77 
Let us now consider the question how, seen from this point of 
view, the radiation emitted by the sun in a direction w (fig. 2) 
must be composed. The outer layers of the sun will emit very little 
energy; proceeding towards lower layers the quantity of emitted 
energy increases, at first slowly, then more rapidly, in consequence 
of the increase of temperature and density. Let us suppose def to 
be the layer outside which no appreciable quantity of energy is 
Fig. 2. 
emitted. Let us first consider the radiation transmitted by the surface 
def in the direction w. We shall take the scattering in the atmos- 
phere outside de f later into account. From the region e, in the centre 
of the sun’s disc w receives a comparatively slight quantity of 
radiation of comparatively low temperature, from e, a larger quan- 
tity of higher temperature, etc. At first the quantity of energy which 
w receives from the different depths, will increase; the deeper we 
get, however, the more will the emitted radiation be weakened by 
absorption, refractional scattering and diffusion. From e, e.g. w will 
again receive less radiation than from e,, but of higher tempera- 
ture, from e, very little, etc. Let us suppose that w does not receive 
an appreciable quantity of energy from layers lying deeper than ). 
