( 106 ) 



The aperture in S^ was closed by a disk of black paper; the back 

 side of A was closed by a metal cover, which might be screwed off. 



The dimensions were chosen in such a way, that the boundary 

 of the beam of Röntgen rays, which passed through the apertures 

 in Si, S^ and S^, lay between the outer side of the carbon bar and 

 the inner side of the tube B. The photographic film covering the 

 inside of B was therefore protected against the direct Röntgen rays. 



If we accept Barkla's supposition on the way in which the 

 secondary beams are generated in bodies of small atomic weight, 

 and if the axis of the primary beam perfectly coincided with that 

 of the carbon bar, then a total or partial polarisation of the Röntgen 

 rays would give rise to two maxima of photographic action on 

 diametrically opposite parts of the film and between them two 

 minima would be found. From the direction of the axis of the cathode 

 rays the place of these maxima and minima might be deduced. 



A very easy method proved to exist for testing whether the primary 

 beam passed symmetrically through the tube B or not. If namely 

 the inner surface of cover D was coated by a jjliotographic plate or 

 film, which therefore is perpendicular to the axis of the carbon bar 

 then we see after developing a sharply defined bright ring between 

 the dark images of the carbon bar and of the ebonite disk. This ring 

 could also be observed on the fluorescent screen — but in this case 

 of course as a dark one, — and the Röntgen bulb could easily 

 be placed in such a way, that this ring was concentric with the 

 images of the carbon bar and of the ebonite disk. 



This ring proved to be due to the rays that diverged from the 

 anticathode but did not pass through the carbon bar perfectly parallel 

 to the axis and left it again on the sides; these raj^s proved to 

 be incapable of penetrating the ebonite, but were totally absorbed by 

 this substance; when the ebonite disk was replaced by a carbon one, 

 then the ring disappeared ; it is therefore a very interesting instance 

 of the selective absorption of Röntgen rays ^). 



When in this way the symmetrical passage of the Röntgen rays 

 had been obtained, then the two maxima and minima never appeared, 

 neither with short nor with long duration of the experiment, though 

 a strong photographic action was often perceptible on the film. Such 

 an action could for instance already be observed after one hour's 

 exposure, if an induction-coil of 30 cm. striking distance was used 

 with a turbine interruptor. A storage battery of 65 volts was used ; 



ij Take for lliis experiment the above described arrangement, but a carbon bar 

 of 1 cm. diameter and 4 cm. long. 



