the Wave-Length of the Soft 7 Rays from Radium B. 863 



reflexions for soft radiations. The crystals employed, how- 

 ever, showed many imperfections, and their behaviour was 

 very different from that to be expected for an ideal crystal ; 

 for example, when the crystal was set at an angle of 12° to 

 the incident beam, and with the width of the pencil such 

 that only radiations between 11° and 13° should be strongly 

 reflected, in addition to the lines in this region other 

 outside lines are observed in varying positions on the photo- 

 graphic plate : for example, in a particular case at 2° 40' 

 and 13° 50'. 



Special experiments showed that the position of these lines 

 on the plate corresponded to a definite frequency of vibration 

 in the incident beam. All our photographs showed similar 

 peculiarities, but the outside lines which appear are very 

 variable for different angles of the crystal. This behaviour 

 of rock salt led us to make many fruitless experiments to 

 obtain a more perfect crystal, but all the crystals of rocksalt 

 we have examined show similar imperfections, though in 

 varying degrees. The crystal of heavy spar employed, for 

 which the face appeared very plane and perfect, also behaved 

 similarly. There appears to be no doubt that many crystals, 

 and especially those of rocksalt, have a contorted or undu- 

 lating surface, and that the orientation of the planes varies 

 within certain limits from point to point of the crystal. 

 At the same time, these irregularities may lead to the 

 absence of a line in the photograph, although the crystal is 

 set at the correct theoretical angle. To avoid this difficulty, 

 it is desirable to keep the crystal in rotation during the 

 experiment. Darwin has examined the consequences of such 

 imperfections in a crystalline structure*, and considers that 

 they offer an explanation of the fact that the intensity of the 

 reflected beam is in general greater than the theoretical 

 value to be expected for an ideal crystal. 



Connexion of Radium JB with Lead. 

 In recent papers f , Moseley has examined the X-ray 

 spectra of a number of the ordinary elements. For this pur- 

 pose, each element either in the state of metal or compound 

 is exposed as anticathode in a focus tube, and the resulting 

 X-ray spectra are obtained photographically by the crystal 

 method. He has shown that the " K " characteristic radia- 

 tion of all the elements between aluminium and silver shows 

 a similar type of spectrum, and the frequency of the corre- 

 sponding lines changes by definite steps in passing from one 



* C. G. Darwin, Phil. Mag. April 1914, p. 975. 

 t Phil. Mag. he. cit. 



