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XXV. 



ANALYSIS OF THE SPECTEUM OF SODIUM, INCLUDING 

 AN INQUIRY INTO THE TEUE PLACE OF THE LINES 

 THAT HAVE BEEN EEGAEDED AS SATELLITES. By 

 G. JOHNSTONE STONEY, M.A., D.Sc, F.E.S. ; Vice-President, 

 Eoyal Dublin Society. (Plates XVI. and XVII.) 



[Read November 18, 1891.] 



Introduction. 



In analyzing spectra, the following definitions and distinctions will 

 be found of use : — 



y, the jot of time, is the time that light takes to advance one- 

 teuth of a millimetre in the open sether. In the open 

 sether all rays ^ travel this distance in the same time, which 

 is approximately one-third of the twelfthet (one-third of 

 the millionth of the millionth) of a second of time. 



The micro- jot is the millionth part of a jot. 



ji, the air-jot of time, is the time that the ray of mean refran- 

 giUlity takes to advance one-tenth of a millimetre in air. 

 Accordingly ji = jUi/, where jUi is the refractive index of 

 air for the ray of mean refrangibility. If we regard the 

 ray whose wave-length is 5000 tenthet-metres as the mean 

 ray, then fxi is very nearly 1-000280. (See British Associa- 

 tion catalogue of oscillation-frequencies in the B. A. 

 Eeport for 1878.) 



X is the wave-length-in-air of a ray, expressed in tenthet- 

 metres, as determined by Rowland."^ 



1 By a ray of light is meant tlie light producing a line in the spectrum. 



2 A is accordingly about l/eOOOt^' more than the wave-length as determined by 

 Angstrom. 



