24 THE POPULAR SCIENCE MONTHLY. 



is thrown on the screen. In this pear-shaped bulb (Fig. 9) the nega- 

 tive pole (a) is at the pointed end. In the middle is a cross {b) cut 

 out of sheet-aluminium, so that the rays from the negative pole pro- 

 jected 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 turn on the coil, and you will all see the 

 black shadow of the cross on the luminous end of the bulb (c, d). 

 Now, the radiant matter from the negative pole has been passing by 

 the side of the aluminium cross to produce the shadow ; the glass has 

 been hammered and bombarded till it is appreciably warm, and at the 

 same time another effect has been produced on the glass — its sensibility 

 has been deadened. The glass has got tired, if I may use the expres- 

 sion, by the enforced phosphorescence. A change has been produced 

 by this molecular bombardment which will prevent the glass from re- 

 sponding easily to additional excitement ; but the part that the shadow 

 has fallen on is not tired — it has not been phosphorescing at all and is 

 perfectly fresh ; therefore, if I throw down this cross — I can easily do 

 so by giving the apparatus a slight jerk, for it has been most ingen- 

 iously constructed with a hinge by Mr. Gimingham — and so allow the 

 rays from the negative pole to fall uninterruptedly on to the end of 

 the bulb, you will suddenly see the black cross (c, d, Fig. 10) change 

 to a luminous one {e,f), because the background is now only capable 



of faintly phosphorescing, while the part which had the black shadow 

 on it retains its full phosphorescent power. The stenciled image of 

 the luminous cross unfortunately soon dies out. After a period of rest 

 the glass partly recovers its power of phosphorescing, but it is never so 

 good as it was at first. 



Here, therefore, is another important property of radiant matter. 

 It is projected with great velocity from the negative pole, and not only 

 strikes the glass in such a way as to cause it to vibrate and become 

 temporarily luminous while the discharge is going on, but the mole- 

 cules hammer away with sufficient energy to produce a permanent im- 

 pression upon the glass. 



