76 
quite honeycombed with irregular gradients of such average 
magnitude, would, as Jurist) has demonstrated, refract, deflect 
and disperse the rays of light that penetrate there, so strongly 
to all sides, that the gas would present itself to a distant spectator 
as a turbid medium; the volume parts in which the density can 
be considered as constant, and hence the light as rectilinear, would 
be too small to be observed separately at such a distance. Since 
the degree of refractional scattering is determined by (m—1), and 
accordingly varies comparatively slowly with 4, the co-operation of 
this kind of scattering with the molecular scattering will weaken 
the mean wavelength effect, peculiar to the latter. 
Besides, the diminution of the intensity from the centre towards 
the limb will be greater than would be the case if only diffusion 
were the cause of this diminution. Irregular refraction, therefore, 
lessens the difference between the rates of darkening towards the 
limb shown by tbe different wavelengths, but at the same time 
strengthens the average contrast between limb and centre. 
As another possible cause of the fact that the observed wavelength- 
effect is slighter than the theoretical, SPIJKERBOER suggests that the 
light of the longer wavelengths, as it gets less greatly weakened by 
diffusion, might come to us from deeper layers of the sun than the 
light of shorter wavelength. This supposition, evidently, excludes 
the older hypothesis that the sun would have to be conceived as a 
well-defined core surrounded by a sharply defined atmosphere. The 
idea that the various radiations originate at different levels is more 
in agreement with the conception of the sun as a glowing gas- mass, 
of which the density and the temperature gradually diminish towards 
the outside. Since the light of greater wavelengths is much less 
weakened than that of shorter wavelengths, it will come to us 
from deeper layers of the sun. The infra-red light will, accordingly, 
be relatively more weakened by molecular scattering than would 
be the case if it only came to us from equally deep layers as the 
violet light. 
From what depth the light comes to us is, however, not only 
determined by the diffusion, but also by the irregular refraction. 
As this is little dependent on the wavelength, the differences in 
depth for the different colours will not be so great as in the case 
that only diffusion played a part. If, as is very probable, irregular 
refraction plays the principal part, especially in the lower layers, 
the difference in depth must be comparatively slight. 
1!) W. H, Juuius, These Proc. 16, 264, 1913. 
