Groove-Form of Diffraction Gratings. 



o3^i 



The report of this work, the development of a simple 

 formula adapted to the experimental conditions of the work, 

 and an analysis of the results obtained with reference to the 

 demands of this formula constitute the subject matter of 

 this paper. 



» §2. 



This investigation had for its object the systematic exam- 

 ination of the energy diffracted from some typical echelette 

 gratings throughout a fairly large region of the infra-red 

 (1-6 jju) ; the determination of the shape of the groove of 

 the gratings used, and, if possible, the discovery of the rela- 

 tion between the shape of the groove and the varying 

 quantity of energy thrown off at the different angles. The 

 gratings selected were two (Nos. 6 and 7) of the gratings 

 ruled by R. W. Wood for the above mentioned work, and 

 one ruled by Dr. C. F. Brackett (No. 10) with a diamond 

 crystal ground so as to rule a groove with one vertical side, 

 and one very gently sloping side. These three gratings 

 combined a fairly high dispersion with a very small angle 

 between the undisturbed surface and the main plane of the 

 groove, these conditions being essential to the crowding of 

 the energy into a single order ; this order being as low as 

 possible. 



Fisr. l. 



The essential parts of the apparatus were two spectro- 

 meters ; referring to fig. 1, the grating Gr was mounted on 

 the divided circle C of the first spectrometer : the light from 

 the Nernst lamp N passing through the svstem by way of 



2N2 



