On the Absorption of X-Rays. 471 



12. The classification which has been adopted for the 

 tracks of particles moving at 10 m./s. holds good for all 

 velocities comparable with wind-velocities and therefore 

 of practical interest. It may be noted, however, that for 

 sufficiently great velocities the classification would break 

 down. Cases III. a, b, and Case II. become identical for 

 the path which crosses the pole and is asymptotic to the 

 equator. Such a particle tends to a position of rest relative 

 to axes fixed in space, and it follows that the velocity of pro- 

 jection from the pole must be equal to the absolute velocity 

 of a point on the equator. 



Cases III. c and d are possible when the relative velocity 

 does not exceed twice the absolute velocity of a point on the 

 equator. For velocities above that limit all tracks cross 

 the equator. 



XLY. Notes on the Absorption of X-Rays. 

 By Tycho E :son AtJREisr, Dr. phil* 



I. Introduction. 



r~pHE coefficient (/jl) for the absorption of X-rays is found 

 JL by means of the law 



l=V-^, (l) 



where I G is the intensity of the incident rays, I that of trans- 

 mitted rays, and d the thickness of the sheet of a definite 

 material. As the above law only holds under the condition that 

 radiation is homogeneous, and a perfectly homogeneous radia- 

 tion of sufficient intensity is not easily brought about, there 

 arise, already from this cause, serious difficulties as to the 

 exact determination of absorption coefficients. Moreover, 

 other difficulties are presented by the fact that the intensity 

 of radiation in the bulb is continually changing, and that 

 the intensity of the rays will be diminished not only by absorp- 

 tion but also by scattering. Thus the determinations made, 

 up to this date, of absolute absorption coefficients seem to be 

 rather uncertain f. Nowadays, the ratio of absorption is 



expressed by the mass-absorption coefficient (— ), which is 



obtained by dividing the absorption coefficients by the 

 den-ity (a) of the absorbing material. 



W. H. Bragg and Pierce f have, instead of the coefficient 



# Communicated by the Author. 



t Compare Bragg and Pierce, Phil. 3Iag. xxviii. p. 626 (1914). 



