364 Dr. J. W. Nicholson on the Scattering 



further, the rays were absent until the bottom m of the tube in 

 the cathode came opposite the opening, and the axis of the 

 opening therefore passed through the Canalstrahlen, when 

 they reappeared ; they became brightest of all when this axis 

 passed through the part of the plate C struck by the Canal- 

 strahlen passing through the tube. The fact that the rays 

 were not visible when the slit was opposite the tube indicates 

 that the rays are not mainly due to the metal plug getting 

 charged negatively by the cathode rays and acting as a 

 secondary cathode. We see from this that particles of positive 

 electricity are shot off in all directions from the gas traversed 

 by the Canalstrahlen. The results just described were ob- 

 tained when the tube was filled with air or with hydrogen ; 

 other gases have not yet been tried. The intensity of rays 

 emitted sideways is small compared with the intensity of those 

 observed in front of the cathode in the preceding experiment, 

 so that there must be considerable reflexion of the direct rays. 

 This view is also supported by the fact that the velocity of 

 these rays is not constant, but increases with the velocity 

 of the Canalstrahlen. 



I wish to thank Mr. Everett for the assistance he has given 

 me in these experiments. 



Cavendish Laboratory, 

 Aug. 6, 1907. 



XXXII. The Scattering of Sound by Spheroids and Disks. By 

 J. W. Nicholson, D.Sc, B.A., Isaac Newton Student in the 

 University of Cambridge* . 



WHEN a plane train of sound waves falls on a small 

 spheroid or disk, formulae expressing the scattering 

 effect have been given by Lord Rayleighf, who employs a 

 method based on an analogy with potential theory. The 

 formulas are first approximations, holding only when the 

 ratio of linear dimension to wave-length is very small. The 

 deduction of more accurate expressions requires the use of 

 harmonic analysis. This analysis has been given by Lord 

 Rayleigh J for the case of the sphere, but the further exami- 

 nation of the problem of the obstructing spheroid or disk 

 does not appear to have been carried out. The object of this 

 paper is to develop a suitable harmonic analysis, and to 



* Communicated by the Author. 



t Phil. Mag. Jan. 1897 ; Scientific Papers, iy. p. 305. 



X 'Theory of Sound/ vol. ii. § 334. 



