300 Photographic Effect of X-rays and X-ray Spectra. 



also. The connexion with the spectral lines (series K) of Br 

 and Ag is shown in the lower portion of the figure. 



As might be expected, the photographic effect is not pro- 

 portional to the absorption of Rontgen rays, in silver, bromine, 

 or silver bromide. For silver bromide is surrounded by gela- 

 tine consisting of light elements. On the one hand, a 

 portion of the photographic effect is undoubtedly due to the 

 secondary rays from this ; and, on the other hand, a portion 

 of the energy originally absorbed in silver bromide is re- 

 emitted and ultimately absorbed by the gelatine. It is 

 significant that when the absorption in silver bromide 

 relative to that in air increases either through the bromine 

 or through the silver, the increase in photographic effect is 

 approximately proportional to the increase in absorption. 



The curve is thus of the same general shape as an absorp- 

 tion curve for silver bromide. This is shown by a broken 

 line in fig. 1. It is probable that there would be a much 

 closer similarity between the photographic curve and an 

 absorption curve for the whole emulsion of silver bromide 

 and gelatine. 



As the values given in column 3, Table I., and plotted in 

 fig. 1, give the photographic effect for equal ionizations pro- 

 duced by the beams in a few centimetres thickness of air, 

 they give us little idea of the relative photographic effects 

 produced in equal times by beams of equal intensity ; for, of 

 course, the more absorbable radiations produce much larger 

 ionizations in an electroscope which absorbs only a small 

 fraction of the radiation passing through it than do the more 

 penetrating radiations of equal intensity. If we assume that 

 the total energy of a beam is proportional to the total number 

 of ions it produces in a given gas when completely absorbed 

 — an assumption which is probably not far from the truth 

 for the radiations considered, — the relative photographic 

 effects of beams of equal intensity are given by the product 

 of the numbers in columns 2 and 3 of Table I. 



The penetrating radiations of course produce small photo- 

 graphic effects owing to the small amount of energy absorbed. 

 The effect of the secondary radiations from other elements 

 in the emulsion will probably be shown by adding salts con- 

 taining heavy elements. We intend to make a more complete 

 investigation of the subject. 



King's College, London. 



Note. — I wish to express my thanks to the Solvay Inter- 

 national Physical Institute for a grant in aid of these 

 researches. — C. G. B. 



