610 SCIENCE PROGRESS 



and Pohl from attempts to obtain diffraction with a wedge- 

 shaped slit, and a similar estimate had been formed from the 

 ratio of the intensity of the X-rays to that of the cathode rays 

 excited by them. 



Soon after Laue's discovery, W. L. Bragg, the son of W. H. 

 Bragg, suggested a different way of regarding the phenomenon. 

 He considered the crystal as containing different series of 

 parallel planes in which the atoms are closely packed ; from 

 these reflection of the rays takes place, for by Huygen's principle 

 a number of points arranged regularly on a plane will give rise 

 to secondary wavelets which build up a wave reflected at the 

 angle of incidence. Now in a crystal supposed built up of the 

 so-called face-centred cubes 1 a system of series of parallel planes 

 rich in atoms can be picked out, from which such reflection, 

 obeying the ordinary laws of optical reflection, takes place. 

 W. L. Bragg obtained a simple geometrical construction for the 

 points that would result from reflection from such planes, and 

 the diagrams he obtained agreed excellently with photographs 

 taken by Laue's method. There is no need to assume the 

 incident radiation homogeneous, or consisting of a few definite 

 wave-lengths ; the crystal structure will account for the sorting 

 out of the general, or " white," radiation, 2 or, in other words, will 

 impress the regularity on it. 



Bragg confirmed his theory of reflection by throwing a 

 beam of X-rays on a cleavage face of mica, cleavage faces of 

 crystals being rich in atoms ; he obtained reflection according to 

 optical laws. W. H. and W. L. Bragg then examined in more 

 detail the reflection of the rays, making use, not of the photo- 

 graphic plate, but of the ionisation produced by the reflected 

 rays in order to detect them. The apparatus resembled a 

 spectroscope in form, in which the collimator was replaced by 

 a lead slit through which the incident rays passed, the tele- 

 scope by an ionisation chamber to which the rays obtained 

 access through a second narrow slit. They found that reflection 

 took place always in accordance with the law of equal angles of 

 incidence and reflection, but that with different angles of incidence 

 the intensity of the reflected ray, measured by the ionisation 

 produced, varied markedly, showing a series of pronounced 



1 A cube with a point at each corner and one in the centre of each face. 



2 By analogy from white light, which can be resolved into a continuous group 

 of wave-lengths. 



