2*2 Alterations ivliich the Light of the Sun undergoes 



On high mountains, at an equal height above the hori- 

 zon, the disk of the sun is constantly seen whiter than iflf 

 valleys and plains. 



In general, the white, yellow, orange, or red colour of 

 the disk depends (if the purity of the air be the same) on 

 the thickness of the strata of air which the ray passes 

 through before reaching the eye of the spectator: the thin- 

 ner the strata which are passed through, the whiter is the 

 disk: aud vice versa ; the disk being at first yellow, then 

 orange, and afterwards red. When the air is filled with 

 exhalations, when it contains solid or liquid substances in 

 suspension, and when its purity is thereby affected, the disk 

 of the sun is sometimes coloured ; but more frequently the 

 intensity of its light diminishes, and the disk remains 

 white*.' 



The cause of the colouring of the sun's disk is one of 

 those problems which ought to occupy the attention of 

 philosopher*, and which Interests them the more in propor- 

 tion to its influence in the phenomena of optics. 



It has been strongly asserted, that this effect was caused 

 bv rays subtracted from the fasciculus during its passage 

 through the air; but we are entirely ignorant what is the 

 number and species of the molecules subtracted. Each of 

 the colours of the disk may result from the separation of 

 one or more coloured molecules : the only condition that 

 ought to be fulfilled, is, that the colours engendered by the 

 subtracted rays should be complementary to those which 

 are perceived. 



The azure colour in which the sky appears to our eyesy 

 has induced some persons to suppose that it was by the 

 subtraction of some blue molecules reflected by the air 

 that the colour of the disk was produced: others have 

 thought that the purple, violet, blue and green rays, being 

 more refrangible and more reflexible than the others, were 

 separated from them in passing through the atmosphere; 

 and that me red, orange and yellow rays combined with 

 them, which had not been reflected, occasioned by their 

 junction the colour of the disk : finally, others have pre- 

 sumed that the violet and green molecule? were reflected 

 by the air at the same time that the red molecules and some 

 green molecules were refracted. 



Although the colour of the sun's disk seemed to an- 

 nounce an action of the air on the coloured molecules ; and* 



* Hist.tfe I ' 'Acad. des Sciences, 1721. De. Maifan relates an observation, 

 of the sun having appeared the whole day through vapours. Its disk wai* 

 white, and its lustre as usual, but without any rays. 



although 



