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Record number: WCP475

Sent by:
John Aitken
Sent to:
Alfred Russel Wallace
19 November 1900

Sent by John Aitken, Ardenlea, Falkirk, Scotland to Alfred Russel Wallace [address not recorded] on 19 November 1900.

Record created:
01 June 2002 by Lucas, Paula J.


Re. light, atmospheric particles and the causes of the blue colour of the sky; 2 ff.

Record contains:

  • letter (1)

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LETTER (WCP475.475)

A typical letter  .

Held by:
Natural History Museum
Finding number:
NHM WP1/8/4
Copyright owner:
Copyright of the John Aitken Literary Estate.

Item notes

Transcription information






19th Mar[ch] 1900

Alfred R. Wallace Esq.

Dear Sir

I very much regret that yours of the 11 inst. 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 and unreasonable, then <names?> is <legion?>, ozone, oxygen, & everything else in the atmosphere which under any conditions some blue have all in turn have been uses by different writers to explain the blue sky.

To me it would appear very evident that the atmosphere is not a blue transparent medium. As you suggest, if it were[,] the moon & distant clouds would appear blue instead of white. Distant landscaped 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 the atmosphere overhead. Then again the sun at sunrise & sunset is much redder than at midday showing that the action of the air is to throw out or in someway destroy the blue & only allow the red rays to pass more freely. The fact that the blue light of the sky is polarized points to the colour being due to the sun’s light being reflected by something in the atmspehere, & 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. We must have some light reflecting surface to reflect the light through the medium to the eye.

[[5]] Though the evidence all seems to point to the blue of the sky being caused by selection 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, as least being described as such by those who have examined it, which examinations 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 to the depth of its colour[.] But supposing we 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 Aitken1 [signature]


1. Aitken, John, (1839-1919); Meteorologist

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