Thin Films of Elements exposed to Ri'mtgenRays. 139 



obtain for the constant* observed in Tables II. and III. for 

 E/» A«* the value 2930. Bohrf, however, calculates from 

 theoretical considerations that Whiddington's ; ' a " should 

 have the value l'lx 10 4 ° for air at a pressure of 76 cm. and 

 at 15° C. for slowly moving electrons. Using Bohr's value 

 corrected to 0° C, we obtain the result 2160 for the theoretical 

 value of the constant tabulated in Tables II. and III. 



It will be seen that the values for the constant in Table III. 

 show better agreement amongst themselves and with the 

 theoretical value than is shown in Table II. The mean 

 value in Table III. is 2000, whilst the separate values in 

 Table II. differ so widely that it does not appear legitimate 

 to take a mean value at all. The direct conclusion is that 

 each atom is associated with the emission of one quantum 

 only, and that entirely different atoms of a homogeneous 

 substance emitting various spectral lines concomitantly are 

 associated with the K, L, or M radiations; but further 

 experiment must decide this important conclusion. 



Finally, it must be clearly stated that the method by 

 which these curves have had to be analysed precludes all 

 possibility of an accurate determination of the ranges of the 

 sub-groups as it turns on the discrimination of those points 

 in the curves of figs. 5 and 6 (PI. III.), where the function 

 represented by the one portion of each curve changes into 

 that represented by the proximate position. 



To me, the surest evidence for the existence of the sub- 

 groups rests in figs. 8 and 9 (PL III.), roughly conforming 

 as they do to the requirements of the photo-electric equation. 

 The ten curves of fig. 8 (PI. III.) divide themselves into 

 two distinct sections : those of the one section being below, 

 and those of the other section above the silver curve, the 

 positions of the minima, representing the range of the sub- 

 group, approaching nearer and nearer to the screen as we 

 pass up from As to Se to Zr, starting out again at Ba and 

 approaching once more as we pass up through An to Pb to 

 Bi. In fig. 9 (PI. III.) the division into sections is at 

 the Ba, the sub-group having a smaller and smaller speed 

 as we pass up from As to Sb, starting out again at Au and 

 diminishing to Bi. All this is exactly as is required by the 

 photo-electric equation . 



Because of the difficulties involved in obtaining experi- 

 mental curves which will bear differentiation with any 



5 — - " . = - — — , from y*=«t7. 



77 ha~ n hv 2 m q" 



t Phil. Mag. ? er. 6, vol. xxv. p. 28 (1913). 



