DR. CHARLES O. BARKLA ON POLARISED RftNTGEN RADIATION. 



179 



reduced. It will be seen from Table III. that in the earlier experiments in which the 

 radiator was air alone the possible error was a considerable fraction of the total 

 variation. 



I give the result of later observations which show the polarisation effect much more 

 clearly : 



The duration of each experiment was regulated to give an approximately constant 

 deflexion of electroscope A, so that, as in the second part of Table III., the numbers 

 give merely the ratios of deflexions. 



I have also studied the radiation from a large number of substances and have found 

 that with an increase in atomic weight of the radiator the disappearance of the 

 variation in intensity does not take place abruptly, but that there are elements 

 having atomic weights between those of the two classes of substances spoken of in this 

 paper which exhibit an intermediate variation. 



Calcium, for instance, when subject to a certain radiation, was the source of a 

 secondary radiation, the variation in intensity of which was about half that exhibited 

 by substances of low atomic weight. 



The change with atomic weight may, however, take place in well-defined steps. 

 Also, I have found that there is a considerable change in the amount of variation with 

 a change in the hardness of a given X-ray tube. 



A full discussion of these results, along with others outside this subject, will be 

 published later in a paper on Secondary Radiation.] 



