PHYSICS IN 1913 613 



of one wave-length, but complex. De Broglie has taken 

 excellent photographs by the reflection method by arranging 

 the crystal to rotate very slowly by means of clockwork, and 

 letting a narrow beam of X-rays strike the crystal face where 

 the axis of rotation passes through it. As the correct angle 

 for any homogeneous ray present is reached, the ray is 

 reflected and recorded on a fixed photographic plate. 



By the experiments described much light has been thrown 

 upon the nature of X-radiation ; the Braggs have applied these 

 results to study the structure of crystals. For this purpose 

 X-ray photographs are taken with the crystal to be examined, 

 either by the Laue method of transmission, or the reflection 

 method ; for the former heterogeneous radiation is required, 

 for the latter a homogeneous beam, such as that found to 

 be emitted from a rhodium anticathode. The reflection 

 method gives the more direct information ; photographs are 

 taken by reflection from the planes richest in atoms in the 

 crystal, the so-called (100), (no), (in) planes, and the given 

 line, corresponding to the homogeneous radiation, is sought 

 at the series of angles corresponding to the first, second, and 

 subsequent orders. In general the line cannot be found in 

 all the orders; for instance, for 'the (in) planes in diamond 

 there is no second order spectrum, although first, third, and 

 fourth are found. There is no space here to enter into the 

 details of the deductions which can be drawn from such 

 evidence ; it will suffice to state that for a crystal of an element, 

 such as the diamond, the absence of certain orders indicates that 

 the series of parallel planes from which the reflection under 

 consideration takes place are not equally spaced from one 

 another, but a series of equally spaced planes are separated by 

 another series of equally spaced planes arranged so as to 

 divide the spaces between the first set unequally. By con- 

 siderations of this kind, the Braggs have obtained a detailed 

 model of the spacing of the carbon atoms in the diamond, which 

 they checked by photographs of the Laue type; the model 

 shows the atoms arranged at the points of two interpenetrating 

 space lattices ; between a series of planes equally spaced other 

 planes are placed so as to divide the distance between them in 

 the ratio of one to three. 



Further very interesting information has been obtained as to 

 the arrangements of the atoms of different kinds in crystals of 



