24 SCIENCE PROGRESS 



a fluorescent spectrum as complex as the absorption one, and 

 probably its complement. If, however, instead of white 

 light, a strong beam of monochromatic radiation is used to 

 illuminate the vapour, we find that it emits a series of bright 

 lines spaced at nearly equal intervals along a normal spectrum. 

 Various series of lines with varying distribution of intensity 

 can be brought out by changing the wave-length of the exciting 

 light. In every case light of the same wave-length as that 

 of the exciting light is emitted by the vapour, and in addition 

 a large number of other frequencies, which bear a definite 

 relation to each other. On account of the equality of wave- 

 lengths mentioned, Wood calls such a fluorescent spectrum a 

 " resonance spectrum." Sodium vapour, for instance, has 

 been excited by the monochromatic radiations from the 

 cadmium-vapour lamp, the bismuth arc, the magnesium arc, 

 the lithium arc, the mercury-vapour lamp, and incandescent 

 sodium itself. Mercury vapour has also been shown to emit 

 a resonance radiation when excited by the 2,536 line of the 

 mercury arc. Iodine vapour was also found to behave in a 

 manner similar to sodium vapour when stimulated with 

 monochromatic light for various metallic arcs. One inter- 

 esting point about these spectra is the approximately equal 

 spacing of the lines of any one spectrum along the scale of 

 wave-lengths, for this is bound to shed some light on the 

 structure of the atom. As Prof. Wood remarks in his Physical 

 Optics — " Prof. Rowland once said that ' a molecule is much 

 more complicated than a piano.' In most cases all that we 

 have been able to do is to strike the entire keyboard at once, 

 but in the case of (the resonance spectra of) sodium it seems 

 possible to strike one key at a time. In the March number 

 of the Phil. Mag., Wood gives new data concerning the reson- 

 ance spectrum of iodine vapour, obtained since his earliest 

 trials (in 19 10) with that substance were made. Using the 

 Cooper-Hewitt mercury- vapour lamp, he finds that the iodine 

 vapour emits a spectrum consisting of a series of doublets 

 (with a separation of 1*5 Angstrom Units), very regularly 

 spaced along the spectrum and separated by intervals of 

 about 70 A.U. As we pass, however, from the green mercury 

 line where the doublet series has its origin to the termination 

 of the series at 7,685 in the red, the interval separating the 

 last two doublets is 102 A.U., and it appears that the fre- 



