various 

 orders was obtained bolometrically for two relatively narrow 

 regions in the infra-red spectrum of mean wave-lengths 

 4*3 [a and 8*6 jjl. In the case of gratings of so large a con- 

 stant the form of groove could be inferred from inspection 

 with the microscope, and yet the ratio of grating space to 

 wave-length of the incident energy is about that which 

 obtains in practice with the usual grating in the visible 

 spectrum ; that is, the grating is about fifteen times as coarse 

 as one suitable for optical work, but the length of wave is 

 from ten to twenty times as great. In the investigation 

 above referred to, the ratio between wave-length and grating 

 space was greater than the value which limits the applica- 

 bility of the simple diffraction theory, and as the results 

 obtained were in fair qualitative agreement with theory it 

 was thought advisable by the present writers to extend the 

 research towards the visible spectrum. 



* Communicated by the Authors. 



t Phil. Mag. [6] vol. xx. pp. 886-898 (1910). 



