292 Miss Marian Baxter on the Dispersion 



crossed by a number of horizontal interference bands, which 

 as the temperature rises show very beautifully the form of 

 the dispersion curve between the first two pairs of lines 

 of the principal series of potassium. The two lines in each 

 or! these pairs are so close together that for a first approxi- 

 mation the pair of lines may be treated as a single line, and 

 the mean wave-lengths for the two pairs taken as 7675 

 and 4045. In order to measure wave-lengths, a piece of 

 iron wire burnt in the arc gave the iron spectrum super- 

 posed on the potassium absorption spectrum. A series of 

 photographs was taken of the fringes at successive steady 

 temperatures, and all measurements were made from the 

 negatives obtained. 



The investigations which have been so far made as to the 

 behaviour of sodium vapour under these conditions appear 

 to show that at first, though the temperature at the middle 

 of the tube is higher than at the ends of the heating spiral , 

 the variation of density which results only occurs parallel to 

 the axis of the tube and not perpendicular to it, and that at 

 the ends of the vapour column distillation goes on very 

 slowly and quietly. The conditions are sufficiently steady 

 over any plane perpendicular to the axis to allow of perfectly 

 good definition in the interference fringes. 



The behaviour of potassium vapour in this respect was 

 found to be precisely similar. But, as in the case of sodium, 

 the range of temperature over which photographs can be 

 obtained is restricted by the fact that when the temperature 

 produced by the heating spiral becomes considerable distil- 

 lation occurs more and more quickly, and the vapour density 

 varies across any plane perpendicular to the axis. The 

 definition of the fringes is thus lost. 



This occurred as nearly as I could judge at a temperature 

 of about 410° C. It w;is impossible, therefore, to take photo- 

 graphs above this temperature. The highest temperature 

 for which measurements were obtained corresponds, however, 

 to considerable dispersion; the fringes near the blue absorp- 

 tion line were almost vertical, and those in the immediate 

 neighbourhood of the line too close together to be resolved 

 by the spectroscope. Near the red line the fringes were 

 entirely obscured for some distance (6000-6800). This is 

 the absorption region originally shown by Carter to consist 

 of a very large number of fine lines. The portion of the 

 fringes which was at first visible between this region and 

 the red line disappeared at an earlier stage. The tempera- 

 ture range is thus much greater than in the previous 

 measurements with potassium by the crossed prisms method. 



