THE X-RAYS. 



151 



From these and similar experiments with glass and tin we may draw 

 the following- result: if a body be imagined to be made up of successive 

 layers with their faces perpendicular to the direction of the X-rays, 

 each of these layers will be more transmissible than the one next pre- 

 ceding. In other words, the specific transmissibility of a body is greater 

 the thicker the body. This "result is completely in accord with obser- 

 vations which may be made by photography of a tin scale as described 

 in section 4 of my first communication, and also with the fact that 

 occasionally in photographic shadow pictures, the shadows of thin lay- 

 ers, as for example the pai)er used in wrapping the plate, came out 

 relatively strongly. 



5. If two plates of different substances are equally transmissible 

 this equality will not in general be retained for another pair of plates 

 of the same substances with thicknesses altered in the same ratio. 

 This fact may be shown very easily by the use of thin sheets, as, for 

 example, of platinum and aluminum. I used for this purpose platinum 

 foil 0.002G millimeter thick and aluminum foil 0.0299 millimeter thick. 

 I found iu one instance that one sheet of platinum was equally trans- 

 missible with six sheets of aluminum j but the transmissibility of two 

 sheets of platinum was less than that of twelve sheets of aluminum 

 and about equal to that of sixteen sheets of the latter metal. Using 

 another discharge tube, I found 1 platinum equal 8 aluminum but 8 

 platinum equal 90 aluminum. From these experiments it follows that 

 the ratio of thicknesses of platinum and aluminum of equal trans- 

 missibility is less the thicker the sheets under examination. 



6. The ratio of the thicknesses of two equally transmissible i^lates of 

 diU'erent material is dependent on the thickness and the material of 

 the body, as, for instance, the glass wall of the discharge tube, through 

 which the rays have to pass before they reach the plates investigated. 



7. The experiments described in sections 4, 5, and 6 relate to the 

 alterations which the X-rays proceeding from a discharge tube experi- 

 ence in their transmission through different substances. It will now 

 be shown that one and the same body may for the same thickness be 

 unequally transmissible for rays emitted from different discharge 

 tubes. 



In the following table are given the values of the transmissibility of 

 an aluminum plate 2 millimeters thick for the rays given out by 

 different tubes : 



The discharge tubes were not materially different in their construction 

 or in the thickness of their glass wall, but varied iu the density of the 



