RADIANT ENERGY 393 



tered as can light. When a wave front strikes a grid, such as a 

 piece of glass upon which many close parallel lines have been 

 etched, the wave is broken up into smaller waves (Fig. 168). 

 The wavelets thus formed combine to form a new wave front. 

 The same diffraction effects can be obtained with sound waves of 

 much greater length. (Diffracted sound waves have been photo- 

 graphed.) If X rays are of the nature of light rays, it should be 

 possible to diffract them. The German physicist von Laue had 

 the brilliant thought of using the symmetrical distribution of 

 atoms in a crystal (Fig. 143) as a superfine grid with which to 

 diffract X rays. The experiment was successful and opened up 

 an entirely new and highly satisfactory method of studying 



' I ' . ' I ' I 



Fig. 168. — The diffraction of light by a grating. 



crystal structure, which has been carried so far and so success- 

 fully by William Bragg. 



Other types of radiation produced by a vacuum tube include 

 cathode rays, or negatively charged electrons, and canal rays, 

 or positively charged ions. Cathode rays are negative electrons 

 and come from the cathode. Canal rays were so called because 

 in order to observe them, it was necessary to drill a hole in the 

 metal of the cathode through which some of the canal rays found 

 their way. Canal rays appear to emanate from the anode but 

 actually do not do so; instead, they originate in the tube, some- 

 where in front of the cathode, as a result of ionization of the gas; 

 being ions, they are obviously much heavier than the electrons of 

 cathode rays. Anode rays are another type of radiation from a 

 vacuum tube which may be regarded as distinct from canal rays, 

 though perhaps at times at least they are the same. They are 

 seldom referred to, because as a distinct type they are not evident 

 in an ordinary vacuum tube with simple metal electrodes; they 

 may be produced by coating the anode with a metallic salt which 

 disintegrates under an electric charge across the tube and gives 

 forth positive or anode rays. Biologically, the X rays of a 



