674 Prof. R. W. Wood on Resonance Spectra of 



greatly underestimated. With any grating of moderate 

 power it appears to be completely resolved, though some of 

 the lines appear broader than others. I find, however, that 

 with every increase in the resolving power that I bring to 

 bear, more structure comes into view. 



The elaborate maps and tables of wave-lengths which have 

 been published from time to time are absolutely worthless. 

 I have made a study of this absorption spectrum only in the 

 vicinity of the mercury emission-lines which I have employed 

 for exciting the resonance spectra, but this study has demon- 

 strated what I have for many years suspected, that tables of 

 wave-lengths of spectra of this type are of no more value 

 than a carefully prepared table of the weights of the individual 

 grains in a cupfull of sand. 



I have employed the highest resolving power that has 

 ever been brought to bear upon the spectrum, and it is still 

 not completely resolved. As an illustration of the amount 

 of value to be attached to tables of wave-lengths and maps, 

 I may mention that I have found seven sharp and cleanly 

 resolved absorption-lines of iodine within the green mercury 

 line emitted by the quartz mercury arc. This group of seven 

 lines is represented by a single line only on Has ; elberg's 

 great map of the iodine spectrum. Fourteen absorption- 

 lines were found in one of the yellow mercury lines and 

 twelve in the other. When, therefore, we excite the re- 

 sonance spectra by one or the other of these lines, from 

 seven to fourteen different frequencies may respond to what 

 we might be tempted to call monochromatic excitation. 



It becomes at once clear why a small change in the dis- 

 tribution of the intensity in the exciting line or in its structure 

 may modify the resonance spectrum. 



Excitation by lines which are broad enough to excite a 

 number of adjacent frequencies I have accordingly named 

 multiplex excitation, to distinguish it from excitation by lines 

 so narrow that they cover a single absorption-line only. I 

 have even succeeded in modifying the line structure of the 

 exciting line by the use of ray filters which remove certain 

 frequencies from it : for example, by passing the green light 

 from the mercury arc through bromine vapour before allowing 

 it to enter the iodine vapour, we can produce a change in the 

 resonance spectrum excited by the green mercury line. The 

 changes produced in this way are very interesting, and the 

 same method could without doubt be applied to the study of 

 the resonance spectra of sodium, by filtering the light through 

 iodine or bromine vapour before allowing the selected line 

 to stimulate the vapour of sodium. 



