the Wave-Length of the Soft y Rays from Radium B. 859 

 Measurement of the Plates. 



The positions of the lines were measured as distances from 

 the fiducial line, which fixes the position of the plate relative 

 to the plate-holder. From this was calculated the angle 

 which the ray corresponding to any line made with the 

 normal to the plate ; the angle which the plate-hoider made 

 with the zero position being known, the angle of reflexion 

 of the given ray followed at once. To correct for possible 

 errors in the fixing of the zero, the same line was photo- 

 graphed twice, the crystal being rotated between the two 

 photographs to a symmetrical position on the other side of 

 the zero, so as to throw the line in the one case to the right, 

 in the other case to the left, of the un deflected beam. This 

 enables the angle of reflexion to be fixed with considerable 

 accuracy if the zero positions are only roughly determined. 



Experimental Results. 



In this paper an analysis will be given of the soft type of 

 •7 radiation from radium B. Evidence of lines corresponding to 

 the more penetrating rays from radium B and the penetrating 

 rays from radium C has been obtained on the photographs, 

 and the spectra have been separated by the interposition of 

 -absorbing screens ; lines have been found, due to radium C, 

 with 6 mm. of lead between the radium tube and the crystal. 

 The spectra due to the penetrating rays from radium B and 

 radium C are faint compared with that of the soft radiation 

 from radium B, and have not yet been fully investigated ; 

 .an account of them is withheld for a future paper. 



The stronger lines due to radium B appeared with great 

 distinctness on the photographic plate, as will be seen from 

 fig. 2 (PI. XII.), which is reproduced from an actual photo- 

 graph; they permir, of accurate measurement. In the photo- 

 graph B is the band made by the direct rays coining through 

 the slit, /3 and a are the two strong lines formed by the 

 reflected rajs, and F is the fiducial line. The fainter 

 lines do not appear on all the plates ; however, no line is 

 given in the table which has not been measured on at least 

 two plates. The main feature of the spectra of the radiation 

 reflected from rocksalt is two strong lines at almost exactly 

 10° and 12° respectively ; they are accompanied by a number 

 of fainter lines at angles of from 8° to 14°. There is also a 

 large group of faint lines between 18° and 22°, which do not 

 permit of accurate measurement, and so are omitted in the 

 table ; some of these, at least, are probably repetitions of the 

 measured lines in the second order. 



Most of the photographs were taken with crystals of 



