862 Sir E. Rutherford and Dr. E. X. da C. Andrade on 



make the strong 10° line coincide with the corresponding 

 platinum line. The dotted lines in the platinum spectrum 

 are taken from a paper of de Broglie * ; as his determination 

 of the strong line differs somewhat from that of Moseley and 

 Darwin, the whole spectrum given by him has been reduced 

 by multiplying by a constant factor chosen so as to make 

 the strong lines agree. 



Structure of the Spectral Lines. 



In the case of the stronger lines from rocksait, viz. the 

 10° or 12° lines, the structure of the lines could be studied 

 in some detail. They consisted of slightly curved bands 

 about U'5 mm. wide, the photographic intensity being greatest 

 at the edge of the bands. A reproduction of part of one of 

 these bands, magnified about five times, is shown in PL XII. 

 fig. 4. With weak intensities only the outer edges of the 

 band could be seen, and the band appeared as a close double. 

 The spectral band appeared to be the exact mirror-image of 

 the source, both as regards magnitude and distribution of 

 intensity in radiation ; the width of the image was the same 

 as the diameter of the a-ray tube, viz. 0'50 mm. The sharp 

 and well-marked edges of the band are due to the fact that 

 the intensity of the radiation is least from the centre of the 

 cylindrical «-ray tube, and increases to a maximum from 

 the edges, owing to the active matter deposited on its inner 

 surface. It is well known that a photograph taken of an 

 a-ray tube by its own rays through a narrow slit parallel to 

 the source always shows these variations of intensity. The 

 fact that the spectral band on the photographic plate is 

 the mirror-image of the source, indicates clearly that the 

 scattered rays forming the band come from very near the 

 surface of the crystal. Attention has been drawn to 

 the completeness of the reflexion of X rays from a crystal 

 at the proper angle by Darwin f, and shown by him to be 

 a necessary consequence cf the mathematical theory. The 

 efficiency of the reflexion is also well shown by recent 

 experiments of W. L. Bragg J. 



Imperfection of Crystals. 



In most of our experiments we have employed a crystal of 

 rocksalt, since its structure has been worked out in detail by 

 "W. H. and W. L. Bragg, and since it gives fairly strong 



* Journal de Physique, loc. cit. 



t C G. Darwin," Phil. Mao-. Feb. 1914, p. 315. 



X W. L. Bragg, ' Nature,' March 1914, p. 31. 



