902 Prof. Wood and Mr. Hemsalech on Fluorescence 



when in motion. The apparatus used in the present investi- 

 gation is shown in fig. 1. 



It was a box constructed of hard wood in the form of a 

 cross, painted black on the inside, and made very nearly 

 gas-tight with putty. The lower compartment S, which will 

 be spoken of as the spark-chamber, communicated with the 

 rest of the box only through the narrow slit at A. This was 

 formed by two jaws of copper about 3 mm. thick, which 

 could be moved to and fro in a brass frame, like the jaws of 

 a spectroscope. The jaws were made of the form shown (in 

 section) by the small diagram of fig. 1, this form being 

 adopted to insure the sparks striking immediately below 

 the slit. If flat plates are used the spark wanders about, 

 and much longer exposures are necessary. The brass frame 

 and copper jaws formed one electrode, the other (B) con- 

 sisted of a wedge-shaped piece of copper mounted as shown. 

 The rays of the spark come up through the narrow crevice 

 between the jaws, and are absorbed by the black walls of 

 the upper portion of ths box. The other compartments of 

 the box were in free communication, the one to the left con- 

 taining the quartz-fluorite achromatic lens L, and the 30° 

 quartz prism P, which was cemented over a circular hole in 

 the side of the box. This optical system formed a spectrally 

 dispersed image R on the photographic plate of any lumi- 

 nosity produced in the air or other gas above the slit through 

 which the spark rays passed ; the compartment K furnished 

 an absolutely black background against which the luminous 

 jet was photographed. When studying the action of moving 

 gas currents on the jet, the current was delivered against 

 the jet by the tube T. The jet-chamber J was filled with 

 the desired gas through the tube E, a shield D insuring 

 thorough washing out of the upper compartment. The 

 spark-chamber was filled through the tube P. Both cham- 

 bers were connected with a water manometer, so that the 

 pressure in either could be measured. A small window at 

 H enabled observations to be made of the interior, to deter- 

 mine the presence or absence of a visible cloud. 



It will be understood that the arrangement of the photo- 

 graphic plate in fig. 1 is diagrammatic only, a small extension 

 camera being arranged for photographing the jets, with its 

 front board removed and the bellows attached to the end of 

 the wooden box. The spark discharge was very powerful, 

 and was furnished by a Hemsalech-Tissot resonance trans- 

 former with a condenser of 0*05 mfd. capacity. We usually 

 kept the spark-chamber open on one side, ventilating it with 

 an air blast, which prevented overheating of the electrodes ; 



