727 



between the two absorption niaxiina. If the al)soi'plion maxima 

 coincided with maxima of thiorescence, the former woiiid either not 

 shift their places, or they would split up. Hence we arrive at the 

 conclusion that at temperatures above about 260" the maxima of 

 absorption do not coincide with maxima of fluorescence, but that 

 the latter maxima, resp. maximum, lie between the maxima of 

 absorption. 



I have indicated two curves in the figure for which the influence 

 of the fluoi-escence light is visible, the curves {D^)' and {DJ. They 

 represent tiie distance of the components of Z)i and Z).^ for a radiated 

 thickness of layer of 40 m.m., in which (he tube of sodium vapour 

 was placed before the entrance slit of the spectrometer: the distance 

 from the tube to the slit was about 10 cm., the opening of the 

 incident beam being about 1 : 10. Under these circumstances it was 

 also possible to observe the fluorescence light by means of a spectroscope. 



If the tube with sodium vapour was placed between object glass 

 and eye glass of the reading glass, the thiorescence can only be 

 brought about by the light that belongs to the narrow spectral range, 

 issuing from the spectrometer, instead of through the undivided 

 white light. It is easy to see that the part of the fluorescence 

 light, which in this case is already very faint, the part that 

 finally reaches the retina, will have to be exceedingly small 

 in comparison with the observed light; the influence of the 

 fluorescence light will, therefore, have to be imperceptible then. I 

 have actually convinced myself that when the tube with vapour is 

 placed between object glass and eye glass of the reading glass the 

 distance of the absorption maxima is independent of the thickness 

 of layer traversed by the rays. For this purpose I compared a tube 

 of 50 m.m. bore with the tube of 28 m.m. used for the measurements ; 

 in these two cases the distance of the components was the same at 

 the same temperature. 



I think myself justified in drawing the conclusion from all that 

 has been observed that the distance of the absorption maxima of 

 the sodium lines is closely connected with the density of the vapour, 

 and that in this way that the splitting up increases with increasing 

 density. That what has been observed is chiefly an effect of density, 

 and not in the first place a temperature effect, is supported by the 

 fact that the influence of the soda mist blown into the arc is for 

 the greater part the same as that of absorbing sodium vapour of 

 much lower temperature. The temperature at which the absorption 

 in the arc takes place, will namely undoubtedly be much higher 

 than that of the vapour in the glass tube. On the other hand the 



