Eclielette Grating for the Lifra-Red. Ill 



covers a range of fully ten degree?, embracing as many as 

 12 or more orders of second class spectra. In the case of 

 the Michelson echelon, the width of the step is from 0*5 to 

 1 mm., and the range of diffraction is so small that but one 

 or two orders of spectra are included within it. By the 

 study of these echelette gratings we can pass gradually from 

 the case of the ordinary grating to that of the echelon. 



The results appear to me to indicate that with a simple 

 groove, such as we have here, we cannot secure a concen- 

 tration of light in a region narrower than the diffraction 

 range from a single reflecting element. This question will 

 be more fully discussed in a subsequent paper treating of 

 the energy distribution among spectra of different orders 

 produced by these gratings, with visible light and very long 

 heat waves. 



Some of the gratings, with a constant of '0123 mm. gave 

 strongly coloured images, and lateral spectra of low order in 

 which a certain colour or colours were wholly absent. The 

 first order spectrum on one side, for example, may contain 

 no yellow-green, a broad dark band bisecting the spectrum. 

 A third order spectrum may have two dark bands, one in the 

 yellow, and another in the greenish-blue. 



In one case the oblique image or the central maximum of 

 the spectra of the first class, instead of being white., v\as 

 distinctly blue, while the maxima immediately to the right 

 and left of it contained only red, orange, and yellow light, 

 as shown in fig. 2 c (PL XVII.). 



This curious distribution of colour was observed in the 

 case of one grating only, and its explanation gave a good 

 deal of trouble. To explain it we must devise some type of 

 reflecting element which will give, in the case of red light, 

 zero illumination at the centre of symmetry, w T ith strong 

 lateral maxima, and w T ith blue light a strong maximum at 

 the centre, bordered by minima which occupy the positions 

 of the red maxima, and maxima in the positions of the red 

 minima. This can apparently be brought about only by an 

 element consisting of two parts, in other words a double 

 reflecting strip, with a half-wave retardation for red light, as 

 in the case of the laminary grating. The central maximum 

 will vanish in this case for red light, as can be easily seen by 

 constructing the diffracted wave-fronts. If blue light is used 

 the retardation becomes very nearly a whole wave, and we 

 have the centre of the system bright. An examination of 

 the grating with a microscope showed that there were in 

 fact two reflecting strips in contact which together formed 

 one side of the groove, the grating being built up of paired 

 reflecting elements separated by inoperative strips of about 



