422 



NA TURE 



\An^tist 28, 1879 



that is very highly exhausted, and as before, will make the fide 

 pole (a') the negative, the top pole (b) being positive. Notice 

 how widely dififerent is the appearance from that shown by the 

 last bulb. The negative pole is in the form of a shallow cup. 

 The molecular rays from the cup cross in the centre of the bulb, 

 and thence diverging fall on the opposite side and produce a 

 circular patch of green phosphorescent light. As I turn the 

 bulb round you will all be able to see 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 there still. I now make the lowest 

 pole (d) positive, and the green patch remains where it was at 

 first, unchanged in position or intensity. 



We have here another property of radiant matter. In the low 

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

 whilst in a high vacuum the position of the positive pole scarcely 

 matters at all ; the phenomena seem to depend entirely on the 

 negative pole. If the negative pole points in the direction of tile 



Fig. 7. 



positive, all very well, but if the negative pole is entirely in the 

 opposite direction it is of little consequence : the radiant matter 

 darts all the same in a straight line from the negative. 



If, instead of a flat disk, a hemi-cylinder is used for the nega- 

 tive pole, the matter still radiates normal to its surface. The 

 tube before you ( Fig. 8) illustrates this property. It contains, 

 as a negative pole, a hemi-cylinder (a) of polished aluminium. 

 This is connected with a fine copper wire, b, ending at the 

 platinum terminal, c. At the upper end of the tube is another 

 terminal, d. The induction-coil is connected so that the hemi- 

 cylinder is negative and the upper pole positive, and when 

 exhausted to a sufficient extent the projection of the molecular 

 rays to a focus is very beautifully shown. The rays of matter 

 being driven from the hemi-cylinder in a direction normal to its 

 surface, come to a focus and then diverge, tracing their path in 

 brilliant green phosphorescence on the surface of the glass. 



Instead of receiving the molecular rays on the glass, I will 

 show you another tube in which the focus falls on a phosphorescent 

 screen. See how brilliantly the lines of discharge shine out, 

 and how intensely the focal point is illtmiinated, lighting up the 

 table. 



Radiant Matter when inteixepted by Solid Matter casts a Shadow 



Radiant matter comes from the pole in straight lines, and does 

 not merely permeate all parts of the tube and fill it with light, 

 as would be the case were the exhaustion less good. Where 

 there is nothing in the way the rays strike the screen and produce 

 ph: sphorescence, and where solid matter intervenes they are 

 obstructed by it, and a shadow is thrown on the screen. In this 

 pear-shaped bulb (Fig. 9) the negative 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 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 phos- 

 phorescent I turn on the coil, and you will all see the black 



Fig. 8. 



shadow of the cross on the luminous end of the bulb (c, J), 

 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 o 1 

 the glass — its sensibility has been deadened. The glafs has get 

 tired, if I may use the expression, by the enforced phosphor 

 escence. A change has been produced by this molecular bo; 



ii 



bardment which will prevent the glass from responding 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 per- 

 fectly fre.sh ; therefore if I throw down this cross— I can easily 

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

 ingeniously constructed with a hinge by Mr. Gimingham — and 

 so allow the rays from the negative pole to fall uninterruptedly 

 on to the end c f the btilb, you will suddenly see the black cross 



