424 Molecular Physics in High Vacua. [June, 
it is also much more strongly phosphorescent to the mole- 
cular discharge in a good vacuum, as you will see when I 
pass the discharge through this tube (Fig. 8). The white 
plate {a, b) in the centre of the tube is a sheet of mica 
painted over with the luminous sulphide of which the 
letter <j> was composed in the diagram you have just seen. 
On connecting the poles with the coil, the mica screen 
glows with a strong yellowish green light, bright enough to 
illuminate all the apparatus near it. But there is another 
phenomenon to which I now desire to draw attention : on 
the luminous screen is a kind of distorted star-shaped 
figure. A little in front of the negative pole I have fixed a 
star (c) cut out in aluminium, and it is the image of this 
star which you see on the screen. It is evident that 
the rays coming from the negative pole project an image of 
anything that happens to be in front of it. The discharge, 
therefore, must come from the pole in straight lines, and 
does not merely permeate all parts of the tube and fill it with 
light as it would were the exhaustion less good. Where 
there is nothing in the way the rays strike the screen and 
produce phosphorescence, and where there is an obstacle 
they are obstructed by it, and a shadow is thrown on the 
screen. I shall have more to say about this shadow pre- 
sently ; I merely now wish to establish the fact that these 
rays driven from the negative pole produce a shadow. 
I must draw your attention to an important experiment 
connected with these molecular rays, but unfortunately it 
is a very delicate one, and very difficult to show to 
many at once ; but I hope, if you know beforehand what 
Fig. 9 a. 
to look for, you will all be able to see what I wish to 
show. In this pear-shaped bulb (Fig. 9 a) the negative 
pole (a) is at the pointed end. In the middle is a cross (6) 
