Astronomy. — “The Structure of the Sun's Radiation’. By 
P. H. van Citrert. (Communicated by Prof. W. H. Juuivs). 
(Communicated in the meeting of May 3, 1919). 
It is known that the intensity of the light on the sun’s disc very 
apprecially diminishes from the centre towards the limb, and that 
this takes place in a different degree for the different colours: 
regions near the limb’) of the sun’s dise are distinctly ruddy as 
compared with the centre. 
By the aid of a spectral photometer Voarr*®) determined this 
distribution of intensity for six regions of wavelength in the visible 
part of the spectrum. The decrease of intensity towards the limb 
appeared to be greater for light of shorter wavelength than for light 
of longer wavelength; the diminution, however, does not take place 
quite regularly with the wavelength, but presents an anomaly in 
the neighbourhood of 5000 A. the contrast of the limb to the centre 
is for this wavelength-region less than the contrasts for the other 
wavelength-regions would lead us to expect. 
Voaer’s observations were repeated by Apsor*) in 1906 by the 
aid of a bolometer. ABBor determined the decrease of intensity for 
a great number of wavelength-regions in the infra-red and the 
visible spectrum. He also found a strongly pronounced wavelength- 
effect: the contrast towards the limb increased very greatly towards 
the violet. Also his observations presented an anomaly in the neigh- 
bourhood of 5000 A, though less pronounced than that in Voerr’s 
observations. In fig. 1 we have plotted as functions of the wavelength, 
what value the intensity has for places which are at a distance of 
0.65, 0.825, and 0.95 of the radius of the sun’s dise from the centre, 
when in the centre the intensity is put equal to 100 for all the 
colours. The data are borrowed from Assor’s tables. It is seen that 
the intensity rapidly decreases towards the limb, and this the more 
rapidly as the wavelength is smaller, but at the same time the fact 
1) In the present paper, the phenomena at the limb itself. and even in regions, 
more than e.g. 5%5, of the radius of the disc removed from the centre, are left 
out of consideration. 
4) H. C. Voce, Berl. Ber., 1877. 
3) CG. G. Aspot. Ann. of the Obs. of the Smiths. Inst., 2, 205, 1908; 3, 153, 1913. 
