156 



NATURE 



\June 6, 1878 



striking result ; that the absence of any rain or deposit of 

 any kind must not be left out of account is clear. The 

 air in the side valleys of Cashmere, although rich in 

 vegetation, is particularly transparent. Strange enough, 

 the principal valley of Cashmere, i.e., the valley of the 

 Jehlum, is generally hazy, although there is a good deal 

 of rain. I have seen the planet Mars look almost white ; 

 Jupiter and the other stars at that time had a bluish tint." 



I have been anxious to give these extracts not only 

 because they form a valuable contribution to science, but 

 because we see here the student of science doing what an 

 artist is generally supposed to do, namely, interesting 

 himself in the colouring of natural objects, and I cannot 

 omit pointing this remark with the statement of my 

 belief that when the artist attacks these also from the 

 scientific point of view as well as the artistic one, his eye 

 will lose nothing of its keenness, and his interest in the 

 glory of nature will be nothing the less. 



Let us consider, then, the action of those molecules 

 which absorb the blue light. 



Now since these molecules absorb blue light we know 

 that they will reflect blue light, and, practically speaking, 

 nothing else. Here, then, we have the cause for the 

 blue colour of the sky. 



Those who are familiar with the brilliant researches of 

 Dr. Tyndall on the action of light upon vapours will 

 recollect that he also has arrived at a somewhat similar 

 conclusion from a different line of reasoning and a 

 different method of experimentation. 



To return one moment to oxygen and nitrogen, the 

 gaseous constituents in our atmosphere, I must here 

 remark that we have no evidence that the pure gases 

 in our air change their molecular constitution; but we 

 know that the aqueous vapour does to an enormous 

 extent, and there is one state to which at present 

 no reference has been made. There is a condition 

 of aqueous vapour which is competent to absorb white 

 light without giving rise to any coloured phenomena ; 

 this is the form of which mist and clouds are built 

 up ; why they are so dazzlingly white in the sunshine ; 

 why we have a dark grey day absolutely devoid of 

 colour when a pall of cloud hangs over the whole sky. 

 In addition to this we know also not only that condi- 

 tion to which I hare already referred, which absorbs in 

 the blue, but certainly of one, and in all probability two 

 which absorb in the red. One of these absorptions indi- 

 cates that the form of vapour which produces it is of the 

 most delicate kind, while that which gives us the continuous 

 absorption in the red end is perhaps the last stage reached 

 before clouds are formed. If this be so, the very complex 

 nature of the true cause of sky colour will be obvious. 

 We have three molecular colour-giving states to contend 

 with, and the action of these will depend largely upon 

 the thickness of aqueous vapour traversed by the sun- 

 light. A diagram will at once explain how the action of 

 these different thicknesses is brought about. 



In the diagram. Fig. 3, we have a section of a part of 

 the earth and its atmosphere, supposing the latter to be 



somewhere between forty and fifty miles thick. On the 

 assumption that the aqueous vapour, which, as I have 

 shown, is the effective absorber in the air, is equally 

 distributed, let us see how the question of thickness of 

 absorbing layers comes in. Take an observer at 0, sup- 

 posing him in the tropics, and that he sees the sun over- 

 head at ^/notice the distance a, which represents the 

 thickness of air traversed when the sun is overhead, and 

 compare it with x, when the sun is rising or setting 

 as at X, and when, therefore, the greatest thickness is 

 traversed, taking no account of refraction. 



The whiteness of the sun at a high altitude and the red- 

 ness of it when rising or setting is associated then with 

 the fact that at these times the light traverses the least 

 and greatest thickness of the atmosphere respectively ; an 

 intermediate height of the sun is represented at F, and 

 obviously the distance oy will vary for intermediate 

 altitudes from a to ox. 



Now the thinner we make our atmosphere the greater 

 will be the difference of the thicknesses a and x, and, 

 as a matter of fact, the effective aqueous vapour lies very 

 low down ; so that the differences will be greater when 

 we consider the aqueous vapour alone than when we 

 consider the Avhole atmosphere. The thickness of the 

 aqueous vapour, therefore, increasing from a to x, let 

 us take the case of a perfectly clear sky at sunset ; the 

 white light reflected, as I have already shown, in con- 

 junction with the blue, will find itself most absorbed in 

 the line ^ ;r, least absorbed in the Xvao. a. In the line 

 X we get everything absorbed but the red ; we get, 

 therefore, a red sky. A little higher everything is ab- 

 sorbed but the red, orange, and yellow ; this will produce 

 a rich golden colour above the red ; higher still, the 

 green and part of the blue is allowed to pass ; in fact, 



only the extreme blue is absorbed; and, as I stated 

 before, when I referred to the absorption of chlorine gas 

 in a tube, the residual light will be green. Above the 

 green we have the blue. 



This is the order of the colours of the sky ; the sun in 

 consequence of its greater brilliancy can overcome this 

 absorption until it has reached a very extreme limit, 

 sunset clouds lighted up by the sun, therefore, mtist put 

 on the colour of the sun, because the light which has 

 reached our eye is red light, which has travelled to us 

 vi& the cloud, hence the green is limited to a band of 

 sky, between the gold and the blue, a green cloud is im- 

 possible, and it is on this ground that I ventured to 

 criticise Mr. Ellis's picture, "The Last of the Wreck," 

 555. Mr. Ellis has painted green clouds ; I am certain 

 he never saw one in his life ; for a similar reason I have 

 objected to Mr. Oakes's picture, "The Dee Sands." Sky 

 colour is begotten by a low sun. 



I do not think that after what I have said it is 

 necessary to point out how it comes that the blue 

 clouds which Mr. Thornburn has chosen to paint are 

 also impossible ; a cloud can only be of a colour which 

 is got from the sun directly or indirectly. Now a blue 

 sun is possible, but clouds illuminated by a blue sun are 

 impossible in a picture, because for the sun to be blue 

 there must be nothing but a thick veil of mist. 



I have drawn another diagram, which, although it looks 

 rather complicated, may, I think, be rendered clear by a 

 short description. The object I have had in view has been 

 to show how the colours of the sky may be complicated after 

 sunset. I believe in three pictures of sunrises or sunsets out 

 of four, the phenomena presented have really been 

 observed after sunset, in fact, in most pictures of sunset, 

 the sun is a little too slow, we get sunset colour too soon. 



