Echelette Grating for the Infra-Red. 77 L 



light. Gratings with constants varying from 0*1 mm. to 

 *01 mm. could be studied by means of residual rays, or 

 narrow regions of the infra-red spectrum, isolated by a salt- 

 prism spectrometer, and the relation between the intensity 

 distribution and the form of the groove determined. Methods 

 were worked out by which a groove of any desired form 

 could be ruled, with optically riat sides (a very important 

 point), the angular slope of each side of the groove measured, 

 and the exact nature of the ruling determined, i. e. whether 

 the metal had been forced up between the grooves, or 

 whether the angle between the opposed faces was equal to 

 the angle between the edges of the ruling knife. This by 

 no means follows, as the ruling of groove No. 2 may force 

 the metal to one side and increase the angle of slope of the 

 adjacent side of groove No. 1. Gratings were finally obtained, 

 which have proved so efficient in the investigation of infra- 

 red spectra that it seems worth while to designate them by a 

 name of their own. They throw a large percentage of the 

 energy into one or two spectra to the left of the central 

 image, and show little or no trace of any energy to the right 

 of it. With visible light they send the greater part into a 

 group of spectra, say, from the 12th to the J 6th, or from the 

 24th to the 30th order. Thev may thus be regarded as re- 

 flecting echelons, of comparatively small retardation, and I 

 propose the name " echelette," to distinguish them from the 

 ordinary grating and the Michelson echelon. 



Various methods were tried for their production. The 

 first were made by punching the grooves with a steel die, 

 two adjacent surfaces of which had been ground flat and 

 highly polished. The die was a block of hard steel measuring 

 3 x 2 x 1*5 cms., and the gratings were punched with an 

 ordinary milling machine, the die being clamped at the 

 proper angle in a fixed position, and a polished plate of some 

 soft metal pushed up against it from below. This method is 

 analogous to the one used by Mr. Thorpe, in making his 

 gratings for the demonstration of predominant spectra, but 

 it did not give very satisfactory results. 



After considerable experimenting with various metals and 

 ruling points, I came to the conclusion that soft alloys must 

 be avoided, for it appeared 1o be impossible to cut a groove 

 with optically fiat sides. The crystalline structure of the 

 metal caused the point to rule a groove, the sides of which 

 undulated more or less, causing more or less reflexion in 

 directions parallel to the grooves. 



The method finally adopted was the following : — A sheet 

 of polished copper plate, such as is used by photo-engravers 



3E 2 



