860 Sir E. Rutherford and Dr. E. N. da C. Andrade on 



rocksalt, the crystal in some cases being a slip less than a milli- 

 metre thick, in others a specimen about a centimetre thick : 

 the results did not differ noticeably for the soft rays. To 

 check the measurements of the angles of reflexion made 

 with the rocksalt, photographs were also taken by reflexion 

 from the cleavage (001) face of a crystal of heavy spar 

 (barium sulphate). Since the scattering by an atom is 

 proportional to its atomic weight, it was thought that the 

 heavy spar might give spectral lines of more intensity than 

 the rocksalt for the more penetrating rays. In order to 

 compare the constants, or " grating space/' of the two 

 crystals, special experiments were made with them both 

 with X rays by the photographic method developed by 

 Moseley, who kindly designed for us an X-ray tube with a 

 nickel anticathode. This was provided with a side tube and 

 slit, and the rays escaped through a thin aluminium window ; 

 the tube emitted an intense beam of soft X rays consisting 

 mainly of the characteristic radiation of nickel. The angle 

 of reflexion for the two strong lines in the nickel rays was 

 directly determined for both rocksalt and heavy spar ; for 

 the (100) plane of rocksalt the angles obtained agreed closely 

 with those determined by Moseley. The angles of reflexion 

 from the (001) plane of heavy spar were found to be 12° 5 r 

 and 13° 23' in the first order. The ratio for the corresponding 

 angles for the two crystals was 1*278. This enables us to 

 compare numerically the photographs taken with the two 

 crystals by the 7 rays. 



The spectra obtained with heavy spar appeared to be less 

 intense than with rocksalt for the soft rays, and did not 

 show the harder rays with much greater clearness. The 

 angle of reflexion for the two strong lines of the radium B 

 radiation were found to be 7° 52' and 9° 28'. Multiplying 

 by the factor 1*278 to express them in terms of rocksalt, 

 these become 10° 3' and 12° 6', agreeing closely with the 

 values obtained directly with this crystal. This puts it 

 beyond doubt that the lines given by rocksalt are true 

 diffraction lines, and do not arise from irregularities in the 

 crystal. 



In the following table the angle of reflexion of the different 

 homogeneous rays which make up the softer y radiation 

 from radium B are given for rocksalt. Their relative inten- 

 sities are denoted by the letters "s." (strong), "m." (medium), 

 and "f." (faint), but this indication is only very rough, 

 as the circumstances conditioning the intensity vary from 

 photograph to photograph. The wave-lengths (in centi- 

 metres) corresponding to the different angles of reflexion 

 are calculated from the formula A = 2d sin 0, the value 



