212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



portant crystal face. In this way reflections from a relatively enor- 

 mous number of different crystal planes are obtained at one time. 

 Some Laue photographs have been prepared which show nearly a 

 thousand reflections; it is very common that the number should lie 

 around three to four hundred. Of course several of these spots may 

 belong to the same crystal form, but in spite of this fact the number 

 of different kinds of planes that give reflections in a single photo- 

 graph is very large. 



In contrast with the method of powders the principal use of the 

 transmission method lies in its application to the determination of 

 the structures of crystals. A study of the Laue photographs from a 

 crystal, when combined with a spectrometer determination of the 

 number of molecules associated with the unit of structure, supplies 

 more data for distinguishing between the various possible arrange- 

 ments of its atoms than does any other single method. Transmission 

 photographs are of particular use at the present time because the 

 relatively large amount of data which they furnish makes it possible 

 to dispense to a large extent with the assumptions concerning the 

 reflecting powers of the atoms and the effect of spacing upon the 

 intensity of reflection. As already pointed out, unless some entirely 

 new procedure is devised it is only by taking advantage of the rela- 

 tively few and scattering instances where structures can be investi- 

 gated without the help of both of these two "laws" that direct 

 progress can be made. It remains to be seen whether the Laue photo- 

 graphs will continue to give the most usable information after the 

 forms of these two " laws " have been determined. 



Whereas the other two methods employ X rays of a single wave 

 length the Laue photographs are obtained by the diffraction of that 

 part of the radiation from an X-ray tube, which, by reason of its 

 analogy to white light, is called the "white radiation." Figure 15 

 shows the spectrum of tungsten. Spectrometer measurements make 

 use of the line radiation, the characteristic radiation ; the Laue pho- 

 tographs are produced by the continuous portion of much shorter 

 wave lengths. This is alike an advantage and a disadvantage — a 

 disadvantage because we must devise methods of studying the Laue 

 photographs which will permit the determination of the wave length ; 

 an advantage (1) because the wide range of wave lengths that are 

 available permit reflections from a large number of planes for a 

 definite setting of the crystal, and (2) because the shortness of the 

 wave lengths makes it possible for planes with very small spacings — 

 and complicated indices — to give reflections that can be readily 

 studied. 



Figure 9 (pi. 4) shows the Laue photograph that is obtained by 

 passing X rays through a thin section of magnesium oxide cut, or 



