264 Sir E. Rutherford and Dr. E. N. da C. Andrade: 



large part, if not all, of these penetrating radiations give 

 definite line spectra and correspond to groups of rays of very 

 high frequency ; but it has been a difficult task to determine 

 the wave-lengths of the lines with the accuracy desired. 

 We have been much aided by the development of a new 

 method for finding the wave-length, which depends on the 

 measurement of absorption as well as of reflexion lines. 



In our first experiments the same general method was 

 employed as in the previous work. A fine glass tube con- 

 taining about 100 millicuries of emanation was used as a 

 source. The distances between the source and crystal and 

 between the crystal and the photographic plate were equals 

 and, as in the previous experiments, about 9 cm. A beam 

 of 7 rays passing through a narrow opening in a lead block 

 fell on the crystal, the arrangement being that shown in 

 fig. 1 of our previous paper. The width of the direct photo- 

 graphic impression on the plate was in general about 3 mm. 

 It was important in these experiments with penetrating 

 Y rays to use a thin crystal, since the rays pass right through 

 the crystal and the exact plane of reflexion is consequently 

 uncertain. The crystal employed was a slip of rocksalt 

 about 3 cm. long, 2 cm. wide, and about 1 mm. thick. For a 

 radiation which is reflected at about 1° from the (100) planes 

 of rocksalt, the diffraction line is displaced on the photo- 

 graphic plate little more than 3 mm. from the centre of the 

 dark band. The fact that some lines may be reflected from 

 near the front face of the crystal, and others from further 

 back, introduces a possible source of error in the determina- 

 tion of the correct angle of reflexion. This difficulty could 

 have been avoided if the lines could have been measured in 

 fhe second order as well as in the first, but the second order 

 lines were too faint to pick out with certainty in the presence 

 of a number of other faint lines. A large number of photo- 

 graphs were taken and measured up, and the mean of the 

 deflexions of the strongest and easily observed lines should 

 not be much in error. 



The crystal was kept in slow rotation by the method 

 described in the previous paper. This prevented the appear- 

 ance of apparent lines due to crystal imperfections. The 

 *' centre " of the crystal was in most cases obtained by 

 observing, on a special photograph taken for the purpose, 

 the position of the strong 10° and 12° lines. This was 

 checked also by obtaining the same line on both sides of the 

 zero by rotating the crystal through about 180° ; as described 

 in the previous paper. 



