1894.] 



On some Phenomena in Vacuum-tubes. 

 FIG. 4. 



239 



visible, with absolutely dark, wide spaces between them. On increas- 

 ing the current, the bright bands expand, the dark spaces growing 

 narrower till at last the bright bands touch one another and then 

 overlap. Then the paradox occurs that the dark bands appear bright 

 and the bright bands appear dark ; i.e., the bright bands overlap 

 where the dark spaces should be, and therefore appear doubly as 

 bright as the remaining portion of the bright bands, which now 

 appear dark by contrast. 



From experiments conducted on other tubes I think it is quite 

 clear that this is the usual condition of things, and that it is exceed- 

 ingly rare that the true black bands are really seen. But in this 

 experiment there is one point of interest which must not be over- 

 looked. It is, that whether the current be small or large, the rule 

 set up by the little rod at the end of the tube cannot be upset. The 



