I02 



NATURE 



\ytinc 3, 1880 



exhaustion was such that the dark space extended about 

 8 millims. from the pole, the first appearance noticed was 

 that of a ray of dark blue light issuing through the hole 

 in the mica screen, and shooting upwards towards the 

 side of the tube, but not reaching it. Fig. 4 shows the 



dark space round the pole, and the ray of blue light. On 

 increasing the exhaustion this blue line of light, and the 

 luminous boundary to the dark space, disappeared, and 

 presently a green oval spot appeared on the side of the 

 tube, exactly on the place previously marked where the 

 rays issuing normal from the surface of the pole should 

 fall. 



It happened that this oval spot fell on a portion of the 

 tube where one of the elliptical projections from the 

 opposite (corrugated) pole also fell when that was made 

 negative. Thu5 by reversing the commutator I could get 

 a narrow band of green phosphorescent light from one 

 pole, or a wider oval of green light from the other pole, to 

 fall alternately on the same portion of the glass. Fig. 5 

 shows these effects, which, however, did not occur together 

 as represented in the figure, but alternately. 



The narrow band shone very brightly with green 

 phosphorescence, but on reversing the commutator and 

 obtaining the oval spot, this was seen to be cut across the 

 middle by a darker band where the phosphorescence was 

 much less intense. The light of the band was always 

 more intense than that from the spot ; the impacts from 

 the one being more concentrated than from the other, 

 owing to the shape and position of the poles ; moreover 

 the experiments had been first tried with the corrugated 

 pole negative. The glass along the band gradually 

 becomes deadened by repeated impact^, and will not 

 readily phosphoresce in reply to the weaker blows from 

 the flat plate, although it still responds to the more 

 energetic bombardment from the corrugated pole. This 

 phenomenon almost disappears at very high e.xhaustions, 

 or if the tube is allowed to rest for some time. The tired 

 glass then recovers its phosphorescent power to some 

 extent, but not completely. 



To obtain this action in a more striking manner, a tube 

 was made having a metal cross on a hinge opposite the 

 negative pole. The sharp image of the cross was pro- 

 jected on the phosphorescent end of the bulb, where it 

 appeared black on a green ground. After the coil had 

 been playing for some time a sudden blow caused the 

 cross to fall down, when immediately there appeared on 

 the glass a bright green cross on a darker background. 

 The part of the glass formerly occupied by the shadow, 

 having been protected from bombardment, now shone out 

 vith full intensity, whilst the adjacent parts of the glass 



had lost some of their sensitiveness, owing to previous 

 bombardment. 



This effect of deadening produced on glass by long- 

 continued phosphorescence was shown in a very striking 

 manner at a lecture delivered at the Royal Institution on 

 April 4, 1879, when the image of a cross was stencilled on 

 the end of a large pear-shaped bulb. 



I subsequently experimented further with this bulb, and 

 found that the image of the cross remained firmly 

 stencilled on the glass. The bulb was then opened and 

 the wide end heated in the blowpipe flame till it was quite 

 soft and melted out of shape. It was then blown out 

 again into its original shape, and re-exhausted ; on con- 

 necting it with the induction coil, the metal cross being 

 down out of the line of discharge, the original ghost of 

 the cross was seen to be still there, showing that the 

 deadening of the phosphorescing powers the glass pro- 

 duced by the fiist experiment at the Royal Institution 

 had survived the melting-up and re- blowing out of the 

 bulb. 



When experimenting with this apparatus a shifting of 

 the line of molecular discharge was noticed when the 

 current was first turned on. The flat pole b (Fig. 6) being 



negative and the line cd being normal to its surface, the 

 spot of light falls accurately on d, when the exhaustion is 

 sufficiently good to give a sharp oval image of the hole c. 

 But at higher exhaustions, when the outline of the image 

 of c becomes irregular and continually changing, the patch 

 of light at the moment of making contact is sometimes 

 seen at c, and then almost instantly travels from e to d, 

 where it remains as long as the current passes. The 

 passage of the spot from e to d is very rapid, and requires 

 close attention to observe it. If the coil is now stopped 

 for a longer or shorter time, and contact is again made 

 ilie same way as before {b being negative), the spot does 

 not now start from position e, but falls on d, in the first 

 instance. This can be repeated any number of times. 



If now the pole b be made positive even for the shortest 

 possible interval, and it then be made negative, the 

 original phenomenon occurs, and the spot of light starts 

 from e and rapidly travels to d. After this it again falls 

 on d, ab initio, each time contact is made, so long as b is 

 kept the negative pole. There seems no limit to the 

 number of times these experiments can be repeated. The 

 explanation of this result appears to depend on a tem- 

 porary change in the condition of the wall of the glass 

 tube when positively electrified molecules beat against it, 

 a change which is undone by subsequent impact from 

 negative molecules. This phenomenon is closely con- 

 nected with some shadow and penumbra experiments 

 described further on, and as the same explanation will 

 apply to both I will defer any theoretical remarks for the 

 present. 



A suggestion was made by Prof. Ma.xwell that I should 

 introduce a third, idle, electrode in a tube between the 

 positive and negative electrodes so that the molecular 

 stream might beat upon it, so as to see if the molecules 

 gave up any electrical charge when impinging on an 

 obstacle. A tube was therefore made as shown in Fig. 7 ; 

 a and b are the ordinary terminals ; c and d are large 

 aluminium disks nearly the diameter of the tube, con- 



