and the Resonance Radiation of Electrons, 523 



system which gives rise to the channelled spectra. Whether 

 J) l and D 2 are connected has not yet been determined. To 

 do this it will be necessary to free the illuminating beam from 

 the D T radiations, and observe whether or not the fluorescent 

 band is double. An attempt will be made to accomplish this. 

 AVe will now consider more in detail the appearance of the 

 fluorescence spectrum, when the illuminating light consists of a 

 more or less monochromatic beam from the illuminator. As 

 has been said, the yellow end of the spectrum appears only 

 when the stimulating light is blue-violet. It is probable lhat 

 the yellow light will also be radiated when the stimulating 

 light is of the same wave-length, though such a radiation 

 has not yet been detected. The absorption for radiation in 

 this region (X = 571 to 560) is very small, and the vapour 

 appeared practically black in all cases thus far observed. 

 The region of maximum fluorescence moves down the spec- 

 trum as the stimulated region moves up, as is well shown 

 in the series of photographs in fig. 3 (PL VII.) . In this 

 series of photographs the stimulated region is indicated by 

 the luminous region to the left. The lowest one, e, shows 

 very clearly that when the stimulation is at the violet end 

 only the first five bands at the yellow end appear. In 

 spectrum u a }> the stimulation is at the centre. The change 

 in the position of the groups or fluted bands is best shown in 

 figs. 4 and 5. The same thing can be seen in fig. 3. by 

 mo vino- a vertical straight edge along the series. That 

 the spectra are in perfect "register"''' can be seen by 

 noting the position of the D lines at the right. These 

 were recorded by holding a soda flame in front of the 

 slit for a few seconds during each exposure. Hadiations 

 both above and below the excited region are present, showing 

 that Stokes's law is not obeyed. This is especially noticeable 

 in fig. 2 (PL VII.), where the excited region is indicated by 

 a white rectangle. The plate-holder of the spectrograph was 

 arranged so that it could be moved up and down, and as 

 many as seven spectra could be photographed on a single 

 plate if necessary. After recording a fluorescent spectrum, 

 the plate was elevated and the beam from the monochromatic 

 illuminator thrown back into the instrument by means of a 

 mirror. In this way it was possible to obtain a record of the 

 extent of the region of fluorescence below the excited region. 



in in 



It was thought desirable to obtain a more homogeneous type 

 of exciting light than that furnished by the monochromatic 

 illuminator, and a mercury arc in a quartz tube was accordingly 

 tried. Though the light was extremely intense, and though 



o o «/ 7 in 



its radiation contained a number of lines in the region de- 

 sired, it was found impossible to obtain the slightest trace of 



