Measurement of the Obliquity Factor of Diffraction. 619 



variation of* the amplitude of vibration on a Huygens's 

 secondary wave. 



My first observations on oblique diffraction by a rectangular 

 aperture or reflecting surface were published in a note in the 

 Phil. Mag. for Nov. 1906. The principal points noted were 

 that the diffraction-pattern was unsymmetrical in character, 

 the width of the bands instead of being the same throughout, 

 as in the case of normal incidence, increasing continuously 

 from one side of the pattern to the other : again, that the 

 number of bands on one side of the pattern was limited. 

 These results were shown to be simple consequences of the 

 formula giving the positions of the minima of illumination in 

 the pattern, i. e. 



sin z — sin 6= -\ . 



— a 



Later observations, in which the unsymmetrical character 

 of the distribution of intensity in the pattern was noted, were 

 described in the second paper quoted above. These obser- 

 vations were inexplicable on the ordinary (non-analytical) 

 theory of diffraction. It was found that the broader bands 

 on one side of the pattern were considerably feebler in 

 intensity than the corresponding bands on the other side ; 

 whereas the ordinary theory required that they should be of 

 equal intensity. The differences of intensity became very 

 large as grazing incidence was approached. It was also 

 observed that for any particular angle of incidence, the 

 illumination in the pattern died away as the plane which sets 

 the limit to the number of bands (on the side of the pattern 

 at which these were broader) was approached. According 

 to the ordinary theory, on the other hand, the intensity should 

 have remained finite right up to this plane (which is the 

 plane of the reflecting surface or aperture) and fallen 

 discontinuously to zero at this point. It was this last, rather 

 anomalous consequence of the ordinary theory which first 

 drew my attention to the fact that the illumination in the 

 diffraction-pattern was actually unsymmetrical in character. 



The only explanation for these effects that I could find was 

 that they were due to the varying obliquities at different 

 points of the pattern ; in other words, that the effects were 

 due to the obliquity-factor of diffraction, of which no account 

 is taken in the expression for the intensity derived from the 

 ordinary theory. This explanation was developed in the 

 paper quoted above, and was shown to be capable of 

 accounting for the unsymmetrical character of the intensity- 

 distribution. 



2 S 2 



