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of the gizzard, the mucus in the stomach, and the inner membrane of that cavity appear equally to have acquired it.

This coagulation appears to be the first change the food undergoes in the process of digestion, and where the digestion is rapid, the coagulated parts are very quickly dissolved.

Mr. BULLOCK was led, by his love of natural history, to spend some time on the Bass Rocks, and has frequently seen a Solan goose swallow a herring, and come immediately to feed its young, and although the time the herring remained in its stomach could not have been more than a few minutes, when it was brought up again, to be given to the young bird, the external covering was entirely dissolved.

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XIII. On some Properties of Light. By David Brewster, LL.D.

F. R. S. Edin. In a Letter to Sir H. Davy, LL.D. F. R. S.

Read January 28, 1819.

DEAR SIR, Having been for some time engaged in a series of experiments on the phenomena of light arising from its transmission through diaphanous bodies, I have taken the liberty of communicating to you, for the information of the Royal Society, a short and general account of the results of my enquiries. In the narrow compass of a letter, it would be impracticable to include the various details of these experiments; the particular methods of observation that were employed; or the numerical results which I have obtained for the refractive and dispersive powers of nearly two hundred substances. As these will form part of a separate work, in which I am now engaged, I shall confine myself at present to some of those results which appear to be most interesting, either from their novelty or importance.

1. On a new Property of refracted Light. As you are already well acquainted with the optical properties of doubly refracting media, and the analogous property of reflected light discovered by Malus, it will be unnecessary to take any notice of these phenomena. After repeating the experiments of MALUS, and measuring several of the angles

of incidence at which this property was communicated to light by reflection from different substances, I made a variety of experiments, with the view of discovering if a similar character could be impressed upon light by its transmission through bodies, either wholly or imperfectly transparent. All these experiments afforded no new result, and every hope of discovering such a property was extinguished, when my attention was directed to a singular appearance of colour in a thin plate of agate. This plate, bounded by parallel faces, is about the fifteenth of an inch thick, and is cut in a plane perpendicular to the laminæ of which it is composed. The agate is very transparent, and gives a distinct image of any luminous object; but on each side of this image is one highly coloured, forming with it an angle of several degrees, and so deeply affected with colour that no prism of agate, with the largest refracting angle, could produce an equivalent dispersion. Upon examining this coloured image with a prism of Iceland spar, I was astonished to find that it had acquired the same property as if it had been transmitted through a doubly refracting crystal, and upon turning the Iceland spar about its axis, the images alternately vanished at every quarter of a revolution. My attention was now directed to the common colourless image formed by pencils transmitted perpendicularly through the agate; and by viewing it through a prism of Iceland spar, it exhibited all the characters of one of the pencils produced by double refraction, the images alternately vanishing in every quadrant of their circular motion.

When the image of a taper reflected from water at an angle of 52° 45', so as to acquire the property discovered by Malus, is viewed through the plate of agate, so as to have its laminæ

parallel to the plane of reflection, it appears perfectly distinct; but when the agate is turned round, so that its larninæ are perpendicular to the plane of reflection, the light which forms the image of the taper suffers total reflection, and not one ray of it penetrates the agate.

If a ray of light incident upon one plate of agate is received after transmission upon another plate of the same substance, having its laminæ parallel to those of the former, the light will find an easy passage through the second plate; but if the second plate has its laminæ perpendicular to those of the first, the light will be wholly reflected, and the luminous object will cease to be visible.

Owing probably to a cause which will afterwards be noticed, there is a faint nebulous light unconnected with the image, though always accompanying it, and lying in a direction parallel to the laminæ. This light never vanishes along with the images, though it is evidently affected by the different changes which they undergo; and in one of the specimens of agate, it is distinctly incurvated, having the same radius of curvature with the adjacent laminæ. This character of the nebulous light I consider as an important fact, which may be the means of conducting us to a satisfactory theory, and I am at present engaged in examining it with particular care.

This remarkable property of the agate I have found also in the kindred substances of cornelian and chalcedony, and it is exhibited in its full effect even when these bodies are formed into prisms, and when the incident rays fall with any angle of obliquity. In one specimen of agate, which has no veins to indicate the direction in which it was cut, the images did not vanish as before; and in another specimen of the same

character the images suffered only an alternate diminution of brightness, in the same manner as a pencil of light receives only a partial modification when reflected from water at a greater or a less angle than 52° 45'.

The different experiments which have now been mentioned were repeated, with the most satisfactory results, by Mr. PLAYFAIR, Dr. Hope, and Mr. John Davy.

Although the preceding results are by no means ripe for generalization, I cannot omit the present opportunity of hazarding a few conjectures respecting the cause of this singular property of the agate.

May not the structure of this mineral be in a state of approach to that particular kind of crystallization which affords double images ? and may not the nebulous light be an imperfect image arising from that imperfection of structure? When one of the images vanishes, the nebulous light which encircled it is then a maximum, and it gradually diminishes during the re-appearance of the image. When the image which had disappeared recovers its full lustre, the surrounding nebulosity is very small, and this remaining light is, in all probability, no portion of the unformed image, but merely a few scattered rays arising from the imperfect transparency of the mineral.

By forming the agate into a prism, the nebulous light should be separated from the image which it encloses, in proportion to the angle contained by the refracting planes; but owing, perhaps, to the smallness of its double refraction, if it has such a property, I have not observed any separation of this kind.

The incurvated form of the nebulous light corresponding with the curvature of the laminæ, seems to connect it with the

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