562 Sir E. Rutherford and Mr. H. Robinson on Mass and 



distance the line of scintillations was not very clearly- 

 defined. A number of determinations were made for dif- 

 ferent voltages, which agreed fairly well with one another. 

 It was found, however, difficult to fix with certainty the 

 centre of the line of weak scintillations, and in consequence 

 the deflexions observed on reversal of the field were found 

 in many cases to be unsymmetrical about the central line. 

 This uncertainty arising in the use of the scintillation method 

 led us finally to adopt a photographic method. The source 

 in the final experiments was a thin- walled a-ray tube con- 

 taining about 100 millicuries of purified emanation. The 

 apparatus as finally used is shown in fig. 2. 



Fir-. 2. 



The electrostatic field was applied between the parallel 

 faces of two silvered strips of plate- glass P, P, each 35 cm. 

 long, 2*5 cm. wide, and 1 cm. thick. These were held 4 mm. 

 apart by ebonite stops and supported in ebonite rockers. The 

 a-ray tube S, which was bent at a right angle at the point 

 where it was joined to the thicker glass, and into which the 

 purified emanation was sealed over mercury, was supported 

 centrally near one end of the glass plates. Across the other 

 end of the plates was arranged a slit, consisting of two plates 

 of thin mica ^ mm. apart, fastened into a mica framework. 

 This apparatus was placed in a large glass tube, closed at the 

 ends by ground-glass plates, and provided with a side tube 

 for exhaustion. Electrical contacts with the two glass plates 

 were made by metal springs passing through ground joints 

 in the sides of the tube. The photographic plate A, wrapped 

 in aluminium leaf to protect it from stray light and from the 

 glow of the source itself, was fixed to one of the glass end 

 plates which closed the tube, and was at a distance of about 

 50 cm. from the slit, or 85 cm. from the source. The path of 



