472 DR. CHARLES G. BARKLA ON POLARISED RQNTGEN RADIATION. 



from the radiator placed in the primary beam opposite apertures C 3 and C' 3 , and that 

 the deflexion of B was due to the narrow beam of primary radiation passing 

 through Of, 



It was found that when the bulb was in a given position, there was a constant 

 relation between the three rates of deflexion when the character of the primary 

 radiation was constant As the radiation emitted by the bulb gradually changed, 

 there was found to be with some radiators a change in the ratio of the deflexions of 

 the secondary and primary electroscopes. This change, however, was a slow one, and 

 the primary electroscope deflexion could always be used to standardize the intensity 

 of primary radiation. 



The method of experiment was then as follows : 



The X-ray bulb was placed with the axis of the conical beam of kathode rays 

 (proceeding from the kathode to the antikathode) in a vertical position, and 

 consequently perpendicular to the line joining the antikathode to the middle of the 

 secondary radiator. The primary beam thus studied proceeded from the antikathode 

 in a direction perpendicular to that of propagation of the kathode rays which produced 

 it. The relation between the rates of deflexion of the electroscopes was obtained 

 when a discharge was sent through the bulb by an induction coil. A number of 

 successive observations showed this to be constant. 



The bulb was then turned round the axis of the primary beam into some other 

 position. Care was taken to keep the source of X-rays in as nearly the same position 

 as possible. To do this the centre of the antikathode was viewed through a telescope 

 when the door of box L was open, and was focussed on the cross- wires of the eye- 

 piece. The telescope was fixed, and when the position of the bulb was changed the 

 adjustment was made so that the centre of the antikathode was again focussed on 

 the cross-wires. It was also kept in a plane parallel to the side of the containing 

 box by viewing it ajong such a fixed plane, so that the source of X-radiation in the 

 experiments was not displaced more than a few millimetres. 



The axis of the kathode stream was kept perpendicular to the direction ot 

 propagation of the primary beam studied, by simply observing that the axis of the 

 tube D was always in a plane approximately perpendicular to what might be called 

 the axis of the primary beam. As neither the kathode rays in the bulb nor different 

 portions of the primary X-beam were parallel, perfect adjustment of the bulb in this 

 manner was not essential. It was also experimentally found that much larger 

 angular displacements of the bulb than might possibly occur in the adjustment did 

 not produce an appreciable change in the ratio of the rates of deflexion of the 

 electroscopes. 



As the bulb was rotated round the axis of the primary beam there was, of course, 

 no change in the intensity of primary radiation in that direction. There was, however, 

 a considerable change in the intensity of secondary radiation in both the horizontal 

 and vertical directions, one reaching a maximum when the other attained a minimum. 



