392 On the Absorption of Narrow X-ray Beams. 



filled the solid angle subtended by the ionization chamber, 

 showed that there was at most a 4 per cent, difference (due 

 probably to the increase in path of rays scattered from the 

 sides). For the 4 mm. slits the vertical plane angle of the 



beam was *5°, and for the "6 mm.". 



•07°. 



Results. 



In the table below the results obtained are tabulated, the 

 absorption coefficient being reduced to atmospheric pressure. 

 The comparison between the various beams was made in two 

 stages — the absorption of the wide beam being compared 

 with that of the 4 mm. beam, and this latter then with the 

 *6 mm. beam. The coefficients for the two narrow beams 

 were almost identical, a small increase of 1 per cent, being 

 shown, on the average, for the finer beam. 



Slit. 



Plane Angle 



of Beam of 



Says. 



Mean 



1 



fi/p 

 (p= -00288.) 



Wide 



13° 

 .50 



•07° 



•0072 

 •0077 

 ■0078 



2-50 

 2-69 

 2-71 



4 mm 



•6 mm 



The use of a gas-filled bulb and the consequent variation 

 of the quality of the X-rays necessarily limited the accuracy 

 of the experiments, and all that can be said is that there does 

 not appear to be any large small-angle scattering of the rays, 

 a result confirming and extending to smaller angles the 

 direct method used by Crowther *. 



My thanks are due to Professor Sir Ernest Rutherford for 

 the suggestion of this research and for the interest he has 

 shown during its prosecution. 



* J. A. Crowther, Proc. Camb. Phil. Soc, vol. xvi., 1911. 



