204 Mr. C. V. Raman : The Experimental 



of the measurements is about 0*1 A.U. ; it will be seen that 

 the measurements of Rutherford and Royds are in good 

 agreement. 



Brief glances at the spectrum when the jar and spark-gap 

 discharge is sent through the vacuum-tube indicate that the 

 spectrum remains unaltered. 



These experiments were made with the aid of the radium 

 loaned to Prof. Rutherford by the Austrian Academy of 

 Sciences. To Prof. Rutherford I am also indebted for 

 the use of the apparatus and methods for purifying the 

 emanation. 



Physical Laboratories, 



Manchester University. 



XIV. The Experimental Study of Huy gens' 's Secondary Waves. 

 By C. V. Raman, M.A* 



[Plate III.] 



IN the Phil. Mag. for Nov. 1906 (pp. 495-498), I published 

 a note on the diffraction-bands formed when a rectangular 

 aperture is held very obliquely in a parallel beam of light. 

 I showed that the bands cease to be of the same symmetrical 

 type as the fringes formed when a rectangular aperture is 

 held normally. They are not equidistant, the band-width 

 increasing progressively from one side of the pattern to 

 the other. Further, the number of bands visible on one 

 side of the pattern is limited. The photographs of the 

 effect published with this paper (Plate III.) exhibit these 

 features. 



Further observation of the diffraction-bands on the 

 spectrometer, made by the methods I described in the paper 

 referred to (i. e. of observing through the telescope the image 

 of the slit of the instrument formed by light reflected very 

 obliquely at the face of a prism, or by light passing through 

 a rectangular aperture cut in a thin sheet of metal and held 

 very obliquely on the table), elicited the following : it was 

 found that the bands on one side of the pattern were fainter 

 than those on the other, the difference becoming very large 

 as grazing incidence was approached. This feature is visible 

 on all the three photographs in the Plate. The effect is 

 inexplicable on the ordinary (non-analytical) theory of 

 diffraction. 



* Communicated by the Author. 



