and the Resonance Radiation of Electrons. 525 



letters at the left-hand side enable the spectra to be 

 identified with the diagrams. 



The line at which the excitation occurs is indicated by a 

 double-headed arrow. This line always appears in the fluo- 

 rescent spectrum. For example, in spectrum B the excitation 

 is at wave-length 4962. In this case three lines of shorter 

 wave-length appear, as well as the line at 50, and the lines 

 at 50-4 and 5071, which belong to the groups of lines or 

 bands already referred to, indicated by brackets. In 

 spectrum C we are stimulating the vapour at 4935, and we 

 obtain the second line in each of the first two groups. In 

 spectrum D we stimulate another member of the same series, 

 and also get the second line in the first two groups ; while 

 immediately above (E) by stimulating 4866 we obtain the 

 third line in the second and third groups, this line being- 

 wanting in the first group, even when the vapour is excited 

 by white light. In spectrum F, with a stimulation at 4839, 

 we get another set of lines. The cause of the shift of. the 

 bands previously observed is at once apparent, the individual 

 lines belonging to different groups of electrons. The number 

 of groups has not yet been ascertained, but the photographs 

 indicate that there are at least four, and probably more. It 

 has not yet been possible to study the minute details which 

 accompany the appearance or disappearance of a line, which 

 are even more interesting, but another year's work on the 

 subject will doubtless yield fruitful results. Longer expo- 

 sures will doubtless enable these relations to be traced 

 throughout a wider range of wave-lengths, but from what is 

 already known it is clear that the shift in the apparent 

 position of a group of lines results from a change in the dis- 

 tribution of the intensity among the individual lines consti- 

 tuting the group. 



These experiments show in a striking manner the com- 

 plexity of the piece of machinery which we will call the 

 sodium molecule. Professor Rowland once said that a mole- 

 cule is much more complicated than a piano. In most cases, 

 all that we have been able to do is to strike the entire key- 

 board at once, but in the case of sodium it seems possible to 

 strike one key at a time. A study of the fluorescent spectra 

 of other vapours will doubtless do much to clear up the 

 mystery of the mechanism of molecular radiation. 



These investigations have been made possible through 

 substantial aid received from the Rumford Fund of the 

 American Academy, before which the paper has been pre- 

 sented. Much assistance has been rendered by Mr. A. H. 

 Pf'und, whose services were secured though a grant made by 

 the Carnegie Institute. 



Baltimore, June 1905. 



