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



edges of the slit S. The spectrum appears as a line since 

 the slit is presented to the spectroscope "end-on." The 

 rather diffuse vertical streamers are the monochromatic 

 images of the jet, dispersed by the spectroscope. I on g 

 wave-lengths are toward the left. The streamer marked 

 with a dot is the water band which is interrupted where the 

 current of nitrogen delivered by the tube T crosses the jet, 

 for reasons which will be further discussed. The position of 

 the tube is indicated. To the left of the water band are 

 the nitrogen bands. In the commercial nitrogen only the 

 first and second, counting from the water band, are present ; 

 while in the purified nitrogen the first (3369) is faint, and 

 the second and third (3556 and 3778) are prominent. Some 

 time after these photographs were obtained, when the appa- 

 ratus had all been taken down, cleaned^ and set up aoain, 

 a different type of spectrum was obtained with purified 

 nitrogen, in which the first and second nitrogen bands were 

 faint and the third strong (fig. 3). The type of spectrum 

 in the commercial nitrogen remained the same. The cause 

 of the difference between fig. 2 and fig. 3 was not discovered 

 even after making a number of exposures under various 

 conditions for the purpose of ascertaining it. 



Some little time was spent in repeating the more important 

 parts of the work of Wood and Hemsalech, done in Paris. 

 With entirely new apparatus, and different sources of supply 

 of our gases, these attempts at repetition often resulted at 

 first in very perplexing failures. Investigation into the- 

 cause of these preliminary failures, however, resulted in most 

 cases in throwing much light on the phenomena themselves.. 

 For example, the first attempts to duplicate the photographs 

 (/. c. figs. 5, 6, 7) obtained with moving and stationary gases 

 were unsuccessful. In these a stream of nitrogen was blown 

 across the fluorescent jet, the stream causing an interruption 

 of the water band, and the appearance of the nitrogen bands, 

 in the moving stream. Later attempts showed that in order 

 to obtain the interruption of the water band it was best to 

 bring the mouth of the tube T close up to the slit S (say 

 within *5 cm.). Experiments made with smoke indicated 

 however, that the stream of gas delivered by the tube at the 

 velocities which we were using maintained approximated its 

 form and area of cross-section, and consequently its velocity, 

 for several centimetres beyond the mouth of the tube. This 

 suggested that the appearance of the water band in the 

 stationary portion of the gas, and its disappearance in the 

 moving stream, might be due, not primarily to the rest or 

 motion of the gas with reference to the fluorescent jet, but. 



