528 Proceedings of Royal Society of Edinburgh. [sess. 
§ 7. According to Sellmeier’s formula the light transmitted 
through a layer of sodium-vapour (or any transparent substance 
to which the formula is applicable) is the same whatever he the 
thickness of the layer (provided of course that the thickness 
is many times the wave-length). Thus the D x , D 2 lines of the 
spectrum of solar light, which has travelled from the source 
through a hundred kilometres of sodium-vapour in the sun’s 
atmosphere, must be identical in breadth and penumbras with 
those seen in a laboratory experiment in the spectrum of light 
transmitted through half a centimetre or a few centimetres of 
sodium-vapour, of the same density as the densest part of the 
sodium-vapour in the portion of the solar atmosphere traversed 
Fig. 6. 
Values arm 
by the light analysed in any particular observation. The question 
of temperature cannot occur except in so far as the density of 
the vapour, and the clustering in groups of atoms or non-cluster- 
ing (mist or vapour of sodium), are concerned. 
§ 8. A grand inference from the experimental foundation of 
Stokes’ and Kirchhoff’s original idea is that the periods of molec- 
ular vibration are the same to an exceedingly minute degree of 
accuracy through the great differences of range of vibration pre- 
sented in the radiant molecules of an electric spark, electric arc, 
or flame, and in the molecules of a comparatively cool vapour 
or gas giving dark lines in the spectrum of light transmitted 
through it. 
§ 9. It is much to be desired that laboratory experiments be 
made, notwithstanding their extreme difficulty, to determine the 
density and pressure of sodium-vapour through a wide range of 
