My Dear Sir,
I should like much to see the fringes you describe. I conceive that they may be produced by Reflexion between thin plates of parallel Glass of equal thickness inclined to each other. The fringes in parallel bands of coloured light may be seen either in the transmitted or in the Reflected rays. You will find them described after my Expts in Sir J. Herschels Treatise on Light, <1> & their theory given.
The village of King ue eusie, as it is pronounced is the chief place in the district of Badenoch in Inverness-shire. It is watered by the Spey. We are near the Root of the celebrated hill of Cairngorm & of Benmacdowie, <2> now found to be the highest in Scotland. We are not far from the Parallel Roads of Glenroy <3> the greatest Geological wonder in the Universe.
I wish something would tempt you to pay us a visit in Autumn. If you do not approve of our Sun, or of our Rocks, we can give you abundance of Grouse & Roe-deer shooting.
It is not known to me that if two Tourmalines <4> are similarly disposed with regard to each other so as to let the light pass freely a transparent crystal will produce darkness. An approximation however to this effect would take place if the interposed crystal polarised the white of the first order, because the complementary tint is nearly black. If the interposed crystal were a simply polarising one, or one in which one of the pencils <5> has been absorbed or dispersed, then blackness would be produced in two rectangular positions of it.
I am My Dear Sir Ever Most Truly yrs
D Brewster
Belleville by Kingussie
March 31st 1834
Notes:
1. Sir John Frederick William Herschel (1792–1871), astronomer & scientist, ‘Light’, in Encyclopædia Metropolitana, v.4, pp. 341–586, 1828.
2. Ben Macdhui. [See also Doc. No: 02873 regarding its being higher than Ben Nevis].
3. Lengthy, wide, and at the time inexplicable, terracing high on either side of Glen Roy, in the Highlands of Scotland northeast of Fort William, that was later identified by J. L. R. Agassiz as representing the various shoreline-levels of a retreating glacial lake.
4. Used as a polariser. [See Doc. No: 06773].
5. Rays.