618 Mr. C. V. Raman on the Photometric 



of the sounding-board which would otherwise be of double 

 frequency. 



While the work of setting up the apparatus for photo- 

 graphing the vibration-curves was in progress, Barton and 

 Ebble white's paper in the Phil. Mag. for September 1910 

 appeared, in which it was shown that the motion of the 

 bridge of the violin in some eases had a frequency double 

 that of the string. This effect seems to be clearly analogous 

 to that shown in my photographs, though the effect is not 

 obtainable in all cases with the bridge of the violin. 



Fig. 10 illustrates still another method, which was devised, 

 of demonstrating the 1:2 frequency relation. This was to 

 compound the motion of the sounding- board and the wire 

 optically. An illuminated horizontal slit is placed just 

 below a point on the wire, transversely to it. The light 

 issuing therefrom after reflexion at a mirror fixed over it 

 and then at the oscillating mirror of the optical lever is 

 focussed by the lens of a camera. The two motions of the 

 shadow of the wire on the slit are at right angles to each 

 other, and its path is the appropriate Lissa jous's figure. It 

 is seen in the photograph that this is approximately a para- 

 bolic arc. 



Nagpur, C. P. India, 

 10th November, 1910. 



LXXI. The Photometric Measurement of the Obliquity Factor 

 of Diffraction *. By C. V. Raman, M.A. f 



[Plate VII.] 



IN an earlier paper on "The Experimental Study of 

 Huygens's Secondary Waves," published in this Journal 

 (Phil. Mag. Jan. 1909), 1 showed that the study of diffraction- 

 patterns formed at oblique incidences is of particular 

 interest, as the phenomena observed throw a fresh light on 

 Huygens's principle and on the theory of secondary waves. 

 Quantitative measurements of the effects described in that 

 paper have since been made with the rectangular aperture, 

 this being the one which lends itself most readily to the 

 work. The present paper deals with the detailed and 

 quantitative verification of the explanation suggested by me 

 to account for the effects observed : its net result is to show 

 that it is possible actually to observe and measure the 



* A preliminary note on this subject was published in 'Xature,' dated 

 the 18th of November, 1909. 

 f Communicated by the Author. 



