Spectrum of Penetrating y Rays from Radium B and C. 2Q7 



face of the cubic crystal), the ray RA making an angle 6 

 with the normal is in position to be strongly reflected in 

 passing through the cubic crystal ; similarly also the corre- 

 sponding ray RA'. The reflected rays from each point of EF 

 cut the normal in the neighbourhood of 0. A photographic 

 plate placed at normal to RD should thus show a narrow 

 dark band on the general background, while if placed at D 

 it should show dark bands at B and B'. If a consider- 

 able part of the rays in the direction RA, RA' are reflected, 

 the lines A and A' should be positions of minimum intensity 

 of radiation, and thus appear as white lines on the general 

 dark background. In other words, reflexion lines appear at 

 B and B' and absorption lines at A and A' *. 



These conclusions have been completely confirmed by 

 experiment. When the photographic plate is placed at 0, a 

 central dark band appears with two absorption lines sym- 

 metrically placed on either side of it. An actual untouched 

 photograph of this kind is shown in fig. 3 (PL V.), and brings 

 out the main points quite clearly f. 



If an emanation-tube, with the emanation compressed into 

 a small length of the tube, is placed normal to the face of 

 the cubic crystal, it corresponds nearly to a point source. 

 Under such conditions, the photographs obtained show a 

 beautifully symmetrical pattern. The two sets of strong 

 absorption bands cut one another at right angles, while the 

 crystal planes at 45° also give bands cutting the main system 

 of bands at an angle of 45° and passing through their points 

 of intersection. The 45° bands are relatively less marked 

 than the bands from the 100 planes. The results obtained 

 are in exact agreement with those to be expected from the 

 geometry of a cubic crystal. Reproductions of two photo- 

 graphs of this kind made with different crystals are shown 

 in figs. 1 and 2, Plate V., fig. 1 showing the 45° lines well. 

 The nearly monochromatic radiation producing the pattern 

 is the "A" line, or doublet; the lines due to the other 

 radiations are too faint to show in the reproduction. 



Considering the fact that no attempt was made to cut off 

 the effect of the general <y radiation on the photographic 

 plate, the sharpness with which the bands show up is very 

 remarkable. Results of this kind bring out clearly the large 



* Professor W. H. Bragg lias found evidence of a similar absorption 

 in passing X rays through a thin crystal of diamond at the reflexion 

 angle, using the electric method to detect the rays. See 'Nature,' 

 March 12th, 1914, p. 31. 



t The three reproductions of Plate V. are of prints from the original 

 negatives : in consequence the reflexion lines show white, and the 

 absorption lines black. 



