860 Prof. Bragg and Mr. Glasson on a Want of 



correct, and is very nearly so when the secondary rays are 

 much less penetrating than the primary, as, for example, 

 when we are considering secondary cathode rays due to X- 

 or 7 rays. 



We have made a large number of measurements by the 

 method described above, using the following metal sheets as 

 radiators : — Pt, weight per square cm., *0150 gr. : Sn, '0096 

 gr. ; Cu, -0083 gr. ; Fe, -0077 gr. ; Al, "105 gr. ; celluloid, 

 '20 gr. As screens we have used various thicknesses of Sn, 

 Cu, and Al. 



The proportion of emergence to incidence radiation differs 

 considerably for the different radiators, but is much the same 

 for different screens or different thicknesses of screen, except 

 that the proportion tends to increase slightly as the screen 

 is made thicker; and the tendency is most pronounced in 

 the case of those metals which give out a quantity of soft 

 secondary radiation. For example, Fe and Cu show little 

 difference between incidence and emergence radiations until 

 the screen is so thick that only a small fraction of either of 

 the radiations can pass through. The results vary somewhat 

 with the state of the bulb ; and since these variations are 

 comparable with those which are met with on changing the 

 nature of the screens, we are not now in a position to discuss 

 smaller variations in detail. We must content ourselves 

 with quoting a few results in order to show the want of 

 symmetry, which is a persistent effect. When, for example, 

 two tinfoils were used as screen (weight per square cm. 

 of each, *0056), we obtained the following figures, which 

 represent movements of the scale in mm. during 10 sees. : — 



Kadiator Sn. Cu. Fe. Al. 



Emergence Current 176 140 39 185 



Incidence Current 122 119 15 60 



With four tinfoils the figures were : — 



Radiator Sn. Cu. Fe. Al. 



Emergence Current 143 24 23 116 



Incidence Current 87 1 34 



Again, using a copper screen *002 cm. thick, we found : — 



Cellu- 

 Radiator Pt. Sn. Cu. Fe. AL loid. 



Emergence Current... S6 140 361 118 80 138 

 Incidence Current ... 65 104 364 118 32 93 



Putting together a number of results for Cu screens of 



