Diffraction Spectra described by Prof. Wood. 61 



the cycle is quite different. In this case we have a pair 

 of unsymmetrical shaded bands which move in the same 

 direction as the angle oil incidence is changed." 



An important observation relates to polarization. " It was 

 found that the singular anomalies were exhibited only when the 

 direction of vibration (electric vector) was at right angles to the 

 ruling. On turning the nicol through a right angle all trace 

 of bright and dark bands disappeared. The bands are naturally 

 much more conspicuous when polarized light is employed." 



The production of effects changing so suddenly with the 

 wave-length would appear to require the cooperation of a 

 large number of grating-lines. But, as the result of an 

 experiment in which all but about 200 lines were blocked out,. 

 Prof. Wood was compelled to refer the matter to the form 

 of the groove. To this cause one would naturally look for 

 an explanation of the difference between this grating and 

 others ruled with the same interval, but it does not appear 

 how the discontinuity itself can have its origin in the form 

 of the groove. 



The first step towards an explanation would be the estab- 

 lishment of a relation between the wave-lengths of the 

 bands and the corresponding angles of incidence ; and at the 

 time of reading the original paper I was inclined to think that 

 the determining circumstance might perhaps be found in the 

 passing off of a spectrum of higher order. Thus in the 

 spectrum under observation of the first order, an abnormality 

 might be expected at a particular wave-length if in the third 

 order light of this wave-length were just passing out of the 

 field of view, i. e. were emerging tangentially to the grating- 

 surface. The verification or otherwise of this conjecture 

 requires a knowledge of the grating interval (e). This is not 

 given in the published paper ; but on hearing from Prof. Wood 

 that there were 14,438 lines to the inch, I made at once the 

 necessary calculation. 



If 6 be the angle of incidence for which light of wave- 

 length \ is just passing off in the nth spectrum. 



e(l + sin<9) = >a. ...... (1) 



In the first diagram the angle of incidence is 4° 12' 

 and the wave-lengths of the bands are given as 609 and 

 517, or in centimetres 6'09 x 10~ 5 and 5*17 x 10 -5 . Also 

 6 = 2-540/14438 cm., and sin = *O732. Using these data 

 in (1), we find for the larger wave-length n = 3'10, or n = 2'68, 

 according as the upper or the lower sign is taken. Again, 

 for the smaller wave-length we find with the upper sign 

 72 = 3*65, and with the lower n = 3*15. To reconcile these 

 numbers with the suggested relation it is necessary to suppose 



