Radiant Emission from the Spark. 709 



with two lateral tubes H and G, can be screwed to the spark 

 chamber when it is desired to study the emission in dry 

 filtered air or some other gas. The tube G is closed with a 

 quartz window, while H terminates in a smaller tube J for the 

 introduction of gas. The emission is quite invisible in dust- 

 free air, yet it can be photographed with an exposure of one 

 or two minutes with a quartz lens of 2 cm. aperture and 

 15 cm. focus. In arranging the position of the quartz 

 camera the focal plane should be examined with an eyepiece 

 in a dark room to make sure that no part of the lens receives 

 light from the edge of the hole ; in other words, the top of the 

 lens must be just below the plane of the aluminium disk A. 



In order to get an idea of the appearance of scattered 

 light, the air around the apparatus was filled with smoke and 

 the spark discharge started. The photograph obtained in 

 this way is reproduced on PL XIV. fig. 1. Fig. 2 was 

 obtained when the air was free from smoke or dust, and 

 shows the appearance of the emission with which we are 

 concerned. A comparison of these two photographs shows 

 us at once that the emission does not extend nearly so far 

 out from the aperture as does the luminous region of light- 

 scattering smoke. It appears as if it were rapidly absorbed 

 by the air. That this is not due to differences in the time of 

 exposure is shown by the original negatives, for fig. 2 has a 

 density nearly double that of fig. 1 in the immediate proximity 

 of the aperture in the disk. An experiment was next made 

 to ascertain the nature of the light given out by the emission. 

 A fine thread of fused quartz, about 2 mm. in length, was 

 mounted at the edge of the aperture by means of a small 

 drop of soluble glass. This scattered the light of the spark, 

 forming a narrow linear source of spark light located at the 

 centre of the base of the emission. The slit tube of a small 

 quartz spectrograph was removed, and the luminous quartz 

 thread brought into its place. The resulting photograph is 

 shown in tig. 3, a continuous band of light, the spectrum of 

 the quartz fibre, with the emission above it and about at its 

 centre. This picture proved that the light given out by Uie 

 emission embraced a limited range of wave-lengths in the 

 region 300-310. This picture was secured with an exposure 

 of only fifteen minutes, which made it seem probable that the 

 spectrum of the emission could be obtained with a fairly 

 narrow slit. Fig. 5 shows a spectrogram obtained with a 

 wide slit, the aluminium lines showing faintly as a result of 

 diffused light : the lower spectrum is that of the spark for 

 comparison. The spectrum of the emission consists of two 

 broad bands, one very strong, the other (to the right) much 



