between Anode and Kathode Spectra. 341 



By still further exhaustion the negative glow is further diffused, 

 the light of the capillary again pales, and the glass about the 

 anode fluoresces under the action of the kathode rays which get 

 through the capillary. 



Schuster distinguishes the spectrum of the negative glow, 

 which he describes as consisting of five bands in the green and 

 orange, from the four lines, which he calls the compound line 

 spectrum. But I am quite unable to make out that these are 

 two independent spectra. I always see both together in the 

 negative glow and have never seen either produced in any other 

 way. When the glow is very faint the three green bands are 

 more easily seen than the lines because they fill a larger space 

 on the retina, not because they are intrinsically brighter. And 

 when the density of the gas is such that the dark space extends 

 but a short distance round the kathode, both the bands and 

 lines are very bright, brighter than they are in the capillary 

 at any stage of the exhaustion. 



Schuster's elementary line spectrum is best seen when the 

 discharge is taken in oxygen at atmospheric pressure, nevertheless 

 it is very well seen in the negative glow along with the bands 

 and lines above mentioned, and can be easily photographed from 

 that source. 



I have reason to think that sulphur resembles oxygen in 

 giving no anode light. Hitherto I have not been able to 

 examine the discharge in sulphur vapour, but in sulphurous acid 

 gas the kathode glow gives a series of bands which are probably 

 those of sulphur, and are entirely absent from the anode which 

 appears to have no spectrum. 



Chlorine, Bromine, and Iodine, closely resemble one another 

 in their behaviour. Each gives a bright green negative glow, 

 of which the spectrum consists of bright lines, described by 

 Salet 1 , of which the groups in the green are most characteristic. 

 The light from the anode seems to be only a continuous spectrum, 

 very bright in the case of iodine and bromine, less bright in 

 chlorine. In chlorine I see no lines at all in this continuous 

 spectrum, which extends through the orange and green. In 

 bromine, and iodine, the disappearance of the green lines at the 

 anode is as well marked a feature as in chlorine, but the orange 

 lines and some others are visible. Nevertheless I do not think 

 that they belong to the anode, for a reason which is peculiar to 

 the halogens so far as I have observed. When the pressure is 

 not much less than that at which the spark will just pass through 

 the gas, the path of the discharge forms a thread through the 

 tube from one electrode to the other and there is no definite 

 kathode glow, but the whole thread of discharge seems to emit 

 1 Ann. Chimie et Physique (4), xxviii. 24, 1873. 



