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



amount of the radiation that is "scattered " or "absorbed" 

 when passing through the crystal at the proper reflexion 

 angle. The distance A, AJ between the absorption bands can 

 be easily measured, and the angle of reflexion is given by 



-A A' 

 tan" 1 2 p^ and can be determined with accuracy. 



The dark lines are fairly sharp, as might be expected for 

 such small angles of reflexion (1° to 2°), even if the rays are 

 reflected throughout the thickness of the crystal. To esti- 

 mate the angle from them, the rays forming them must be 

 considered as proceeding from the point 0, whose distance 

 from the photographic plate was obtained by subtracting 

 twice EH, the mean distance of the emanation-tube from the 

 crystal, from RD, the distance of the tube from the plate. 

 When this was done, the angles of reflexion as given by the 

 narrow black bands, or lines, agreed excellently with those 

 given by the white absorption lines, 



It is seen that the precision of these photographs depends 

 upon the theoretical perfection of the crystal planes. If, for 

 example, the planes of the crystal happened to be distorted 

 into a curved shape, the distance apart of the absorption 

 lines would be different if the crystal were reversed so as to 

 have the other side towards the source of the rays. With 

 some crystals the lines were slightly bent, but with those 

 selected for the final work the lines were straight, and no 

 measurable differences in the distances apart of the lines 

 were observed when the crystal was reversed in the manner 

 indicated. 



In the actual experiments, it was found that the A line gave 

 a very marked absorption band for a thickness of crystal of 

 8 mm. and also a visible band in the second order. The first 

 order of this band was observed on a crystal only two milli- 

 metres thick. It is obvious, however, that if the spectrum of 

 the rays is complicated, a dark line may in some cases fall on 

 an absorption band and mask it. For this reason, it is some- 

 times desirable to take photographs at (fig. 1) where there 

 is no chance of confusion due to these causes. 



In order to overcome this difficulty, recourse was also had 

 to another method involving; the measurements, of the dark 

 bands arising from reflexion. The general arrangement of 

 apparatus is clearly seen in fig. 2. The emanation- tube was 

 placed close to the crystal, and a lead screen LL, with a 

 narrow slit to allow the passage of the reflected rays, was 

 placed at the focus 0. The photographic plate was placed 

 some distance away at D. A dark central band was observed 

 in the photograph produced by the direct radiation through 



