3 oo SCIENCE PROGRESS 



utilised their ionising properties for purposes of measurement, 

 though some methods have also been devised which take 

 measure of the heating produced when the rays are absorbed 

 by a metal. 



The nature of the rays. — There are very few scientific 

 discoveries which have excited the imagination and interest 

 of the world in the way that Prof. Rontgen's announcement 

 did. Speculation was rife, and many were the theories put 

 forward of the nature of the new rays. 



By some they were identified with extremely short light- 

 waves, but these were met with the argument that nothing 

 corresponding to regular optical refraction or reflection has 

 ever been detected with X rays. It is questionable, too, as will 

 be seen on further reading, whether any genuine diffraction 

 effects have been observed. 



On the other hand, in many of their properties the rays 

 strongly resemble the cathode rays ; but no observer has ever 

 noticed the slightest deviation in a. magnetic or electric field, 

 a result which indicates the absence of a free electric charge. 

 In this particular, then, there is an outstanding difference 

 between the X ray and its creative agency, the negatively 

 charged cathode ray. 



Rontgen himself proposed in the third of his Memoirs a 

 theory of neutral material particles. It did not receive general 

 acceptance, for it was then felt that the difficulty of accounting 

 on such a theory for the great penetrating power of the rays 

 was insuperable. 



Two theories at present hold the field : the ether pulse 

 theory put forward by Stokes in 1897, and since elaborated by 

 J. J. Thomson and Sommerfeld ; the neutral pair theory which 

 has lately received the advocacy of Prof. Bragg. On the one 

 view the Rontgen rays are pulses of electromagnetic disturbance 

 propagated with the velocity of light through the ether, on the 

 other they are doublets made up of a positive and an equal 

 negative charge travelling with great velocity, and possessing 

 energy and momentum. We shall for the present withhold the 

 discussion of these theories, and proceed on lines directed 

 to some portions of the experimental side of the subject which 

 have intrinsic or theoretical interest. 



A Rbntgen-ray tube. — The modern form of X-ray tube has 

 a concave cathode, with the object of focussing the cathode 



