Transformation of Resonance into Band Spectrum, 265 



the yellow lines from the light of the arc. The colour of the 

 fluorescence becomes at once greener. The 5796 line is still 

 visible in the resonance spectrum, but the 5774 line has 

 disappeared, which is what we should expect. The spectra 

 are so faint, with any considerable dispersion, that visual 

 observations are unsatisfactory. As soon as I am able to 

 photograph them with my large three-prism spectrograph, 

 we shall be able to tell better just what we have. Tn studying 

 the spectra I now place a plane mirror behind the bulb, in 

 such a position that the fluorescent sheet of vapour and its 

 reflected image are seen superposed. Much could be gained 

 by employing two mercury arcs, one on each side of the 

 bulb. 



Paris, Jan. 16, 1911. 



XXIX. Transformation of a Resonance Spectrum into a Band- 

 Spectrum by Presence of Helium. By R. W. Wood and 

 J. Franck '*. 



[Plate II.] 



AS has recently been shown by Wood, the band-spectrum 

 of iodine vapour, brought to a state of fluorescence by 

 excitation with w hite light, can be analysed into simple series 

 of very sharp and widely separated lines by monochromatic 

 excitation. With the mercury arc, the green line being the 

 only one capable of exciting fluorescence, we obtain a series of 

 15 lines, about 70 A.E. apart, and of very variable intensity. 

 Each band of the channeled spectrum is made up of a large 

 number of fine lines, and we have, roughly speaking, one 

 line of the resonance spectrum for each one of the bands. 

 As we have already shown, the reduction in the intensity of 

 the fluorescent light by the presence of helium, is much less 

 than that produced by any other gas. In helium at 80 mins. 

 pressure, the fluorescence can still be seen though rather 

 feeble and quite red in colour, in marked contrast to the 

 green colour shown by the vapour in vacuo. In chlorine, 

 however, at a pressure of 4 or 5 mm. the fluorescence has 

 disappeared entirely. 



If we have our iodine vapour in a bulb containing helium 

 at a pressure of only 2 or 3 mm., there is no apparent 

 change in the intensity or colour of the fluorescence, regard- 

 less of whether we excite it with white light or monochro- 

 matic light. The spectroscope shows, however, that a most 

 remarkable change has taken place. Exciting, with the green 



* Communicated by the Authors. 



