1881.] 



Capt. Abney. On the Spectrum of Sodium. 



443 



tend to introduce greater or less errors. For example, the flow in the 

 second medium must have been somewhat disturbed by the circum- 

 stance that the non-conducting walls of the intermediate cell were 

 parallel to the direction of the incident, not refracted flow, which may 

 possibly have disturbed the course even as far as the middle of the 

 cell. Again, there would have been a much larger resistance in the 

 porous wall than in a stratum of equal thickness of one of the electro- 

 lytes, and the thickness or porosity of the wall, or the proportion of 

 the two electrolytes imbibed by it, may have varied somewhat in a 

 lateral direction. The numbers obtained cannot therefore be deemed 

 sufficient to decide between two such laws as that of sines and that of 

 tangents. In case of the second medium being the better conductor, 

 it is evident that the law of sines would lead to extravagant results, as 

 there can be no such thing as total internal reflection. The alteration 

 in the direction of the equipotential surfaces, and, therefore, of the lines 

 of flow, in passing from one metal into another of different conducting 

 power, has already been investigated experimentally by Quincke, and 

 the results of experiment compared with theory. (" Pogg. Ann.," 

 vol. xcvii [1856], p. 382.) — Gr. Gr. S., 14 June, 1881.] 



XX. " Note on the Spectrum of Sodium." By Captain W. de 

 W. Abney, R.E., F.R.S. Received June 14, 1881. 



On examining the spectra of different metals, there is one point 

 which is striking in the extreme, viz., the absence of any very marked 

 lines in the region between \ 7000 and X 7600, which latter number 

 we may take as the visible limit of the spectrum. With the exception 

 of the well-known pair of lines of potassium, I am not aware that any 

 lines in metallic spectra, which have been carefully studied, have been 

 found below this limit, though recently, in the spectra of some of the 

 rarer earths, I believe some few lines have been recorded. 



Having photographed the emission spectra produced in the arc of 

 several metals, it appears, so far as examination has been made, that 

 only those which can be. volatilised at a low temperature have any 

 lines in the infra-red region. Sodium is an example of this. It has 

 a pair of lines at wave-length of about 8187 and 8199 of an intensity 

 of about 3, taking the intensity of D lines as 10. It will be noted 

 that the difference in wave-length between this pair is greater than 

 that of the D lines. They do not seem to have any corresponding 

 dark lines in the solar spectrum, though there are three faint lines 

 which lie close to these wave-lengths. 



In the calcium spectrum there is a pair of very faint lines which 

 lie between \ 8500 and 8600. Their exact wave-lengths have not at 

 present been determined. 



VOL. xxxii. 2 u 



