Photographic Effect of 'X-rays and X-ray Spectra. 297 



which it traversed. The angles which the axes of these two 

 pencils of radiation — one producing photographic action, the 

 other ionization — made with the normal to the plate were 

 approximately equal. 



The photographic action and the ionization were pro- 

 duced simultaneously until the electroscope deflexion reached 

 a suitable definite value obtained by preliminary experiments. 

 The radiating plate was replaced by another, the photo- 

 graphic plate was moved over its aperture so that a fresh 

 portion was exposed to the fluorescent radiation, and the 

 electroscope was re-charged. Again, the exposure lasted 

 until the same deflexion was produced in the electroscope by 

 the fluorescent X-radiation from the second substance. In 

 this way the photographic plate was exposed to various 

 homogeneous radiations for periods sufficient for these to 

 give identical ionizations in the air contained in the electro- 

 scope. After development of the plate it was seen that the 

 various radiations had not produced equal photographic 

 effects. In order to get a standard for comparison of these 

 photographic effects, a photographic wedge was made with 

 a similar plate exposed to a uniform light for periods pro- 

 portional to the distance from one end of the wedge. By 

 comparing the various exposed portions of the plate affected 

 by X-rays with this wedge, the relative exposures required 

 to produce the effects observed for the various homogeneous 

 X-radiations were obtained. The results given in Table L, 

 column 3, are the mean values obtained from a number of 

 experiments, the number varying from three to eight for 

 different radiations. 



X-racliation producing 

 photographic action. 



Mn X-rays (series K) 

 Zn 



Mo 



A S 



Sn „ „ 



Sb „ „ 



■*■ J) 55 



Ce 



Table I. 



Absorbability 

 of radiation 



47-7 

 39-4 

 16-4 

 5-3 

 2-5 

 1-57 

 1-21 

 •92 

 •6 



Relative Intensity of Photo- 

 graphic Effect (measured as 

 indicated) for equal ioniza- 

 tions in air (8 cm. thickness). 



1-77 

 1-73 

 1-67 

 1-75 

 2-25 

 2-25 

 2-75 

 4-22 

 4-62 

 4-67 



If we attempt to plot the absorbability of the homogeneous 

 Phil. Mag. S. 6. Vol. 25. No. 146. Feb. 1913, X 



