WCP475

[1]^{1, 2}

Ardenlea

Falkirk

19^th Nov[embe]^r 1900

Alfred R. Wallace Esq^r.

Dear Sir

I very much regret that yours of the 11^th ins^t. should have remained so long unanswered owing partly to my absence from home & many things requiring attention on return[.]

So far as I know there is no complete or satisfactory account of the blue colour of the sky in any book or [2] paper that I know of. Of suggestions, reasonable & unreasonable, their name is legion, Ozone, Oxygen, & everything else in the atmosphere which under any conditions gave blue have all in turn have been used by different writers to explain the blue sky.

To me ^it appears very evident that the atmosphere is not a blue transparent medium<.>³ As you suggest, if it were <,> the moon & distant clouds would appear blue instead of white. Distant landscapes would also always be deeply coloured blue. A mountain at a distance [3] of 40 or 50 miles ought to look intensely blue as between it and the eye there will be two or three times the amount of air there is [in] ^the atmosphere overhead. Then again the sun at sunrise & sunset is much redder than at mid-day, showing that the action of the air is to throw out or in some way destroy the blue & only allow the red rays to pass more freely[.]

The fact that the blue light of the sky is polarized⁴ points to the colour being due to the sun's light being reflected⁵ by something in the atmosphere, & as there is always a great number of [4] small particles floating in the atmosphere they probably play an important part in the phenomenon. Whether gaseous molecules play any part in the reflection I cannot say but think unlikely[.]

If we were to accept the idea that the atmosphere was a blue transparent medium we would still require something more to explain the blue sky. A blue medium in front of a black background would be black.⁶ We must have some light reflecting surface behind the blue medium to reflect the light through the medium to the eye.

[5]⁷ Though the evidence all seems⁸ to point ^to the blue of the sky being caused by selective reflection & not to selective absorption. Yet that does not entirely exclude the idea that air may be a blue transparent medium, but if it is so its powers of absorption must be feeble otherwise the red rays would be unable to pass through so freely as they do[.]

I have never seen the apparatus for producing liquid air so cannot answer your question about precautions taken [6] to exclude the fine dust particles, but will make enquiries.

It is probable that liquid air being a blue transparent medium, at least being described as such by those who have examined it <,> which ^examination we may presume was conducted in such a way as to prove it transmitted blue light most freely. If then, air, when liquid [,] is a blue transparent medium[,] it is probable that it will have similar properties when gaseous. But how far this blueness, supposing such to exist, will affect the question I am unable to say[,] never having seen [7] liquid air so cannot speak of the depth of its colour[.]

But supposing one knew the depth of its colour I don’t know that it would help one much as I don’t know whether there is any definite relation between the depth of colour of a liquid & ^of its gas.

I am sorry I am unable to give you more satisfactory information on this most interesting subject. If there is any point I have not put clearly in the above I hope you will let me know that I may do my best to assist you[.]

Yours truly | John Aitken [signature]