250 Fluorescence of Crystals under the Rontaen Rays. 



that obtained by Hopkinson. In his experiments the maxi- 

 mum occurred at about 800 volts, and thereafter the sensibility 

 went on decreasing much more rapidly than in Ayrton and 

 Sumpner's experiments. Further^ Hopkinson's instrument 

 was much more sensitive. Again^ Ayrton and Sumi)ner 

 made some experiments with a single- fibre suspension, and 

 although they obtained greater sensibilities as the potential 

 of the needle was increased^ they did not obtain a maximum. 



Returning to their experiments with the bifilar suspension 

 and the unsymmetrical guard-tube, it is almost obvious that 

 moving the quadrants away from the guard-tube would 

 reduce the disturbino^ effect. 



I therefore think that their experiments apply only to an 

 extremely unsymmetrical instrument of comparatively low 

 sensibility, and that the maximum considered in this paper is 

 beyond the range of their instrument. 



XXIX. Fluorescence of Crystals under the Rontgen Rays. 

 By Hon. R. J. Strutt, Fellow of Trinity College, Camhridge^. 



A FEW months ago M. Blondlot published an account 

 of some experiments which led him to conclude that 

 the Rontgen rays from a focus-tube are polarized. It has 

 since appeared that he was (very naturally) misled by the 

 presence of a new kind of refrangible rays from the tube, 

 which are capable of brightening a minute eleotric spark. 



The question of whether the Rontgen rays are really 

 polarized remains untouched. 



It is known t that the fluorescent crystals of magnesium 

 platinocyanide light up with a different colour according to 

 whether the plane of polarization of the incidenc light is 

 perpendicular to the optic axis, or parallel to it. The change 

 is from orange to scarlet. There seemed to be a possibility 

 that a difference of the same kind might appear when the 

 crystal was fluorescing under the action of the Rontgen rays. 



I am indebted to Messrs. Johnson and Matthey for the loan 

 of three laro-e crvstals of the salt. Ronto-en ravs from a 

 focus-tube were allowed to fall normally on the prism faces, but 

 the fluorescence was not found to depend on the angle between 

 the optic axis of the crystal and the cathode stream in the 

 tube. So that the experiment, for what it is worth, tells 

 against the polarization of the Rontgen rays. 



It was incidentally noticed that the prism faces were more 



* Communicated by the Author. 



t Lommel, Ann. Phys. Cliem. viii. 1879, p. 634. 



