Published online by Cambridge University Press: 25 April 2016
The analysis of the Sun by spectroscopic methods refers to an exceedingly thin layer, just above the photosphere, which has a thickness of only a few hundreds of kilometers. The deeper layers are inaccessible to our investigations, because there the matter becomes too opaque. The higher layers of the chromosphere and the corona again are less well known, because they are too transparent, so that deviations from thermodynamic equilibrium conditions may occur. So it is understandable that just the layer where the equilibrium is nearly established must be the only layer which can be fully investigated. Probably this is the best analysed sample of the universe. Whether the composition here found is really representative for the Sun as a whole depends on the importance of the convection, which tends to stir the gases into one homogeneous mixture.