454 Mr. C. F. Meyer and Prof. R. W. Wood on 



of nitrogen delivered by the tube T is perfectly dry, but is no- 

 longer interrupted if the nitrogen is moistened, would lead 

 us to believe that if the stagnant gas in the jet-chamber 

 consisted of entirely pure dry nitrogen, then the fluorescent 

 spectrum would consist of the three nitrogen bands only. 



Experiments tried with gases other than nitrogen and air 

 were not carried far enough to lead to results of sufficient 

 interest and certainty to warrant discussion, except in the 

 case of iodine vapour. Some crystals of iodine were placed 

 in a glass tube through which nitrogen was passed, and the 

 resulting mixture of iodine vapour and nitrogen was blown 

 from the tube T across the slit. The iodine fluoresces in 

 the visible region with a bluish-green light as was noted by 

 Wood and Hemsalech (/. c. p. 907), and in the ultra-violet. 

 A spectro-photograph of the streamer in ultra-violet is repro- 

 duced in fig. 6. This fluorescence was so bright that it could 

 be easily photographed by throwing an image of the fluorescent 

 jet upon the slit of a spectroscope with a quartz lens, thus 

 obtaining its spectrum in very much greater detail. This 

 was very kindly done for us by Mr. Voss of this laboratory, 

 and the result is shown by the photograph of fig. 7, which 

 was made with an improvised quartz spectrograph of the 

 Littrow type, furnished with two Gornu prisms and a lens 

 of about 2 metres focus. The comparison spectrum is that 

 of the iron arc. The band is highly monochromatic, as will 

 appear from the wave-lengths of the principal lines which 

 make up the band : they are as follows : — 



3379-7 34.14-8 



3388-8 3418-5 



3400-0 3423-9 



3406-3 3426-4 



3408-8 3431-4 



3413-3 3435-3 



The spectrum of the visible bluish-green light has not yet 

 been photographed, as it is very much fainter than the ultra- 

 violet band. 



Transmission, Refraction, and Reflexion. 



In the study of the transmission of the rays exciting the 

 fluorescent jet an entirely new jet-apparatus was used, the- 

 spectroscope and electrical equipment remaining the same. 

 This new apparatus was based on the same general principles 

 as the old one, but differed in being smaller ; in providing 

 for the delivery of the fresh gas from the rear through a large 

 tube (1*5 cm. in diameter), w T hich at the same time formed a 



