150 



angle, and even then it can only give a spectrum of one order at 

 the same time ; in this respect it differs principally from an ordinary 

 line-grating which may give at any angle of incidence spectra of 

 different orders simultaneously. It follows from this that in the 

 L a u e-patterns previously considered, where the crystal-plate 

 remained in a fixed position during the experiment with "white" 

 Rontgen-radiation, each spot on the photographic plate corresponds 



to a series of wave-lengths A, -, -, etc. If we could distinguish the 



"colours" of a Rontgen-radiation with continuous spectrum, as 

 we do in-visible light, we should see that the pattern would be a multi- 

 coloured one, every spot having its own colour, wholly depending 

 on the direction of the set of net-planes in the crystal from which 

 the ray which produced that spot, took its origin. 



23. For our purpose the facts referred to in the preceding 

 paragraphs may be considered sufficient. Indeed, if Rontgen- 

 radiation of known wave-length A be used, observation of the angles 

 of reflection on a definite set of net-planes will give us a relation 

 between A and d, and by doing this for various known directions 

 of a crystal, for instance for the three pinacoides 100} , 010} , and 

 001} , etc., we shall gain an important insight into the dimensions 

 di, d 2 , d 3 , etc., related to them. 



The equation deduced in the above includes all we have to know 

 for the study of the special arrangement of the net-planes in such 

 a crystalline medium with respect to each other, and it was in this 

 way, that W. H. and W. L. Bragg made their successful investi- 

 gations of the internal structure of a number of crystals. Without 

 considering their special methods of experimenting l ) in detail, 



l ) The directions in which, on turning the crystal about an axis, real inter- 

 ference-maxima occur, can be found by several methods. The English investigators 

 mentioned used for this purpose the ionising effect R on t gen-rays have on gas- 

 molecules. The "indicator" of their ^"-ray-spectrometer was an ionisation- chamber. 

 filled with a suitable gas, as for instance sulphur dioxide. However, it is equally 

 possible to make use of other effects produced by Rontgen-rays, when these 

 effects can be measured or observed, even if they be very feeble. Many investigators 

 have in recent times made use of the photographic plate (De Broglie, Friman 

 and Siegbahn, Debije, and others), especially for the study of wave-lengths. 



Some few indications may be sufficient concerning Debije's beautiful method of 

 investigation, which undoubtedly is of highest importance for future researchwork 

 in this direction. The advantage of this method is, that no well-developed crystals 

 or accurately cut plates are any longer necessary : the substance to be investigated 



