266 ProE. Wood and Mr. J. Franck on Transformation 



Hg line we find that the presence of the helium has caused 

 the complete band-spectrum to appear, the isolated lines o£ the 

 resonance spectrum being still visible though greatly reduced 

 in intensity. In helium at 10 mm. there is very little trace 

 or the resonance spectrum left, the fluorescence spectrum 

 being almost identical with that excited by white light. 



In chlorine, however, which, as we have shown, reduces 

 the intensity of the fluorescence by its electro-negative 

 properties, we do not find this effect. The weakening of the 

 lines of the resonance spectrum is not accompanied by the 

 appearance of the band-spectrum. Photographs of the spectra 

 are shown in fig. 1 (PI. II.) . They were made withaHilger 

 wave-length spectroscope. Fig. 1 a shows a portion of the 

 resonance spectrum of the iodine vapour in vacuo excited by 

 the green mercury line ; immediately below it is the Barium 

 arc for comparison. Fig. 1 b is the same as "a" except 

 that it was made with a slightly wider slit, and with a plate 

 having its maximum of sensibility in the yellow-green. On 

 this plate we find a fairly strong line which is wholly absent 

 on plate a, which was sensitized for the red and probably 

 had a minimum of sensitiveness at the wave-length in 

 question. 



Fig. 1 c shows the effect of introducing helium at 2 mm. 

 pressure into the bulb. The band-spectrum comes out dis- 

 tinctly with the lines of the resonance spectrum superposed on 

 it. Just below, fig. 1 d, we have the spectrum excited by white 

 light with the iodine in vacuo. The duration of the exposures 

 were, 1 hour for the white light fluorescence, and 15 hours 

 for the excitation with the mercury lamp. As will be seen, 

 there is a slightly different distribution of intensity in the 

 band-spectrum excited by white light with the iodine in 

 vacuo, and the spectrum excited by the green mercury light 

 with the iodine in helium, apart from the presence of the 

 lines of the resonance spectrum. It is probable that similar 

 effects will be found with sodium vapour, the numerous 

 resonance spectra of which were discovered and studied by 

 Wood. 



If we regard each one of the many resonance spectra as 

 due to the vibration of one system of electrons, the effect of 

 the collisions with, or proximity of, the helium molecules, is 

 to couple all of the systems together so to speak. An isolated 

 molecule can perhaps be thought of as containing a large 

 number of electron systems. Monochromatic radiation 

 excites one of these electrons which emits radiation of the 

 same wave-length as that of the exciting light (resonance 

 radiation), and in addition communicates a disturbance to 



