Evening Glow and analogous Phenomena. 281 



or liquid (water droplets or extremely minute water vesicles, as 

 they are probably formed by incipient condensation of aqueous va- 

 pour) . When the sun is high above the horizon, the groups of 

 small screens which can be formed at right angles to its rays are 

 not numerous enough to produce a perceptible action. But when 

 the sun is near the horizon, its rays must traverse a sufficiently 

 long path of the lower layers of the atmosphere (which are just 

 those which contain such small corpuscles) to experience to an 

 appreciable extent the diffracting action of the groups of screens 

 which those particles can form. Hence each point of the sun 

 must thereby appear reddish and surrounded by more strongly 

 reddened diffracted light; for now, the red halos of adjacent 

 points becoming superimposed, with the direct light of each point 

 of the sun the diffracted light of the adjacent points will be as- 

 sociated and thereby, again, deepen its redness. Hence with a 

 luminous surface the red colour must be more strikingly conspicuous 

 than with an isolated point of light. While the luminous disks 

 of the sun and of the moon shine on the horizon with a splen- 

 did orange-red, the reddish colour of rising and setting fixed 

 stars is scarcely noticed. Distant white surfaces like the gla- 

 ciers and the fields of snow of the Alps, and clouds near the 

 horizon, when illuminated by the setting sun, often show a 

 purplish-red colour, while a white wall in the neighbourhood 

 only appears of an orange-red. The light reflected from the 

 former must, before reaching our eyes, traverse a sufficiently thick 

 layer of air to experience once more a diffracting action from 

 the particles suspended in it. It has been hitherto left unde- 

 cided whether solid corpuscles or fine vesicles of water exert the 

 diffracting action. The solid particles of dust have doubtless a 

 considerable part in it; but assuming, with Forbes, that it 

 belongs mainly to aqueous vapour at the commencement of its 

 condensation (that is, when it, though previously quite gaseous, 

 begins to deposit liquid water in the form of extremely fine ve- 

 sicles or droplets), our theory differs from his merely in explaining 

 from the nature of light the property which aqueous vapours in the 

 act of condensing have, of transmitting red rays more freely, 

 and in not ascribing it to aqueous vapour alone, but equally to 

 any other medium made turbid by fine particles. From the 

 present theory, therefore, may be deduced the same conclusions 

 as from that of Forbes — for example, as regards the meteorolo- 

 gical significance of the redness of morning and evening. 



Moreover it seems to me that the redness of morning and of 

 evening is only to be ascribed to the action of aqueous vapour in 

 the atmosphere when it appears especially brilliant and the 

 whole morning or evening sky is of a fiery glow. If, on the 

 contrary, the rising or setting sun simply appears as a reddish 



Phil. Mag. S. 4. Vol. 34. No. 230. Oct. 1867. U 



