154 THE X-RAYS. 



different absorbability, and the investigations of Hertz and Lenard 

 show that the cathode rays are simikirly to be discriminated. While 

 the "softest" tubes investigated generated rays much less subject to 

 absorption than any cathode rays investigated by Lenard, yet there is 

 no reason to doubt the possibility of X-rays of greater absorbability, 

 and cathode rays of less. It therefore api^ears probable that in future 

 investigations rays will be found bridging over the gap between X-rays 

 and cathode rays, so far as their absorption is concerned. (2) We 

 found in section 4 that the specific transmissibility of a body becomes 

 less the thinner the plate passed through. Consequently, had Ave made 

 use in our experiments of plates as thin as those employed by Lenard 

 it would have been found that the X-rays were more nearly like those 

 of Lenard in their .absorbability. 



10. Besides the tiuorescent phenomena, there may be excited by 

 X-rays photographic, electric, and other actions, and it is of interest to 

 know how far these various manifestations vary in similar ratio when 

 the source of the rays is altered. I must restrict myself to a compari- 

 son of the first two phenomena. * * * 



A hard and a soft tube were so adjusted as to give equally bright 

 fluorescence as compared by means of the photometer described in sec- ' 

 tion 2. Upon substituting a photographic plate in the place of the 

 fluorescent screen it was found, on development, that the portion sub- 

 ject to the rays from the hard tube was blackened to a less degree than 

 the other. The rays, though producing equal fluorescence, were thus 

 for photographic purposes unequally active. * * * 



The great sensitiveness of a photographic plate even for rays from 

 tubes of medium hardness is illustrated by an experiment in which 9G 

 films were superposed, placed at a distance of 25 centimeters from the 

 discharge tube, and exposed five minutes with due precautions to pro- 

 tect the films from the radiations of tlie air. A photographic action 

 was apparent on the last film, although the first was scarcely over- 

 exposed. * * * 



If the intensity of the radiations is augmented by increasing the 

 strength of the primary current, the i^hotographic action increases in 

 the same measure as the intensity of the fluorescence. In this case, as 

 in the case where the intensity of the radiation was increased by an 

 alteration of the distance of the fluorescent screen, the brightness of 

 the fluorescence is at least approximately proportional to the intensity 

 of the radiation. This rule should not, however, be too generally 

 applied. 



11. In conclusion, mention should be made of the following particu- 

 lars : 



With a discharge tube of proper construction, and not too soft, the 

 X-rays are chiefly generated in a spot of not more than 1 or 2 milli- 

 meters diameter where the cathode rays meet the platinum plate. This, 

 however, is not the sole source. The whole plate and a part of the tube 



