292 . Dr. C. G. Barkla on 
sensitive the ratio of deflexions would therefore have been 
7-45x18-05 
i.e. 
12-5 
10-76 : 5-575 
: 5*575. 
or 1-03 : 1, 
That is, the average intensities of the secondary beams from 
R received by the electroscopes A x and A 9 were in the ratio 
1-93 : 1. 
As this ratio appeared to vary considerably in different 
experiments by an amount much greater than the possible 
experimental error would account for, it was apparent that 
it depended to a certain extent on the character of the primary 
radiation. 
Gas was therefore admitted into a regulating X-ray tube, 
until it was brought to that state when a discharge through 
it did not produce X-rays which could be detected by the 
electroscopes at all. A discharge was then passed through 
until rays were emitted in sufficient intensity to be experi- 
mented upon. The ratio of intensities was then found to be 
2*0 : 1. When the bulb was somewhat " harder " this became 
1'93 : 1, and when " harder" still the ratio became 1*80 : 1. 
In order to experiment on a beam of much greater average 
penetrating power, the more absorbable portion of this beam 
was then cut off by a plate of aluminium -04 cm. in thickness. 
This diminished the ionizing power of the primary to about 
30 percent, of its original value, while the transmitted beam 
was on the average about twice as penetrating. Using this 
penetrating primary beam, it was found that the ratio of 
intensities had dropped to 1*51 : 1. 
These experiments were repeated with other X-ray tubes 
and the results were confirmed, the ratio never exceeding 
2 : 1, and decreasing with increasing penetrating power of 
the primary rays used. 
It must, however, be observed that the primary beam used 
in these experiments was partially polarized, the variation in 
intensity of secondary beams from carbon in the two principal 
directions at right angles to the direction of primary propa- 
gation amounting to about 20 per cent, when the beam was 
of the very absorbable type, but only to 6 or 7 per cent.* 
when the rays were much more penetrating. Thus the 
* There is, of course, probably no lower limit to the variation, but 
these were approximately the limits found for the different beams of the 
type used in these experiments. 
