Notices respecting New Boohs. 471 



parallel to the wires. The form, magnitude, and position of the 

 dark surfaces are independent of the position and magnitude of the 

 anode, and of the direction in which the positive discharge takes 

 place, but may be largely altered by varying the form, magnitude, 

 and relative position of the two kathodes. These changes are mi- 

 nutely followed through a large number of widely varied experi- 

 ments. The dark surfaces are in all cases such as can be explained 

 by a repulsion exerted by each electrode on the kathode-rays pro- 

 ceeding from the other, so that the (comparatively) dark spaces are 

 due to the non-production of phosphorescence on those portions of 

 the glass wall not reached by the deflected rays. It should be noted 

 that the boundary of the dark surfaces possesses increased bril- 

 liancy, so that the dark surfaces appear bounded by a sharply- 

 defined line of light. 



If the kathodes are of unequal thickness the dark surfaces are 

 of unequal breadth, that nearest the thicker electrode being the 

 largest. 



The Second part of the work is devoted to the examination of the 

 cause of these phenomena. First of all, it is shown, by making one 

 of the kathodes of a twisted aluminium wire, that it is only light 

 from the further kathode which is deflected from each dark sur- 

 face, and that each is illuminated by light from the nearer electrode, 

 just as when it alone was excited. It may be explained, in passing, 

 that the phosphorescence excited by the discharge from a twisted 

 wire is not uniform, but exhibits a spiral line of greater brilliancy. 



If a diaphragm having a small square hole in it be placed between 

 two parallel kathodes with its plane at right angles to their plane, 

 then, when only one is excited, a small pencil of rays passes through 

 the opening and forms a phosphorescent speck on the glass, which is 

 crossed by a fine dark line, the shadow of the second wire. If the 

 second wire be also excited, the phosphorescent speck divides into 

 two, which separate from each other to a distance of some 12 mil- 

 lim. on the walls of the tube, thus clearly proving the repulsion 

 exerted upon the rays by the second kathode as they pass it. In 

 order to determine whether the pencil is deflected as a whole, or 

 whether it is bent into a curve, or whether it is so bent as to form 

 a sharp angle at the point nearest the electrode, the following ex- 

 periment was made : — A movable electrode, so arranged as to emit 

 a small concentrated cone of light, is hung on to the bent end of a 

 wire forming one of the kathodes, and in consequence of its weight 

 always hangs vertically, and emits a cone of light whose axis is 

 vertical. The other kathode is a wire crossing the tube. By 

 slightly altering the position of the tube the cone of light may be 

 made to impinge on the second kathode, or to pass close to it on 

 either side, or at a little distance from it. It is found that when 

 both kathodes are excited, the cone of rays is bent sharply at an 

 angle of about 30° in passing the horizontal kathode, or is divided 

 into two portions forming an angle of about 70° with each other, if 

 it is allowed to impinge on the horizontal kathode. The deflection 

 is found to be greater the closer the ray passes to the deflecting 

 kathode. 



