770 Mr. H. Pealing on Distribution and Quality of 



held in the vertical plane by means of a thin brass rod 

 which was capable of rotation in a circular block of wood D. 

 The rod was provided with an index (shown in din gram) 

 moving on a scale marked in divisions of 5 degrees, to 

 determine the angle the plane of the radiator made with the 

 primary beam. The secondary electroscope S, which was of 

 the simple Wilson type, was attached to a block of wood, and 

 was capable of rotation about the same axis as R. The 

 electroscope which was used to standardize the primary 

 beam was placed on the top of the lead box used to contain 

 the bulb, and a vertical beam or the rays was admitted through 

 a window of thin tissue-paper at the base of the electroscope. 

 The width of this beam could be varied by a screw. 



The secondary electroscope was fixed so that the perpen- 

 dicular to the centre of the window (consisting of aluminium 

 foil between two sheets of tissue-paper ) admitting the rays 

 passed through the axis of rotation of the radiator, thus 

 ensuring that S was always symmetrical about the axis of 

 rotation of the radiator, uo matter what angle the axis of the 

 electroscope made with the direction of the primary rays. It 

 was provided with an index moving over the scale on D to 

 measure this angle. The window of S was 2*4 cm. wide and 

 5' 5 cm. high, and was distant from the centre of R 17 cm. 

 S was provided with a lead tube T lined with aluminium, 

 which admitted nearly all the rays from the carbon which 

 would have fallen on the window of S but cut off most of the 

 air and other stray radiations. These were also limited by 

 the tube T 2 which allowed a beam of rectangular cross-section 

 to fall on the radiator. The anticathode A of the bulb 

 contained in the lead box B was distant from the centre of R 

 40 cm. The bulb was held in a wooden stand with the cathode 

 stream inclined at an angle of 45° to the vertical. 



This was a very convenient arrangement, and was devised 

 by Sadler and used by him with Bowen three years ago to 



determine the variation of -— a , using soft beams. Later the 



ho 



writer was asked by Sadler to complete the experiments; 

 but on the publication of an investigation on the same subject 

 and by essentially the same method by Barkla and Ayres, on 

 his advice the experiments were abandoned. Later, the 

 writer observed several apparent anomalies in the results, 

 so the apparatus was reconstructed. The dimensions are 

 given above. They are slightly different from these of the 

 former apparatus. 



