484 SCIENCE PROGRESS 



several theories awaited the test of some crucial experiment. 

 In particular it was known that, if they were of the same nature 

 as light waves (i.e. electromagnetic waves in the aether), their 

 wave length must be of the order of io -8 to io - ' cm. Now 

 this is so small that the waves could not be diffracted by any 

 ruled diffraction grating, for the spacings in a grating need to 

 be of about the same magnitude as the wave length of the 

 rays it diffracts. The conjectured wave length was, however, 

 of the same order as the distance between the atoms or molecules 

 of a crystal, and Dr. Laue of Zurich, who received the Nobel 

 Prize for 1914 for his share of the work, first realised the possi- 

 bility of using a crystal as a three-dimensional grating for the 

 rays. The mathematical problem is a very difficult one ; but 

 Laue was able to show that, if a narrow pencil of parallel rays 

 were passed through a thin plate of a crystal, the emergent beam 

 should give, on a photographic plate, not only a central patch 

 corresponding to the undeviated beam, but a symmetrical 

 diffraction pattern round it. This theory was tested experi- 

 mentally, under Laue's direction, by Friedrich and Knipping 

 in 19 1 2, and with complete success, so that the problem of the 

 nature of the rays was solved. It was, however, exceedingly 

 difficult to deduce the arrangement of the atoms in the crystal 

 from the pattern obtained, indeed almost impossible save in the 

 simplest cases. Moreover, Laue had to assume that the X-rays 

 incident on the crystal contained only a few different wave 

 lengths, i.e. that they had a " bright line " and not a " con- 

 tinuous " spectrum, and even then many of the spots demanded 

 by theory did not appear on the plate. The subject had reached 

 this stage of its development when Prof. Bragg suggested to his 

 son that he should take it up. After making a few experiments 

 W. L. Bragg put forward a new explanation of the formation 

 of the diffraction pattern, namely that it was due to the reflexion 

 of waves, of a wide range of wave lengths, by the various 

 parallel planes of atoms which exist in the crystal. This ex- 

 planation is simpler and far more successful than that given by 

 Laue ; but it owes its chief importance to the fact that it em- 

 bodies the idea of the reflexion of the waves by the successive 

 layers of atoms inside the crystal. It follows immediately, as 

 was suggested by C. T. R. Wilson, that, by increasing the angle 

 of incidence of the rays on any particular set of parallel layers 

 of atoms, they should be reflected back selectively just as light 



