Radiation by Impact of Positive and Slow Cathode Rays. 625 



effect behind a film of collodion thin enough to show the 

 colours of thin plates, paraffin-wax 4 p thick, mica, or thin 

 fluorite *. When the energy corresponds to 80 volts the effect 

 behind the paraffin and collodion is appreciable, while with 

 200 volts and more there is very considerable penetration of 

 the collodion and paraffin by the rays. 



The great opacity of very thin films suggests that the 

 frequency of the rays may be within tho limits of those 

 vibrations which, according to the usual theory of dispersion, 

 are totally reflected by a medium. 



According to this theory, if the medium has only one free 

 period of frequency n, it is impervious to light whose frequency 

 p is between the limits given by the equation 



p — n 

 and fr = )r + — , 



when N is the number of electrons per unit volume, e and m 

 are respectively the charge and mass of an electron. This 

 relation applies when the wave-length is large compared with 

 the distance between the molecules; and the limits of opacity 

 depend on the degree of closeness with which the molecules 

 are -packed : for example, in a gas they depend upon the 

 pressure. In the case of Rontgen rays when the wave- 

 lengths are small, or even comparable with the distance 

 between the molecules, the case is different. The effect of 

 matter in this case is not so much to increase the refractive 

 index as to scatter the radiation, and this scattering will be 

 greatest when the atom is impervious to the radiation. If we 

 apply the equation to the atom itself, it indicates that the atom 

 would be impervious to and therefore scatter strongly rays 

 whose frequency is between limits which depend on the 

 density of the electrons within the atom, and not on the close- 

 ness with which the atoms are packed. 



When there are more frequencies than one intrinsic to the 

 atom, there will be several regions of great opacity separated 

 by intervals of comparative transparency. 



I am indebted to my assistants Mr. Everett and Mr. Engle 

 for the assistance they have given me in these experiments. 



* With larger currents from the Wehnelt I have been able to detect 

 the photographic effect of the 40 volts rays behind thin collodion and 

 mica, and also to detect, the photoelectric effect and ionization due to the 

 rays which had passed through the films. 



PA/7. Mag. S. G. Vol. 28. No. 1G6. Oct. 1914. 2 S 



