H. V. Gill — Electric Discharge in Geissler Tubes. 411 



H. Vincent's* paper on the " Photography of Ripples," and 

 which are striking illustrations of the laws of acoustics. 



The first consisted in obtaining the circular rings which cor- 

 respond to the rings on a lake when a stone is thrown in. In 

 this case the formation of the heaps of powder presents a diffi- 

 culty, as there does not seem to be any reflection of the wave. 

 For many reasons, however, they would seem to be due to the 

 oscillations of the spark as distinct from the sudden movement 

 of the air caused by the explosion. We have repeated the ex- 

 periments here described, substituting explosions, caused by a 

 mixture of hydrogen and oxygen, for the spark, — in many cases 

 more intense than those due to spark, — but in no case was the 

 powder divided as when sparks were employed. This seems 

 to prove, that the effect is not due to the mere current of air 

 caused by the explosion. Another time we hope to examine 

 this question with greater detail. A hole was cut in a piece 

 of sensitive paper ; lycopodium pow T der was scattered as evenly 

 as possible all round. The two terminals were arranged so 

 as to allow a spark about l cm long to pass vertically through 

 the hole in the paper. The waves caused by the oscillations 

 will of course be circles concentric with the spark. Fig. -1, 

 Plate IV, shows the result. It will be noticed that the rings 

 are nearer together some distance from the spark than near 

 to it. There is an area round the spark from which the 

 powder has entirely disappeared. This is due not only to the 

 violence of the spark explosion, but also to the fact that a 

 considerable portion of the powder is attracted by, and adheres 

 to, the discharging terminal. Could not this throw some light 

 on the 'negative dark space' in vacuum discharges? The 

 only difficulty in obtaining this and the following results is in 

 distributing the powder evenly over the surface at the begin- 

 ning. Another point of importance is that the terminals 

 should stand quite vertically, and not pass near the surface 

 of the paper, as the powder 'in this case is attracted by them. 



The effect due to reflection is well shown in Fig. 5, Plate 

 V. In this case the " mirror" was a piece of thin metal about 

 l-5 cm high placed on the surface of the paper. The spark 

 passed at some distance, as is evident from the photograph. 

 The caustic appears very clearly defined, and the focus is quite 



distinct. 



The interference of the air waves due to two 8imultane< 

 sparks was also investigated in the Bame way. In this 

 the lines of interference were well marked and formed a 

 hyperbola with the two sparks as foci. 



In all these photographs it is noticeable that the lines are 

 farther apart near the spark than at some distance. 1 his 



* Phil. Mag., June, 1 



