242 Dr. E. Goldstein on the Electric 



reference to the wall of the tube chosen by Crookes, under 

 certain circumstances the phosphorescent surface of the 

 kathode-light was reduced to a point ; but without alteration 

 of the position of the mirror it was possible, either by alteration 

 of the density of the gas or by interpolation of sparks in the 

 discharge, to obtain surfaces of considerable and very various 

 diameters as foci of the mirror, instead of a point. I venture, 

 for the sake of clearness, to give a few data of the experiments. 



The distance of the centre of aperture of the mirror from 

 the wall was 15 millims. ; a small Ruhmkorff s coil which 

 gave sparks about 1-J centim. long in air was discharged 

 through a cylindrical tube of 4 centims. width, whose axis 

 was cut at right angles by the axis of the mirror. At a pres- 

 sure of about ^ centim. there appeared as focus a bright 

 round phosphorescent disk of 4 millims. diameter. If there 

 were now interposed in the external circuit an air-spark of 

 varying length, the diameter of the phosphorescent disk 

 increased as the interpolated air-spark increased in length. 

 The diameter of the surface (measured by means of a divided 

 strip of paper laid on the outside of the tube) increased thus 

 up to 1 centim. At ^ millim. pressure the diameter with- 

 out the spark was 2J millims., and when the spark was inter- 

 polated rose to 8 millims. At -^V millim. the smallest dia- 

 meter of the image was 1 J millim. 



When, instead of the small Ruhmkorff apparatus, a larger 

 inductorium, giving much longer sparks, was employed, the 

 diameter of the surface varied even when the position of the 

 mirror and the density of the gas remained unaltered, even 

 with the same length of interposed air-space. (It is probable 

 that this variation is connected with the fact that sparks of 

 different length may leap across an air-space of constant 

 length, in consequence of the curvature of the sparks, the more 

 strongly-curved sparks representing greater tensions.) 



Thus with constant air-space and constant pressure the 

 diameter of the luminous surface varied rapidly from 

 2 millims. to more than 1 centim. 



In another case the centre of the mirror was at a distance 

 (25 millims.) from the wall equal to twice the radius of cur- 

 vature of the mirror. According to the law of divergence of 

 the rays supposed by Crookes, the phosphorescent surface 

 ought to have had a diameter equal to that of the mirror 

 (21 \ millims.), and ought to have remained constant. 



At a pressure of \ millim. the kathode-light produced no 

 phosphorescence when the exterior circuit was altogether 

 metallic ; but when sparks were interposed, phosphorescent 

 surfaces up to 26 millims. diameter were obtained. 



