WCP474

[1]

ARDENLEA

FALKIRK

17 April 99

Alfred R. Wallace Esq^r.

Dear Sir

I have yours of the 13th inst. and have read the article by Tyndall¹ in Vol. II of "Nature."² I am sorry I do not possess Vol. I³ so have not seen the other paper you refer to.

I have never got any results like Tyndall's when sending a beam of light through water, possibly because I do not possess the electric light [2] that he would get some effect of the kind described I should quite expect. Indeed in my complete paper I state that there will probably a small amount of du blue due to the selective reflection from any small particles in suspension in the water, but it is evident that the amount of blue from this source can be but slight & Tyndall admits this when he says "the light transmitted by a column of water would be yellow, orange, or red," though at the beginning of the same paragraph he claims "the small "particles…. demonstrably sufficient "to produce the blueness referred to. " Then If the chief cause was the blue from the very small [3] particles, the colour of water might to show brightest when looked at at right angles to the incident light through a nicol prism & be quenched when looked at with the prism turned at right angles – I dont know of any test such as this ever having been made, I think I tried it but yet no satisfactory results, thought I was working at the time at the polarized light of the sky then.

As I have said the amount of this polarized blue light must be small otherwise the transmitted lights would be ^the complimentary colour. To illustrate this point I have described in my complete paper some experiments made in water in which was intentionally introduce a large quantity of very small particles [4] so small that they only reflected the blue waves & the light water was made distinctly blue by this presence. But on testing this water by means of coloured surfaces the following was the result. The whole, as it sunk, gradually became yellow. The yellow surface changed to orange, while the purple changed to red. Now when these colours are sunk in water the white changes to blue, the yellow to green, & the purple to blue, It is therefore very evident that the amount of blue due to selective reflection in water can be but very slight.

In Tyndall's experiments the water had been kept for some time in bottles & that [5] (5) may to some extent have altered it, but the most difficult part of such an experiment to estimate what proportion this selectively reflected light bears to the total, with a brilliant light & the exp^t made in a dark room one is inclined to overestimate its importance. While I have admitted its existence, still, for the reasons given, I do not think the amount is of [6] much importance.

There is one thing certain which is that no green water can be have its colour, due to the selective reflection. We can understand how the larger particles gradually fall out till at last only the smaller remain, causing the water to change from whitish, that is colourless, to blue; but we can't imagine any process by which only the intermediate sign of particles required to reflect the green could be [7] retained whilst the smaller ones fell.

I may remark that Tyndalls statement that "In no respects could I discover the blue of the water was different from that of the firmament" is evidently much too strong & very misleading. He knew perfectly well the blue of the sky is entirely due to selective reflection, whereas & he also knew that only part of the blue of water was due to this cause & partly to the "blue arising from true molecular absorption." When he wrote the above sentence I expect his mind was concentrated on the polarisation phenomena & not on the general question.

I have turned up Tyndalls paper [8] on Dust and Haze in one of his books. We now know that he was entirely wrong in his deductions from these exp^ts The night temp. does not destroy the dust motes, it simply breaks them up into very small non reflecting particles. He got a blue haze when the air entered somewhat quickly. No particles if very slow^ly & little effect if quick. At the medium rate it looks as if the organic matter had only been half burned & vapours formed which caused a kind of blue smoke when condensed in the apparatus. When it went in very slowly all the particles were burned & a vast increase in the numbers resulted, & if the particles had been of thin [9] (9) right kind & size he might have got a fine blue[.]

I do not think you will anywhere find an exp^t in which air alone shows ^{a decided} blue. Indeed it seems quite impossible that it can, otherwise all the exp^t in which blue hazes are produced would require to be made in some gas other than air, & Tyndall says that when the air that passed slowly in over the red hot platinum [10] shows the tube to be optically empty[.]

I have glanced over Tyndall paper on "The blue colour of the sky" but did not notice any result such as you describe. He used vapours in this exp^ts to get his blue reflected polarized light which he compared with the blue of the sky[.]

Yours truly | John Aitken⁴ [signature]