470 Notices respecting New Books. 



number, however, is always the same, not varying either with the 

 nature of the gas or its density, nor with either the dimensions of 

 the tube or the position of the electrodes. There are always four 

 layers of the kathode light : the first is of a chamois-yellow colour ; 

 the three outer ones are blue. Like the layers of the positive light, 

 those of the negative light expand as the pressure decreases. "When, 

 at pressures of less than 1 inillim., the negative light extends through 

 the whole tube and plays upon the opposite wall, it excites there 

 phosphorescence, of colour depending on the nature of the glass. 

 Thus the German glass commonly used for vacuum-tubes phospho- 

 resces with green light, the intensity of which increases as the pres- 

 sure decreases ; while at the same time the optical intensity of the 

 kathode-light itself decreases until at last the eye can scarcely per- 

 ceive that any light at all is emitted by the kathode. The negative 

 light and the positive light do not, at small pressures, occupy di- 

 stinct portions of the tube, but may each fill the whole tube. The 

 presence of the brighter positive light thus increases the difficulty 

 of observing the phenomena taking place at the negative pole ; so 

 that for successful experiment it is necessary to get rid of the posi- 

 tive light altogether. This can be managed by having the distance 

 between the electrodes not greater than twice the diameter of the 

 tube. The smaller the distance between the electrodes, the higher 

 the density at which the positive light disappears. It is sometimes 

 desirable to place the anode close to the kathode. The radiation of 

 the negative light from the kathode takes place, as Hittorf showed, 

 without reference to the position of the anode — not even ceasing 

 when it reaches the anode, but extending beyond it through the 

 whole of the tube. Thus, if both poles are placed at the same end 

 of a cylindrical tube, the kathode-light stretches right through the 

 tube till it reaches the end which contains no electrodes, where it 

 excites phosphorescence in the glass. 



Pliicker and Hittorf have already shown that the kathode-light 

 radiates in straight lines. If the kathode be a straight wire, and if 

 the tube contain a second similar wire parallel to the first, a sharp 

 shadow of the second is seen on the phosphorescent surface excited 

 by the discharge from the first. 



This Introduction occupies 16 pages; and the work itself is divided 

 into four parts. 



In the First part the nature of the phenomena observed by Dr. 

 G-oldstein is expounded. In the simplest form of the experiment, 

 a cylindrical tube contains at one end two parallel wires, one or 

 both of which may be connected with the negative pole, and at the 

 other end of the tube a wire to serve as anode. If one only of the 

 negative wires is excited, the kathode-light from it excites green 

 phosphorescence in the glass all around, on which is seen the sharply- 

 defined shadow of the unexcited wire. If, now, the second nega- 

 tive wire be excited at the same time as the first, there appear im- 

 mediately two dark spaces in the green phosphorescence of the 

 glass, each so situated as to resemble nearly an enlarged shadow of 

 one of the wires produced by the discharge from the other. The 

 dark surfaces may be described as oval, but with straight sides 



