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



From the analysis of the types of 7 rays, it appeared 

 probable that each corresponded to one of the characteristic 

 types of radiation of the element in question. It was conse- 

 quently to be anticipated that each of these radiations would 

 give definite line spectra when reflected from the surface of 

 crystals. 



In order to examine this question, experiments were begun 

 to determine the wave-lengths of the 7 radiations from the 

 products radium B and radium C. For this purpose, a thin 

 walled a-ray tube, filled with a large quantity of emanation, 

 served as a source of 7 rays. The rays were allowed to fall 

 at a definite angle on a crystal, generally rocksalt, and the 

 intensities of the " reflected," or rather diffracted, rays were 

 examined by a photographic method. 



The determination of the 7-ray spectra is in some respects 

 far more difficult than similar measurements for X rays. In 

 the first place, the photographic effect of the 7 rays, even 

 from the strongest source of emanation available, is verv 

 feeble compared with that due to the X rays from an 

 ordinary focus tube. For example, using a source of 100 

 millicuries of radium emanation, an exposure of 21 hours 

 is necessary to obtain a marked photographic effect due to 

 the reflected 7 rays. Under similar conditions, 10 minutes 

 exposure suffices to obtain a well-marked X-ray spectrum. 

 In the second place, special precautions have to be taken to 

 screen the photographic plate from the effects of the very 

 penetrating 7 radiation from radium C. The greatest 

 difficulty of all, however, is to get rid of the disturbing 

 effect of the very swift primary ft particles emitted from 

 the source and the swift ft particles emitted from all material 

 through which the 7 rays pass. This can only be accom- 

 plished by placing the source of radiation, absorbing screens, 

 and crystal in a strong magnetic field, so that practically all 

 the ft rays, both the primary ones and those excited by the 

 7 rays in matter, are bent away from the photographic 

 plate. 



Method of Experiment. 



The straight emanation tube, A, about 0*5 mm. in diameter 

 and 1 cm. in length (fig. 1), was fixed behind a massive 

 block of lead BB, so that the rays from it passed through a 

 horizontal slit in the block ; a square vertical hole in the 

 lead allowed screens SB to be interposed in the path of 

 the rays when desired. The length of the block from back 

 to front was 6 cm., the width of the slit 3 mm. The 

 emergent pencil of rays fell on the crystal CO, which was 



