43 



quently, the radiation measured on the far side of a lead 

 plate is relatively large when the plate is thin, and falls off 

 more quickly at first than it does subsequently, the plate 

 being gradually thickened. The logarithmic curve of Pb has 

 a rapid initial fall, as several observers have shown. But 

 there is a difference in the case of Al or any substance of 

 small atomic weight. The soft y rays give rise to much less 

 secondary radiation, and, moreover, they are not used up so 

 fast. Both these causes operate to make the logarithmic ab- 

 sorption curve of Al more nearly a straight line, as is actu- 

 ally the case. 



It appears that a screen of any material absorbs the soft 

 rays faster than the hard. Consequently, for example, a zinc 

 screen or a lead screen may be used indifferently in order ta 

 reduce the ratio of the emergence radiation of lead to that 

 of zinc. But a screen of large atomic weight acts more 

 rapidly. If the screens of lead and zinc are chosen of such 

 thicknesses that they absorb hard rays to an equal extent^ 

 then the former absorbs more of the soft rays than the latter. 

 The curve CG (fig. 2) shows the results which were obtained 

 when a zinc plug of 205 cm. thickness was placed in the coni- 

 cal opening. In both CC and A A the value for Sn is a little 

 too small ; we were not aware at the time that our Sn plate 



Fig. 3. 



