54: C. D. Coohsey — Secondary Cathode Rays. 



and he and Barkla* have shown that the ratio of the coeffi- 

 cients of any two fluorescent beams is constant for all absorb- 

 ing substances in which these beams do not excite a fluorescent 

 radiation. AVe can, therefore, calculate the values of X., in air 

 from those in aluminium and in air given by Barkla and 

 Sadler. 



The fluorescent rays used were those from chromium, iron, 

 zinc, and tin, and with the Exception of tin Barkla and Sadlerf 

 have measured their absorption coefficients in gold and silver. 

 Their value for the coefficient of the rays from chromium in 

 gold is unreliable according to their statement, but it will be 

 seen that \ 1 is always so small with respect to /3 that an exact 

 knowledge of this quantity is not important. 



The absorption coefficients of the rays from tin in gold and 

 silver are not given b} r the above writers, but may be obtained 

 approximately from the constant ratio between absorption 

 coefficients cited above. Tin rays, being more penetrating 

 than those characteristic of silver and gold, will excite their 

 fluorescent radiations and, therefore, will probably be less 

 penetrating to these metals than the above calculation would 

 indicate, but an inspection of the curves given by Barkla will 

 show that the increase of absorption is not very great in this 

 part of the spectrum, and it will be seen that the effect of X, is 

 negligible in the case of the rays from tin. 



The accompanying tables give the value of the quantities 

 occurring in the formula as calculated from the data given by 

 Barkla and Sadler and the observed values of e/i in gold and 

 silver. The last column gives the values of B, as calculated 

 from these results. Most of the values of e/i were obtained 

 using only one sheet of metal foil, but in some cases more than 

 one sheet was used. As the terms in the formula containing t 

 drop out wdien t is equal to or greater than two sheets of the 

 metal, and as no consistent variation with the thickness of the 

 sheets was observed, the mean of all the values of e/i using 

 more than one sheet and the resulting values of R are given 

 separately in the tables following the greater value of t. 



It is at once apparent from the tables that the ratio of 

 emergence to incidence cathode rays does not vary with the 

 penetrating power of the exciting X-rays in the range of pene- 

 trating powers used ; that is, for an increase of absorbability as 

 measured in aluminium of about 8000 per cent there is no 

 measurable variation in the ratio. 



It is interesting to note in this connection the values of the 

 ratio compared to those found by Stuhlmann for the cathode 

 rays produced by ultra-violet light. The value he gives for 



*Phil. Mag., vol. xvii, p. 739, 1909. fLoe. cit. 



