156 



domain of the red lithium line in the second order, the distance oi 

 the components of the natural lithium doublet (6708) was now 

 measured and found to be 0,144 Angstroms. We can be sure only 

 of the second figure. 



A reproduction of the lithium doublet is given in Fig. 1. of the 

 Plate. Fig. 2 is considered in the communication, immediately following. 

 It exhibits the outer components of the magnetically divided i)-lines 

 in the inverse effect, but it may be used now to give an idea of 

 the closeness of the lithium doublet, the two narrow sodium lines 

 due to the arc-light being also present. As the sodium- as well 

 as the lithium-doublet has been photographed in the second order, 

 the scale value can be derived from the first mentioned pair. 



Rydberg, Kayser and Runge in their researches on spectrum series 

 have emphasised that the distances between the components of pair 

 lines in a group of chemically related elements increase with the 

 atomic weights. With sodium, potassium, rubidium and cesium, the 

 atomic weight is approximately proportional to the square root of 

 the distances between the pair lines. 



The ratio of the difference I'l between the frequencies of the 

 components of the pair lines and the square of the atomic weight ft 

 should therefore be a constant. According to our observation the 

 difference of the frequencies for the lithium doublet is 0.32. 



RiTZ ^) came to the conclusion, a result to which my attention 

 has been kindly drawn by Dr. Ladenburg, that for related elements 

 the difference of the constants p^ and p, <^'f Ritz's spectrum formula 

 divided by the square of the atomic weight is even more nearly 



constant than the ratio — . 



f* 

 Taking for Li') p=—O.OA.lbiO—p„ I calculate p, = — 0.047521. 



We thus find for the alkalis: 



U Na K Rh CS 



^!lJ 3) 6.53 32.3 37.8 32.3 31.6 



II- 



iP2—Pi) • IQ' Q_o22 0.142 0.189 0.180 0.186 



It appears that, if the data for lithium be included, there is no 

 question about a constancy of the proposed ratios in the group of 

 the alkalis. 



1) RiTz. Oeuvres. p. 145. 



2) RiTz. 1. c. p. 57. 



8) The last four numbers according to Rydberg. Rapports, etc. T. II. p. 219. 

 Paris. 1900. 



