A Source Book in Geology [1400-1900]

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Author: William Nicol  | Date: 1829

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Geology

Nicol

THE Nicol PRISM

1829

From Edinburgh New Philosophical Journal, Vol. VI, pp. 83–84, 1829.

The following simple method of constructing a prism of calcareous-spar, so that only one image may be seen at a time, will, perhaps, prove interesting to those who are in the habit of examining the optical properties of crystallised bodies by polarised light.

Let a rhomboid of calcareous-spar one inch tong be reduced in breadth and thickness to three-tenths of an inch; let the obliquity of its terminal planes be increased about three degrees; or, in other words, let the angles formed by the terminal planes, and the adjoining obtuse lateral edges, be made equal to 68° by operating on the terminal planes: These planes may now be polished. The rhomboid is then to be divided into two equal portions, by a plane passing through the acute lateral edges, and nearly touching the two obtuse solid angles. The sectional plane of each of the two halves must now be made to form exactly an angle of 90° with the terminal plane, and then carefully polished. The two portions are now to be firmly cemented together by means of Canada balsam, so as to form a rhomboid similar to what it was before its division.

If a ray of common light fall on the end of such a rhomboid in a direction parallel to the lateral edges, the two rays into which it is divided, in passing through the spar, will deviate so far from each other, that only the ordinary image will be seen. That image, too, will appear exactly in its true position, and free from colour. The range of the ordinary ray will be found considerably greater than the whole field of vision, as may easily be seen by making the rhomboid revolve on an axis parallel to the longer diagonal of the terminal planes. There is a tinge of blue where the ordinary ray vanishes on one side, and a tinge of orange, accompanied by a number of extremely minute obscurely coloured fringes, where it terminates on the other. If the rhomboid revolve beyond the fringes, the ordinary image will disappear, and the extraordinary image come into view. The latter, however, from the great obliquity of the incident light, occupies a smaller range, and is less distinct than the other. The ordinary ray passing out of the rhomboid in a direction parallel to its lateral edges, is therefore the best adapted for analytical purposes; and as calcareous-spar, when pure, and free from flaws, is not only transparent, but perfectly colourless, a rhomboid of that substance, of the above construction, developes the coloured rings of crystallized bodies with a degree of brilliancy not to be equalled by a plate of tourmaline, or perhaps by any other substance.

With the view of rendering the structure of the analyzing rhomboid more easily understood, I have supposed a piece of the spar to be divided into two equal portions. Such a division, however, would be a difficult task; but if two similar pieces of spar be taken, it will be found a very easy matter to remove one-half of each of them, either by grinding, or by the action of a file. The pieces of spar should not be much less than an inch long, and they need not be longer than 1.4 inch. If the latter dimension be adopted, the breadth and thickness will require to be about .48 of an inch.

In cementing the two pieces together, it will be proper to let the pointed end of the one project a little over the terminal plane of the other. By so doing, a more firm contact is obtained at the edges; and when the cement is sufficiently indurated, the whole of the projecting parts may easily be removed, according to their cleavages. The lateral planes should be left quite rough, to prevent the reflection of extraneous light.

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Chicago: William Nicol, "The Nicol Prism," A Source Book in Geology [1400-1900] in A Source Book in Geology [1400-1900], ed. Kirtley F. Mather and Shirley L. Mason (New York: Hafner Publishing Company, 1939), 176–177. Original Sources, accessed February 23, 2024, http://www.originalsources.com/Document.aspx?DocID=96VBFHY3S755FZW.

MLA: Nicol, William. "The Nicol Prism." A Source Book in Geology [1400-1900], Vol. VI, in A Source Book in Geology [1400-1900], edited by Kirtley F. Mather and Shirley L. Mason, New York, Hafner Publishing Company, 1939, pp. 176–177. Original Sources. 23 Feb. 2024. http://www.originalsources.com/Document.aspx?DocID=96VBFHY3S755FZW.

Harvard: Nicol, W, 'The Nicol Prism' in A Source Book in Geology [1400-1900]. cited in 1939, A Source Book in Geology [1400-1900], ed. , Hafner Publishing Company, New York, pp.176–177. Original Sources, retrieved 23 February 2024, from http://www.originalsources.com/Document.aspx?DocID=96VBFHY3S755FZW.