of Gases excited by Ultra- Schumann Waves. 903 



the partition between the two chambers was made of brass, 

 which carried off the heat from the jaws of the slit. 



For future work we recommend a metal box, of the same 

 general form as the one which we have used ; the chambers 

 K and D are essential, as it is necessary to keep the light of 

 the spark away from the background against which the jet 

 is photographed. The position of the lens L should be 

 arranged so that the spectrum of the light from the jaws 

 of the slit is just barely visible in a dark room with an 

 eyepiece. 



Small tubes were inserted at the ends of the box to 

 facilitate the washing out of the gases. 



The spark-chamber was provided as well with a large 

 hole for the exit of the air, when making experiments with 

 the spark in air. In this way the formation of a cloud of 

 metallic dust was obviated. 



It is very important to make sure in each experiment 

 that the "jet-chamber" is free from dust or smoke which 

 scatters all of the light of the spark. In some gases, initially 

 clean, a cloud speedily forms under the influence of the 

 ultra-violet light, and this must be continuously swept away 

 by a fresh supply of the gas. It was found that the faintest 

 cloud visible to the eye, required an exposure of at least 

 three-quarters of an hour to register itself on the photo- 

 graphic plate ; and since most of our exposures were of only 

 twenty minutes duration, we could make sure in every 

 instance, that scattered light played no part in our results. 



A quartz prism of 3(T was placed in front of the quartz- 

 Huorite achromatic lens used for making the photographs. 

 This spreads out the image of the small point of light formed 

 by the illumination of the walls of the narrow aperture 

 through which the spark rays pass, into the short horizontal 

 spectrum, which appears at the bottom of each one of the 

 figures on PL XIV. The rays from the spark pass up in the 

 form of a thin sheet, rendering the gas fluorescent, and 

 the monochromatic images of the luminous sheet of gas are 

 separated by the prism, appearing on the plate as vertical 

 streamers of light. 



Similar streamers of course appear if the air or gas in the 

 jet-chamber is charged with dust or smoke, but in this case 

 they are located at the regions of the spectrum where the 

 strongest groups of spark-lines fall. 



In the photographs which are reproduced on PI. XI Y. the 

 violet region of the spectrum is to the right, the ultra-violet 

 to the left ; a small black dot marks the position of the 

 so-called water-band at wave-length 30(31. Fig. 1 was made 



302 



