May 1st 1834
My Dear Sir,
Now that you have mentioned the exact phenomenon which you have observed, I can give you the precise theory of it, and point out to you many analogous facts. I consider Biots <1> observations which you quote as erroneous in principle, for it is impossible that a white of the first order is exhibited in a crystal like Sulphate of Lime in which there is no deviation, or next to none, from the Newtonian tints can have total darkness or blackness for its complementary colour. In bright lights there must be some complementary remaining as the white of the first colour can never contain all the difft colours in the exact proportion which composes a pure white. Now this residual colour is in your experiment absorbed by the peculiar
of colour of the crystal if it is a crystal with one axis and with the regular Newtonian tints; or if it is a crystal with two axes the complementary black may arise either by absorption, or what is more likely from the unbalanced disparate action of the two axes, on the different rays of light which may produce a much more pure white of the 1st order than is done in uniaxal crystals.
In certain specimens of apophyllite, too such as those whh Sir J. Herschel <2> calls Leucocyclite, the polarising action upon all the rays of ye spectrum is so nearly the same, that the Rings are a succession of Black & White circles; so that with this mineral your phenomenon is produced
is produced not only at one but at many thicknesses; and at every thickness which gives the maximum tint of the White Ring your phenomenon must be observed in perfection.
Your experiment on the Double Refraction of Indurated Gum deserves to be followed out. The images may arise from inequal density, & not from double refraction, and I have often been deceived in appearances of this kind. You will have no difficulty however in determining whether they are composed of light circularly polarised, or of ordinary light. I think I have somewhere mentioned having observed traces of circular polarisation in Amber, which renders your result the more probable; but if you are able to separate the two circularly polarised pencils <3> the discovery will be a very curious one which no other person so far as I know has anticipated. My observations on the trace of circular polarisation in Amber is of no value whatever, as I have seen the very same thing in thick masses of unannealed Glass and near the optic axes of some biaxal crystals.
In the Great Grampian Range opposite to this on the other Side of the Spey there are three noble mountains Ben-MacDowie, <4> Brae-riach and Cairngorm, composed I believe of granite. They are all nearly of the same height, & the first has been found by the most accurate observations, I believe by those of Colonel Colby, <5> to be about 40 or 50 feet higher than Ben-nevis. <6>
When on the top of Cairngorm, this time I discovered in Loch-Avon, a splendid mountain lake the bright Green tint of Pure water which I believe was never before observed in any lake or stream in Great Britain. The Guides insisted that it come from a Green Weed, but this was not the case.
I am My Dear Sir Ever Most Truly yrs
1. Jean-Baptiste Biot (17741862), French scientist.
2. Sir John Frederick William Herschel (17921871), astronomer & scientist.
4. Ben Macdhui.
5. Thomas Frederick Colby (17841852), major-general, director of the Ordnance Survey. He surveyed northeastern Scotland in 1819.
6. Subsequently established as being the highest peak in Britain.