482 Prof. Thomson, On the phosphorescence observed etc. 



On the phosphorescence observed on the glass of vacuum tubes 

 when the pressure is not very low. By Sir J. J. Thomson, Caven- 

 dish Professor of Experimental Physics. 



[Read 14 March 1910.] 



If an electric discharge is sent through a vessel from which 

 the air is gradually exhausted, at a certain stage of the exhaustion 

 the whole of the walls of the tube will be found to be phos- 

 phorescent. This phosphorescence is of quite a different colour 

 from that produced by the cathode rays and occurs at a much 

 higher pressure, the pressures at which it is brightest vary with 

 the dimensions of the vessel and with the gas inside it, but they 

 are of the order of 1 mm. of mercury. The colour of the phos- 

 phorescence with soft soda glass is an olive-green, quite distinct 

 from the yellowish-green of the phosphorescence due to cathode 

 rays, with lead glass this phosphorescence begins by being greenish 

 but gets blue as the pressure diminishes. The following experi- 

 ments show, I think, that the cause of the phosphorescence is 

 ultra-violet light produced by the electric discharge. A large 

 vessel was divided into two parts A and B, separated by an 

 opaque screen which was perforated with a long narrow channel, 

 thus if any light were produced in A it would enter ^ as a fine 

 pencil. The discharge was sent through A and when the stage 

 was reached when the walls of A phosphoresced the place where 

 a pencil going through the channel would strike the walls of B 

 became phosphorescent with a well-defined spot, a little powdered 

 millemite placed on the glass greatly increased the brilliancy of 

 the spot. Various substances were placed in the path of the 

 pencil, glass was found to be fairly opaque to these rays, although 

 the amount of phosphorescence produced after the pencil has 

 passed through a cover slip is quite appreciable, and with care 

 the pencil can be detected after it has passed through the walls 

 of a vacuum tube. Quartz is much more transparent to the 

 pencil than glass, and white fluorite than quartz. The refraction 

 of the pencil by the fluorite was quite marked : a plate of fluorite 

 with parallel sides supported by a glass rod working in a ground- 

 glass joint was put in the way of the pencil, as the plate was 

 rotated the spot of light due to the pencil on the screen moved 

 backwards and forwards just as a spot due to visible light would 

 have done. 



