WCP473

[1]¹

Ardenlea

Falkirk

7 April [18]99

Alfred Russel Wallace Esq.

Dear Sir

I have yours of the 1^st inst[ance].

I rather think your doubt in accepting the idea that water is a blue medium and not coloured by reflected light, are founded on a misunderstanding of Tyndall's experiments with air. I have no recollection of any result as you have indicated [2] in any of his experiments. In all the cases I can remember in which blue reflected light was produced in air; the particles scattering the light were intentionally manufactured in the tube, generally out of some vaperer [sic] which he caused to condense in the tube. Pure air under the conditions you mention would pass the ray of light without reflecting any of it. I would very much like if you could give me a reference to the experiment to which you refer, as if you are correct one would require to [3] reconsider the whole matter.

I may mention in reference to the second part your letter that a beam of light passes through without scattering blue light. A very little light is scattered, but very little & not blue, unless at the far end of the tube where it is bluish. This scattered light is due to the presence of floating particles [but] excessively small ^ones from which it seems to be almost impossible to free water however carefully it is prepared. The light from these particles near the source of liquid is white & at the end [4] of the tubes furthest from the light — if the tube be long — the reflected light is slightly blue owing to the beam of light becoming coloured by the time it gets to the far end of the tube[.]

If the blue of water was caused by the selective reflection of very small particles in suspension, then it follows that light in passing through such a medium would become poorer in the light of the blue end of the spectrum and any light transmitted [5] would be deficient in blue & would look therefore yellowish. Since the light transmitted by water is blue it is clear that the blue rays have not been scattered by very small particles.

In the air the very small particles reflect the blue rays, & the light that passes through the atmosphere is deficient in that colour [6] & is more or less yellow. & if the particles be very numerous, as at sunrise & sunset where the rays have to pass a greater thickness of air, the colour of the transmitted light is nearly red. It is well known that our atmosphere robs the sin's light of much of its blue & that the sun appears almost bluish when [7] seen at greater elevations that is through less air[.]

Again all distant lights on the surface of the earth of which we know & are become yellower as they receed [sic] into the distance; Whereas the more water a light is seen through the bluer it becomes[.] The one absorbs the one end of the spectrum the other absorbs the other end. Kindly let me know if I have made my meaning clear.

Yours truly | John Aitken [signature]

[8] See Tyndall — in Nature

vol. I. p.340²

'' II p. 489-904³

colours of water and air