us 



Mr. William Crookes 



[April 4, 



Fig. 8. 



rescence is excited the letters sliine for several hours. I will put the 

 diagram at the back, and we shall see how it lasts during the re- 

 mainder of the lecture. This substance, then, is phosphorescent to 

 light, but 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, h) in the 

 centre of the tube is a sheet of mica painted 

 over with the luminous sulphide of which the 

 letter (^ was composed in the diagram you have 

 just seen. On connecting the poles with the 

 coil the mica screen glows with a strong yellow- 

 ish 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 ex- 

 haustion 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 presently; 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 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) cut out of sheet 

 aluminium, so that the rays from the negative pole projected along 

 the tube will be partly intercepted by the aluminium cross, and will 

 project an image of it on the hemispherical end of the tube which is 

 phosphorescent. I think you will all now see the shadow of the cross 

 on the end of the bulb (c, d), and notice that the cross is black on a 

 luminous ground. Now, the rays from the negative pole have been 

 passing by the side of the aluminium cross to produce the shadow ; 



