152 THE X-KAYS. 



gas witliiu tliem, and hence in the potential required to produce dis- 

 charge. Tube 1 required the least and tube 5 the greatest potential, 

 or, as we may say for sliort, the tube 1 is the " softest" and tube 5 the 

 "hardest." The same Ruhmkorff in direct connection with the tubes, 

 the same circuit breaker, and the same current strength in the primary 

 circuit were used in all cases. 



Various other substances which I have investigated behaved simi- 

 larly to aluminum. All are more transmissible to rays from harder 

 tubes. This fact seemed to me particularly worthy of attention. 



The relative transmissibility of plates of different substances proved 

 also to be dependent on the hardness of the discharge tube employed. 

 The ratio of the thickness of platinum and aluminum plates of equal 

 transmissibility becomes less the harder the tubes from which the rays 

 ])roceed, or, referring to the results just given, the less the rays are 

 absorbed. 



The different behavior of rays excited in tubes of different hardness 

 is also made apparent in the Avell-known shadow picturing of hands, etc. 

 With a soft tube a dark shadow is obtained in which the bones are little 

 prominent; when a harder tube is used the bones are very distinct and 

 visible in all their details, whereas the softer portions are less marked, 

 and with very hard tubes even the bones themselves become only weak 

 shadoAvs. From these considerations it appears that the choice of the 

 tube must be governed by the character of the objects which it is desired 

 to portray. 



8. It remains to remark that the quality of rays iiroceeding from one 

 and the same tube depends on various conditions. Of these the most 

 important are the following: (1) The action of the interrupter,' or, in 

 other words, tlie course of the primary current. In this connection 

 should be mentioned the phenomena frequently observed that particular 

 ones of the rapidly succeeding discharges excite X-rays which are not 

 only more intense, but which also differ from the others in their absorp- 

 tion. (2) The character of the sparks which appear in the secondary 

 circuit of the apparatus, (3) The emi)loyment of a Tesla transformer. 



(4) The degree of evacuation of the discharge tube (as already stated). 



(5) The varying, but as yet not satisfactorily known, procedure within 

 the discharge tube. Separate ones among these conditions require 

 further comment. * * * 



The hardness of a tube had been considered to be brought about 

 solely by the continuation of the evacuation by means of the punq>; 

 but this characteristic is affected in other ways. Thus a sealed tube 

 of medium hardness becomes gradually harder by itself — unfortunately 

 to the shortening of the period of its usefulness when used in a suitable 

 manner for the production of X-rays, that is to say when discharges 

 which do not cause the platinum to glow or at least to glow only weakly 

 are passed through. A gradual self evacuation is thus effected. 



' A good Duprez iaterriipter Avorks more regularly than a Foucault interrupter; 

 the latter, however, conserves the primary current better. 



