Energy Distribution of Diffraction Gratings. 891 



In fig. 2 let AB be a portion of the plane wave AD inci- 

 dent upon the grooved surface. After the first reflexion it 



C, 



Fig. 2. 



B, 



4, 



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Tnei d HW _ 



will occupy the position A'B', and after the second A"B /r . 

 The portion BO will be reversed in the same way, and the 

 two portions will unite into the wave C^Ai. It seems, 

 therefore, as if a surface of this nature would not interfere 

 with the constancy of the phase along the wave-front, not- 

 withstanding the fact that the wave has been chopped to 

 pieces, and the pieces made to change places. This being 

 the case, it appears as if we should have no diffraction spectra 

 at all, in spite of the deep furrows. 



Just how a surface ruled with grooves of this type, with 

 perfectly smooth reflecting sides meeting in a sharp edge, 

 would behave is perhaps open to question. Whether a wave 

 can be broken up into paired strips, reversed, and reunited 

 into a plane wave without suffering diffraction, is a question 

 which can probably be answered only by experiment. It 

 seems possible that many of the anomalies exhibited by 

 reflecting gratings can be explained by a two-fold or even 

 multiple reflexion from the groove. It is doubtful, however, 

 if multiple reflexions can be considered as taking place in a 

 groove commensurable in size with the wave-length. The 

 investigation of gratings of this type will be taken up later. 

 The present paper deals only with the behaviour of the 120° 

 groove. 



We will now take up the individual behaviour of the 

 gratings which have been investigated up to the present time. 



The arrangement of the apparatus was as follows : — The 

 light from a Nernst filament, rendered parallel by a concave 

 mirror, was reflected from three large polished surfaces of 

 quartz and focussed upon the slit of the vacuum spectro- 

 bolometer by a second concave mirror. The diffraction 



3N2 



