778 Mr. H. Pealing on Distribution and Quality of 



therefore, absorbed in much larger proportions. This explains 

 the results in Table III. 



In the case of the forward radiation, the amount of the 

 " extra " radiation is much greater than the sum of the other 

 two, and so the fluorescent radiation has very small influence 

 on the ratio unless very penetrating rays are used. This 

 explains at once the results in Table V. when the hard 

 portions of a primary beam are used. 



The increase in the ratio of the extra radiation masks the 

 decrease in the ratio of the remainder due to the presence 

 of hard fluorescent carbon radiation until very penetrating 

 rays are used. 



The results given in Table IY. may be explained best by 

 considering the effect of the portions of the beam harder 

 than the carbon fluorescent radiation separately. This hard 

 portion will excite hard fluorescent, and also a consider- 

 able quantity of " extra " radiation. We should, therefore, 



expect a low value of ^ TOr this portion of the secondary beam 



J-90 



after absorption by the thick aluminium. The other portion 

 of the primary beam, and especially the hard part of it, 

 will produce a very large amount of extra radiation. The 



ratio j— for the soft part of this portion of the beam will 



J 90 

 then be much less than for the hard part of it. Hence, when 

 this portion of the beam is sent through thick aluminium 



the ratio y 1 for the portion that emerges will be much 



greater than for the total beam before passing through the 

 aluminium. Now as the amount of this passing through the 

 aluminium will be large compared with the part due to the very 

 hard rays of the beam, it is clear that the ratio for the two 

 together will alter in the same direction as the ratio for the 

 sotter portion. That is, the ratio will rise. When the pro- 

 portion of the hard rays increases the ratio will fall again. 

 Hence both the results indicated in Table IV. are explained. 

 There remains now to be explained a result which has not 

 yet been mentioned above, namel} r , when a thin radiator is 

 used the ratio in the forward direction is higher. The 

 radiator used was thin, *3 mm., but gave a deflexion of about 

 the same magnitude as the thick radiator. The explanation 

 is that this is due to the smaller percentage of fluorescent 

 radiation present in the former case. The proportion in the 

 case of the thick radiator would be between 2 and 3 times 

 that for the thin radiator. 



