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LXIII. The Production of Soft Mont gen Madiation by the 

 impact of Positive and Slow Cathode Mays, By Sir J. J. 

 Thomson, O.M., F.M.S* 



A GAP of about eight octaves separates the softest 

 characteristic Rontgen radiation yet investigated, that 

 of aluminium, and the shortest waves in the Schumann 

 region, those recently discovered by Professor Lyman; the 

 latter have a wave-length of about 9 x 10~ 6 cm., the former 

 one of 3*6 x 10 ~ 8 cm. Very little is known about any radia- 

 tion of intermediate wave-length, and yet the study of such 

 radiation is essential for the determination of the structure 

 of the atom. By its aid we might hope to gain a knowledge 

 of the distribution of the electrons in the atom, to determine, 

 for example, how many rings of electrons there are in the 

 atom and the number of electrons in each ring. We asso- 

 ciate the K and L types of radiation with the vibrations of 

 the two rings nearest the centre of the atom, and the visible 

 spectrum with those of the outermost ring. By using 

 Rontgen radiation ranging in hardness from that charac- 

 teristic of aluminium to the hardest we can produce, we can 

 detect the existence and study the properties of the two rings 

 nearest the centre. By using the light of the visible and 

 ultra-violet region we can find out a. good deal about the 

 outermost ring, but to study the intermediate rings, and thus 

 get even an approach to the constitution of the atom, we 

 require radiation intermediate between Schumann and 

 ordinary Rontgen rays, a type of radiation which has not 

 hitherto attracted much attention. 



I wish in this paper to describe some experiments recently 

 made in the Cavendish Laboratory on methods of producing 

 and studying radiation of this type. 



In the first place I wish to describe a new method of 

 producing this radiation, for I find that when positive rays 

 impinge against a solid, radiation of this type is produced. 



The apparatus is shown in fig. 1. The positive rays are 

 produced in the bulb A, they pass through a tube about 

 2 mm. in diameter and 5 cm. long in the cathode C, 

 and strike against a platinum disk P. B is a side tube, at 

 the end of which is the arrangement described in my 

 book on ' Positive Rays,' by which a photographic plate 

 could be exposed to the radiation in the tube: a slit was 



* Communicated by the Author, having been read before the Physical 

 Society, June 20, 1914. 



