238 Sir David Salomons, Bart. [June 21, 



structed in this manner, the bright bands are repeated along them, 

 the distance corresponding with the distance between the bright 

 bands formed at each side of any disk. On increasing the current, 

 very considerably in consequence of the bands on the glass disks 

 receding from the latter, a confusion is set up throughout the tube, 

 i.e., the bands all mix together, especially as they become somewhat 

 wider at the same time as they recede from the glass disks, till finally 

 the whole tube appears generally lit without any bands being seen. 

 Some tubes will melt before obtaining this effect. 



I, therefore, conclude that the absence of bands is generally due to 

 this cause, i.e., the bands are really there, but in consequence of too 

 much current they have become expanded and overlap one another 

 so often as to render their existence invisible. Some very instruc- 

 tive experiments for the examination of this effect will shortly be 

 described. 



Tubes with three and four disks on the rods were also used, and 

 these produced similar effects. (See figs. SA and 3s.) 



Experiment 4. 



This experiment and some of those which follow are made with a 

 view of determining the possibility of producing a given number of 

 bands in a vacuum tube. The possibility of doing this might already 

 be inferred from the experiments described. If it is generally true 

 that any two bands produced at one end of a tube are repeated 

 throughout the tube at the same distance apart as the first pair 

 formed, and if it is true that any impediment within the tube will 

 produce a pair of bands, then the problem is solved ; and I think the 

 following tubes show that such is the case. 



The tube shown in fig. 4 has its ends contracted and re-expanded 

 into bulbs, in which the electrodes are placed. At one end, where 

 the contraction exists, a short tube is melted on, which carries a little 

 hollow sphere of glass with three holes so situated that, if a line be 

 drawn from the centre of any hole to the centre of the glass sphere, 

 an angle of approximately 45 is made with the axis of the tube. 

 There is melted on the top of the glass sphere a short rod projecting into 

 the free part of the tube, the axis of the rod coinciding with the axis 

 of the tube. When the current is passed there should be formed a 

 bright band at the base of the rod close to where the current issues 

 from the three holes in the sphere, and another one at the free end of 

 the rod. Then throughout the tube there should be produced bright 

 bands equidistantly placed, such distance being equal, or approxi- 

 mately equal, to the length of the little rod ; and, on passing the 

 current, this proves to be the case. When the amount of current 

 passed is exceedingly small, distinct, narrow, bright bands become 



