THE X-RAYS. 



139 



thickness that all appeared to be equally transmissible. The following- 

 table gives the measured thickness in millimeters, the relative thickness 

 compared with platinum, and the specific gravity : 



From these values it may be seen that the transmissibility of plates 

 of difierent metals so chosen that the product of the thickness and 

 density is constant would not be equal. The transmissibility increases 

 much faster than this product falls oft". 



6. The fluoreseuce of barium-platinum-cyanide is not the only action 

 by Avhicli X-rays maybe recognized. It should be remarked that they 

 cause other substances to fluoresce, as for example the photophores- 

 ceut calcium compounds, uranium glass, common glass, calcspar, rock 

 salt, etc. 



It is of particular importance from many points of view that photo- 

 graphic dry plates are sensitive to X-rays. It thus becomes possible 

 to fix many phenomena so that deceptions are more easily avoided j and 

 I have where practicable checked all important observations made with 

 a fluorescent screen by photographic exposures. 



It appears questionable whether the chemical action upon the silver 

 salts of the photographic i)late is produced directly by the X-rays. It 

 is possible that this action depends upon the fluorescent light which, as 

 is mentioned above, may be excited in the glass plate, or perhaps in 

 the gelatine film. " Films " may indeed be made use of as well as glass 

 plates. 



I have not as yet obtained experimental evidence that the X-rays are 

 capable of giving heat. This characteristic might, however, be assumed 

 as present, since in the excitation of fluorescent phenomena the capacity 

 of the energy of the X-rays for transformation is proved, and since it 

 is certain that of the X-rays falling upon a body not all are given up. 



The retina of the eye is not sensitive to these rays. Nothing is to be 

 noticed by bringing the eye near the vacuum tube, although according 

 to the preceding observations the media of the eye must be sufficiently 

 transmissible to the rays in question. 



7. After I had discovered the transmissibility of various bodies of 

 relatively great thickness I hastened to investigate whether or not the 

 X-rays were refracted in passing through a x)rism. Experiments with 

 water and carbon bisulphide in mica prisms of 30 degrees refracting 

 angle showed no deviation either when observations were made with the 

 fluorescent screen or with the photographic plate. For comparison, the 

 deviation of light rays was observed under the same conditions. The 



