938 Dr. J. \Y. Nicholson on the Approximate Calculation 



would give out a characteristic radiation. Experiments show 

 that with proper conditions every substance can be so chosen 

 as to give a similar type of radiation. It is important to 

 notice that Bragg and Madsen (Phil. Mao-. Oct. 1908) have 

 shown thai the character of the j3 radiation caused by 7-rays 

 is independent of the atom in which it arises, and depends 

 solely on the nature of the y-rays to which it is due. The 

 present investigation shows that this is also true for the 

 -1 condary 7 radiation. 



The quality of the secondary 7 radiation shows no sudden 

 change Erom thai of the primary. There is simply a gradual 

 softening the more the secondary radiation is deflected from 

 it- original direction. The gradual softening is the same for 

 every radiator, Other Investigators have shown that jS-rays 

 arc scattered in their passage through matter. The scattering 

 of y ravs appears to be analogous to the scattering of /3-rays. 

 The primary 7-rays possess a wide range of penetrating 

 power. The softening of the secondary radiation that has 

 been observed is the result of this heterogeneity of the 

 primary rays. The softer radiation is more scattered than 

 the harder radiation; as the radiator Is increased in thickness 

 more of the harder gets turned aside, and in consequence 

 we get both the hardening of the primary and of the 



ondary. The hardening is due in the one case to the 

 cut tine; out of the softer radiation, and in the case o£ the 

 -."■ondary to the addition of a more penetrating scattered 

 radiation. There is no evidence of selective absorption. 

 The prodoction of this secondary 7 radiation is undoubtedly 

 a scattering effect, as Madsen had concluded from previous 

 experiments. 



I desire to thank Professor Rutherford for the use of large 

 quantities of radium and of radium emanation, and also for 

 his suggestions in the course of this work. 



DO 



Physical Laboratories, Manchester. 



CV. Ihe Approximate Calculation of Bessel Functions of 

 Imaginary Argument. By J. W. NICHOLSON, M.A., D.Sc* 



IN the British Association Report for 1908, some formulae 

 were given suitable for the rapid tabulation of Bessel 

 functions whose argument is purely imaginary and large, 

 and whose order may be of any magnitude. The same results 

 apply if the order is large, and the argument of any magni- 

 tude. A proof was not appended, and the object of the 



* Communicated by the Author. 



