ESSAY-REVIEWS 



HOW THE PROBLEM OF THE X-RAYS HAS BEEN SOLVED, 



by Prof. Frederick Soddy, F.R.S. : on X-Rays and Crystal Structure, 

 by W. H. Bragg, M.A., D.Sc., F.R.S., and W. L. Bragg, B.A. 

 [Pp. vi + 228, with 4 Plates and 75 Illustrations.] (London : G. Bell & 

 Sons, Ltd., 1 91 5. Price "js. 6d. net.) 



The title of this book connects two branches of science that 

 have not been previously connected, but its subject-matter is 

 an essential link in the connection between, not two branches, 

 but almost the whole range of modern physical science. There 

 never was any doubt about the nature of the electric waves 

 discovered by Hertz, now, in wireless telegraphy, so powerfully 

 applied to the arts of war and peace. A simple magnification 

 of the scale, a multiplication of wave-length from ten thousandths 

 of a millimetre to metres, and the mind passes easily from the 

 old to the new, from light to electric waves. A magnification 

 of the scale is easy to conceive and easy to put to the test, but 

 the opposite process, though easy enough to imagine, is by no 

 means so easy experimentally to put into evidence. Fraun- 

 hofer's original diffraction grating consisted of fine silver wires 

 wound regularly upon a frame, and with this rough instrument 

 he diffracted and measured for the first time the wave-length 

 of sodium light. A grating capable of diffracting electric waves, 

 on the other hand, would have to be an enormous structure in 

 which the width of the space between the wires was comparable 

 with the wave-length of the waves. 



An extension of the scale of known wave-lengths, similar to 

 that in passing from light to the waves of wireless telegraphy 

 but in the opposite direction, occurred at about the same time 

 as wireless messages first began to be transmitted through the 

 ether. But many years had to elapse before this new excursion 

 of science into the region of the infinitesimal was clearly 

 understood or before the new rays deserved any other than 

 their original name, signifying, in algebra, an unknown 

 quantity. 



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