482 Prof. E. Rutherford : Some 



that the a. or easily absorbed rays of radium are deviated by 

 an intense magnetic field. In the course of his paper 

 M. Becquerel states, " M. E. Rutherford, avec une grande 

 habilete, et par une methode electrique relativement grossiere, 

 a, reconnu un phenomene d'une extreme delicatesse. Cepen- 

 dant la methode employee laisserait prise a diverses objec- 

 tions et a un doute sur l'existence du phenomeme en 

 question si l'on n'en apportait pas d'autre preuve. L'une 

 des objections resulte de la disposition experimentale qui fait 

 traverser des espaces laminaires par un rayonnement dont la 

 partie cathodique est rejetee sur les parois, et les rayons 

 secondaires qui en resultent peuvent donner lieu a des efFets 

 dans le sens observe par M. Rutherford. Je me suis alors 

 propose de mettre en evidence le phenomene par une expe- 

 rience plus simple et plus sure. J'ai eu recours a l'une des 

 dispositions pbotographiques que j'emploie depuis longtemps 

 et qui permettent certaines distinctions qui n'apparaissent pas 

 toujours quand on emploie exclusivement la methode elec- 

 trique." 



I fully recognize the great simplicity and utility of the 

 photographic method, so ably developed by M. Becquerel 

 himself, for determining the magnetic deviation of rays and 

 determining accurately the curvature of their path, provided 

 that the rays emitted are sufficiently intense to affect a photo- 

 graphic plate without too long an exposure. 



The difficulty of obtaining satisfactory photographs in cases 

 of less active material is clearly shown by M. BecquerePs own 

 ■experiments on the rays of polonium described in the same 

 paper. I have unfortunately not had at my disposal very 

 active preparations of radium, and in consequence have had 

 to adapt my methods to obtain effects from such active 

 matter as I possessed. It was for this reason that I employed 

 the electric method, which is capable of extreme refinement, 

 and can be used to compare rapidly the intensities of radia- 

 tions which would require very long exposures to act appre- 

 ciably on the photographic plate. As an example of the 

 comparative sensitiveness of the two methods I may recall 

 some of M. Becquerel's own experiments (Comp. Rend. p. 209, 

 1902) in which he was unable to detect photographically any 

 action of the a rays from uranium, or any evidence of the 

 existence of very penetrating rays from it, although the times 

 of exposure in two experiments were as long as 20 and 

 42 days. 



By the electric method the a rays from I milligramme of 

 uranium can be quickly measured ; and I have recently 

 found that, using 100 grs. of uranium oxide, the existence of 



