780 Mr. H. Pealing on Distribution and Quality of 



of the bulb. It was necessary to have the orifice of E small, 

 otherwise the saturation current would not be obtained, a 

 source of error that was kindly pointed out to me by A. I. 

 Steven, M.A. It is clear, however, that the orifice of E 

 could not be much reduced in size, because the marginal 

 portions of the beam entering S, which have had their softer 

 components absorbed by the lead screen E, will bear an 

 appreciable ratio to the total beam. 



(b) Absorbability of the Secondary Beam. — The radiator 

 R was placed in position and the screen at the tube T 2 

 taken away. The same sheets of aluminium were used 

 to determine the percentage absorption of the secondary 

 radiation from R. The air and other stray radiations were 

 allowed for. Now it is at once obvious that this will not 

 give an absolute comparison of the absorbabilities of the 

 primary and secondary beams. We shall have to correct for 

 the absorption of the two beams in the air and radiator, and 

 also for the obliquity of the secondary beam. If, however, 

 we compare these absorbabilities, using a thin radiator 

 and the bulb only when very soft, we obtain a measure of 

 the correction necessary, as it has been shown by much 

 direct and by an overwhelming amount of indirect evidence 

 that the scattered radiation is of the same penetrating power 

 as the primary*. Much thicker radiators could be used 

 when experimenting with the hard portion of the primary 

 beam, as then the beam is not very heterogeneous. 



IX. lie suits for the Absorbabilities of the Primary and 

 Secondary Radiations. 



When the bulb w;is very soft the results in Table VI. were 

 obtained after subtracting the air effect, using four sheets of 



Table VI. 



(a) 



Percentage absorption of 

 beam by aluminium 

 0*15 mm. thick. 



32-15 



31-8 



33-15 



Percentage absorption of 

 beam by aluminium 

 0*15 mm. thick subsequent 

 to absorption in column 1. 



Nature of 

 beam. 



25-55 



24-6 

 24-35 



Secondary. 



Primary. 



Secondary. 



(&) 



29-95 



27-8 



23-75 

 21-9 



Primary. 

 Secondary. 



* Barkla, Phil. Mag. Sept. 1911, pp. 396-412. 



