270 Prof. Barkla and Miss White on the 



requires more careful installation for the best work in the 

 laboratory. It consists of a simple pendulum, with a light 

 lath for its rod, from a movable stud on which hangs the 

 cord pendulum. 



Even if the lengths are made equal, and also the masses, 

 the vibrations of this pendulum are not interchangeable. 

 The effects of various initial conditions are accordingly more 

 striking. 



It may be used for couplings varying from about 10 to 

 55 per cent. 



Photographic reproductions are given of ten single and 

 four double traces obtained with this pendulum. 



4. The mathematical theories of both pendulums are deve- 

 loped and compared with each other and with the theory of 

 the electrical case it is sought to represent. The experiments 

 are in satisfactory agreement with theory, though this is not 

 always immediately obvious. 



5. In the experiments and most of the theory the lengths 

 of the pendulums have been equal and also the masses of the 

 bobs. A large field lies ready for exploration in the cases 

 where the quantities of both classes are unequal and varied 

 at will. These cases are reserved for later papers. 



University College, Nottingham, 

 July 18, 1917. 



XXIX. Notes on the Absorption and Scattering of X-Rays-> 

 and the Characteristic Radiations of J Series. By C. Gr. 

 Barkla, F.R.S., and Margaret P. White, M.A., B.Sc, 



University of Edinburgh* '. 



Introduction. 



THE experimental investigation of the absorption of 

 X-rays of short wave-length is of importance in a 

 study both of the theories of electromagnetic radiation 

 and of atomic structure. The experimental work on this 

 subject, the results of which are given in this paper, 

 was suggested — indeed forced upon us — by the discovery 

 that the observed intensity of corpuscular radiation from air 

 was not what was to be expected on the simple theory of 

 emission of fluorescent and corpuscular radiations as given 

 in the Bakerian Lecture for 1916 1. An irregularity appeared 

 in the results which could be explained on the assumption that 

 the light elements emitted under suitable stimulus a hitherto 

 unobserved characteristic X-radiation — (a J radiation). Such 



* Communicated bv the Authors. 



t Proc. Koy. Soc. A. xcii. May 25, 1916 ; Phil. Trans. A. 217. pp. 315- 

 360. Preliminary notes in 'Nature/ Feb. 18 and March 4, 1915. 



