Symmetry slioivn by Secondary X-Rays. 859 



that this was the case. The radiating sheets were o£ thin 

 metal, about 1-J in. square. In fig. 2 the cylinder is shown 

 as arranged for the measurement of emergence secondary 

 radiations : it hardly requires further explanation. 



We expected that this arrangement would show up the 

 want of symmetry better than the former, because the 

 portions of the emergence and incidence beams under com- 

 parison would be more nearly normal to the plate. Looking 

 upon the radiations as material, we should naturally expect 

 the intensity of the secondary radiation to decrease gradually 

 as its direction increased in inclination to the forward 

 direction of the primary ray. The emergence rays lie, 

 in inclination, between 0° and 90° ; the incidence between 

 90° and 180°. In our first arrangement we compared the 

 emergence rays between about 40° and 90°, with the incidence 

 rays between about 90° and 140°. There should be a larger 

 ratio of emergence to incidence with the newer arrangement, 

 since the emergence rays between about 30° and 50° would 

 be compared with the incidence between about 130° and 150°. 

 This proved to be the case ; the improvement was consider- 

 able. Again, with the new arrangement, the current with 

 no radiator in position became relatively far smaller. For 

 example, when the radiator was Al, *4 mm. thick, and the 

 absorbing screen DD of tinfoil (two thin sheets), the 

 currents with and without the radiator at B in fig. 1 caused 

 deflexions of 86 and 26 mm. in ten seconds respectively ; 

 the currents with and without the radiator at B in fig. 2 

 were 220 and 35 respectively. There could be very little 

 error, therefore, in taking the incidence and emergence 

 radiations as 60 and 185 respectively ; and the want of 

 symmetry is beyond doubt. 



It should be observed that the emergence radiation can 

 never be shown to an unfair advantage in these experiments, 

 and is often at a disadvantage, for the radiator, when placed 

 as in fig. 2, cuts down the very primary rays to which the 

 secondary radiation is due. It is not difficult to show that 

 if the thickness of the radiator is so adjusted as to give the 

 maximum emergence current (it can of course be too thick 

 or too thin), then the ratio of this maximum to the maximum 

 incidence current (which can be obtained simply by making 

 the radiator thick enough) is only 2/e of the true ratio of 

 emergence to incidence ; provided that the secondary rays 

 are as penetrating as the primary, and that we are con- 

 sidering homogeneous radiations. But if, other conditions 

 being the same, the secondary rays are less penetrating 

 than the primary, then the ratio, as found, is more nearly 



