m 



\ did not however carry out aceiirale measureraentfi about this. They 

 further seemed to me slightly less sharp ihaii tiiose whieli were 

 observed for sodium vapour of low temperature, though this difference 

 was not very conspicuous. If the resolution were really only little 

 dependent on the temperature and possible other factors, we might 

 draw the conclusion from this that the density of the sodium xapour 

 in the chromosphere of the sun is as great as that of saturate sodium 

 vapour of about 270°. 



I further made some obscr\'ations on the emission lines of sodium 

 vajiour. P^or this purpose I used a glass discharge tube which con- 

 tained some sodium, and which was heated to 200° or 300°. 



It then appeared that the emission lines, both D^ and D^, were 

 double, and that the distance of the two light lines increased with 

 increasing temperature. It is not impossible that self-reversal plays 

 a part in this ; it was, however, peculiar in this that chiefly for D^, 

 the two light lines on either side of the dark core differed in intensity, 

 which would not have to be the case for self-reversal. 



With regard to the emission lines /), and D^ Michelson ') has 

 pronounced the opinion that they would each consist of four com- 

 ponents, two intenser ones, and two very faint ones, the distance 



o 



of the intenser ones amounting to about 0.15 A. U. Fabry and Pekot^) 

 are, however, of opinion, that reversal phenomena would play the 

 principal part in this. 



In conclusion I will still state that already a long time ago I 

 observed for the emission lines of copper when this is in the light 

 arc, resolutions of entirely the same order of magnitude as those 

 which I have now described for the absorption lines D^ and D^ of 

 the sodium, also with the aid of the échelon spectroscope. I then 

 observed with a pretty iiigh degree of certainty that these resolutions 

 were greater as the density of the copper in the arc increased. I 

 could not carry out measurements about this, however, as the amount 

 of the resolution was very variable, and besides I had no means (o 

 determine the density of the copper in the arc. 



Haarlem, February- A pril 1914. Physical Labora tori/ 



of "Teylkr's Stichting." 



1) A. A. Michelson and E. W. Morlev. Amer. J. (8) 34. p. 427; 1887. Phil. 

 Mag. (5) 24 p. 463. 1887. 



A. A. Michelson. Rep. Biit. Ass. 1892 p. 170. Phil. Mag. (5) 34 [). 280. 18!. 2. 



2) Ch. Faery and A. Perot. G. R. 130 p. 653. 1900. 



