1879.] 



On the Spectra of Sodium and Potassium. 



401 



The thirteenth group is a diffuse broad band nearly bisected by the 

 iron line 4325. 



The successive groups become fainter and more diffuse as they are 

 more refrangible, at the same time the distance between successive 

 groups diminishes. Their positions are shown on the accompanying 

 diagram to a scale of wave-lengths. It is worthy of note that every 

 alternate group is much more sharply denned than the others. More- 

 over, it is only the diffuse groups (3) (5) (7) which show reversal 

 except the first group, in the orange, which, however, is more difficult 

 of reversal than the others. The whole series, exclusive of D, looks 

 very like repetitions of the same set of vibrations in a harmonic pro- 

 gression ; the first (visible) term consisting of the six vibrations 

 represented by the orange pair (6160, 6154) and the four lines of 

 group (3) ; the next term of the five lines of the fourth and fifth groups, 

 one of the six vibrations being now too faint to be seen ; the next 

 three terms, of each of which only four lines are visible, consisting of 

 the sixth and seventh, and eighth and ninth, and the tenth and 

 eleventh groups, and the last term of the two faint bands of the 

 twelfth and thirteenth groups. 



Simple harmonic relations can be found to subsist between some of 

 the groups ; for instance, the wave-lengths of the fifth, seventh, and 

 eleventh groups are very nearly as : T \ : yV, but the whole series cannot 

 be represented as simple harmonics of one set of six vibrations with any 

 probability. The smallest numbers, which are nearly proportional to 

 the reciprocals of the wave-lengths of groups (1), (4), (6), (8), (10), 

 (12) are 81, 97, 105, 110, 113, 115; and these numbers are only 

 approximately in the same ratios as the reciprocals of wave- 

 lengths. 



The potassium spectrum as seen in the arc, leaving out of account 

 the two pairs of lines in the red and that in the violet, consists of a 

 series of groups of four lines each, succeeding one another at shorter 

 intervals and becoming fainter as they are more refrangible. They 

 all are more or less diffuse, markedly more so on their less refrangible 

 edges. They are shown on the accompanying diagram to a scale of 

 wave-lengths. Lecoq de Boisbaudran gives a more complete list of 

 potassium lines than other observers, but while the spark which he 

 used brought out several lines which are not seen in the arc, the higher 

 terms of the rhythmical series brought out very distinctly by the arc 

 .are not so well seen in the spark. 



The first and least refrangible group of this series consists of the 

 four lines to which Lecoq de Boisbaudran assigns the wave-lengths 

 5831, 5812, 5801, 5783. The second of these lines (5812) is much 

 less strong than the others as seen in the spark. In the arc fchey are 

 all nearly equal in brightness, but we have not seen the second line 

 reversed. 



