Natural philosophy. On the Existence of the Colour Brown.

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Brown is wanting in the prismatic spectrum, and its relation to the colours of the spectrum is as yet unknown. Any one may, however, easily convince himself that brown is nothing more than the complementary colour to that of Herschel's lavender-gray rays, i. e. white light from which these rays have been removed.

For this purpose, separate plates should be split from crystallized gypsum in such a manner that on one side they are as thin as possible, and from it gradually increase in thickness in broad terraces. One of these plates is placed under the microscope, which must be furnished with two Nichol's prisms, one beneath the object-glass, and one in the eye piece, and so arranged, the prisms being parallel, and the linear magnifying power being about twenty diameters (at a distance of eight French inches) that the above-mentioned thin side is in the field. If it is sufficiently ain, no colour is perceived immediately at the side; but as we proceed towards the thicker part, at first a pale brown tint becomes visible, as if we were looking through a very thin plate of horn, and as the thickness of the plate gradually increases in broad and low terraces, the brown continues to become darker until it assumes a deep and pure nut-brown colour, without the intervention of any of the prismatic colours which the thicker parts of the plate exhibit.

It is evident that the plate at the margin where it appears colourless is so thin, that the difference of the path of the ordinary and extraordinary ray on their exit does not amount to half the length of a wave for any colour. Thus interference of the most refractive rays does not occur until the thickness is greater, and the brown colour must therefore be produced by the disappearance of the lavender-gray rays from the compound light.

The correctness of this conclusion is readily tested. On crossing the prisms, it is seen that whilst in the case of all the other colours of the plate the well known complementary colours appear, that portion which was previously brown becomes coloured lavender-gray, and the intensity of this colour is in proportion to the depth of the brown previously observed at the same spot.

— From Poggendorff’s Annalen: read before the Physical Society of Berlin: Phil. Mag. No. 222.

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