244 Dr. E. Goldstein on the Eleclrw 



as is well-known, the diminution of the density causes an 

 increase in the brilliancy of the phosphorescence. But an 

 appearance which is decisive on this point, results from a 

 slight imperfection in the mirror employed; the mirror, struck 

 with a die and subsequently polished, was not perfectly 

 smooth near the edge (in consequence of imperfect malleability 

 of the iron out of which it was made), but showed there short 

 slight wrinkles. In consequence of this, the phosphorescent 

 disk does not appear bounded by a smooth curve, but the 

 periphery of the image shows small teeth and protuberances. 



If, now, in consequence of interpolation of sparks or change 

 of density, the image alter its diameter, the same teeth and 

 projections appear at the corresponding points in the same 

 relative positions, only larger when the surface is larger and 

 smaller when the whole surface has diminished. 



It may then be considered definitely proved that the observed 

 variation in the magnitude of the surface corresponds to an 

 actual change in diameter of the section of the bundle of rays 

 emitted by the kathode by the glass-wall ; or that the direc- 

 tion of the rays emitted by a concave kathode is not constant, but 

 varies with the density of gas, and with the change of conditions 

 caused by intercalating sparks. 



It is on account of phenomena of this kind that, in various 

 papers on the radiation of the kathode-rays and the intensity 

 of the phosphorescence produced, I have distinguished the 

 cases of convexo-concave and plano-convex kathodes from 

 those of concave kathodes. 



It has long been known that, in accordance with Doppler's 

 principle, the velocity of translation of the luminous gas- 

 molecules in the discharge must influence the spectrum of 

 the gas. As it appeared to me that the experimental treatment 

 of this point might furnish a new criterion whether the dis- 

 charge consists of a convective transport of electricity by the 

 gas-molecules or not, I was glad to avail myself of the oppor- 

 tunity offered me by Prof. Helmholtz to make use of a 

 spectroscope of powerful dispersion. I constructed a dis- 

 charge tube, in which the electrodes were two flat pieces of 

 metal at right angles to each other. The connexions could 

 be rapidly changed, so as to make either of the two pieces 

 the negative pole. The tube contained rarefied hydrogen. 

 The part of the tube containing the kathode was so placed 

 in front of the slit of the spectroscope that the axis of the 

 collimator was at right angles to the surface a, and conse- 

 quently parallel to the surface b. The direction of the bulk 

 of the rays emitted by a coincided with the axis of the 

 collimator; the rays from b were at right angles to the colli- 



