304 DISCOVERY CH. 



therefore, be obtained, showing the internal structure 

 of the hand or other parts of the human body. 



These properties of X-rays seemed very wonderful 

 when they were first made known ; but, as we have 

 seen, their discovery was only a natural consequence 

 of earlier scientific work. Without Sir William Crookes's 

 investigations on the electric discharge in high vacua, 

 there would have been no Crookes's tube and no X-rays. 

 Rontgen suspected that an excited Crookes's tube would 

 cause fluorescence, that is, make certain substances 

 shine faintly ; and he noticed that paper covered with 

 a certain substance did show a faint shimmer when the 

 tube was in action, although the tube was covered so 

 that no light could be seen from it. Many other people 

 had used the tubes for lectures and experiments, and the 

 effects visible in it are so beautiful that it is not strange 

 that attention was concentrated upon them. It would 

 have seemed strange at that time to cover up a luminous 

 tube and look not at its own brilliant colours, but at 

 something else, yet that is what Rontgen did, and he 

 found that the unseen rays emitted by the tube were of 

 far greater interest than those which affected the human 

 retina. 



Most of the tubes now used for the production of X-rays 

 are modifications of the " focus tube " devised by Prof. 

 Herbert Jackson, of King's College, London, for the 

 study of phosphorescence, before Rontgen's discovery 

 was announced. Prof. Jackson might have made a 

 fortune by patenting his invention, but he decided 

 deliberately not to do so, and for the sake of scientific 

 knowledge left his device unprotected. In thus sacrific- 

 ing personal profit to scientific needs, he manifested a 

 spirit which the commercial world may appreciate but 

 cannot understand. 



