520 Prof. Wood on the Fluorescence of Sodium Vapour 



greatest in the region between \ = 515 and \ = 545, where it 

 may amount to the full width of the band or 25 A.E. The 

 bands in the vicinity of \ = 555 remain fixed in position, as 

 is probably true of all the less refrangible ones. It was at 

 first thought chat the head of the group remained fixed, the 

 group opening out, or expanding so to speak, but this appears 

 not to be the case ; and the law governing the motion of these 

 bands, if one exists, cannot be determined until a larger 

 number of photographs have been obtained. The probable 

 cause of the motion will appear presently. The wave-lengths 

 of the lines which collectively make up the fluted bands were 

 determined from the calibration curve of the spectrograph, 

 and are recorded in the following table. It was at first 

 thought that they would obey the law of constant second 

 differences, but such was not the case. It has not yet been 

 possible to measure the lines above X = 555, though the bands 

 extend to 571. 



Wave-lengths of bright lines in Fluorescence Spectrum of 

 Na Vapour. 



Faint 



5553 



5315 





5040 



48 



09 



34 



43-5 



02 



25 



39 



5296 



5000 



35 



89 



4995 



31 



83 



88 



14 



76 



So 



10 



70 



04 



05 



64 \ 



59 



5499 



58 \ 



34 



95 



r>2 \ 



25 



91 



4(5 





12 



86 



39 





04 



81 



32 





4895 



77 



26 





85 



62 



19 





75 



58 



13 



<>7 



53 



06 / 



60 



47 



Faint 52/ 



50 



42-5 



5194 \ 



39 



37 



88 \ 



21 



31 



80 



12 



26 



73 



03 



21 



06 / 



4795 



5399 ■ 



Faint 59 ' 



85 



93 ■ 



49 \ 



78 



87 \ 



41 \ 



09 



81 : 



34 



59 



76 i 



27 



53 



70 f 



20 / 



13 / 



43 



64 



39 



59 ••' 



03 33 



t 51 



5096 



28 



44 



88 



16 



38 



80 



04 



32 



72 4695 



27 



49 84 



21 









