146 



Mr. William Croohes 



[April 4, 



exhausted, and, as before, will make the side pole (a') the negative, 

 the top pole (h) being positive. Notice how widely different is the 

 appearance from that shown by the last bulb. The negative pole is 

 in the form of a shallow cup. The bundle of rays from the cup 

 crosses in the centre of the bulb, and thence diverging falls on the 

 opposite side as a circular patch of green light. As I turn the bulb 



Fig. 6. 



round you will all be able to see the faint blue focus and the green 

 patch on the glass. Now observe, I remove the positive wire from 

 the top, and connect it with the side pole (c). The green patch from 

 the divergent negative focus is still there. I now make the lowest 

 pole (c?) positive, and the green j^atch still remains where it was at 

 first, unchanged in position or intensity. 



This, then, gives us another fact which brings us a little nearer to 

 the cause of this green phosphorescence. It is this — that in the low 

 vacuum the position of the positive pole is of every importance, 

 whilst in a high vacuum it scarcely matters at all where the positive 

 pole is ; the phenomena seem to depend entirely on the negative pole. 

 In very high vacua, such as we have been using, the phenomena 



