$66 Sir E. Rutherford and Dr. E. N. da C. Andrade on 



was as shown in fig. 5 ; the slit was narrowed down to about 

 0*8 mm. by means of aluminium strips N, N, and the 

 radiator R placed opposite it in the position indicated. The 

 emanation tube A was fixed to one side, so that no direct rays 



Fi 



?• 5- 



P A°\R 



» 







from it could strike the photographic plate. A second block 

 of lead P was placed behind it, to increase the intensity by 

 means of successive "reflexions'*' of the /3 rays between 

 this block and R. 



The spectrum of the radiation excited in the lead plate L 

 was then determined under as nearly as possible the same 

 conditions as for the y rays from the emanation tube. For 

 a given source, the photographic effect of the spectrum lines 

 from lead only showed up faintly against the general 

 blackening of the plate, but was sufficiently clear to admit 

 of measurements of some of the angles of reflexion. Only a 

 few lines of lead could be measured ; two of these gave 

 reflexion angles of 10° 2' and 12° 0' in good agreement with 

 the strong lines of the radium B spectrum. Other faint lines 

 were also observed but were difficult to measure. There was, 

 however, a possibility of error in such an experiment. It 

 was conceivable that the spectrum lines observed were not 

 due to the characteristic radiations from the lead but were to 

 be ascribed to some of the soft 7 radiations from radium B 

 scattered by the lead plate. To test this point, the lead plate 

 was replaced by one of platinum of the same dimensions and 

 the spectrum again measured. The positions of the lines 

 were quite distinct from those observed with the lead 

 radiator, and the measurements of the reflexion angles of two 

 of the strongest lines were in fair agreement with those 

 given by Moseley and Darwin for platinum. 



According to Moseley's results, the frequency of a 

 reference line for the X-ray spectra of successive elements 



