but that it acts much more energetically upon the Red extremity, and much less energetically upon the Blue extremity of the spectrum. The yellow rays being thus solicited by equal and opposite forces, the crystal will exercise over them no polarizing energy. Thered rays being subjected to a greater polarizing energy from the Negative than from the Positive axis, will give rings corresponding to the difference of their opposite actions, and the characters of these rings will be Negative. The blue rays, on the contrary, being much less energetically acted upon by the Negative than by the Positive axis, will form rings proportional to the difference of their actions, and these rings will be Positive, from the predominating influence of the positive axis. In this way, a particular crystal of Apophyllite may exercise over the red of the polarized beam a negative influence; over the blue rays a positive influence; and over the yellow rays no influence at all *; while it is the general character of the mineral to exert an attractive doubly refracting force over all the rays of the spectrum. The ray of compensation, in place of being yellow, may have any position in the spectrum, and those on each side of it will afford positive or negative tints, according as the positive or the negative axis exercises over them a predominating influence.

rays

"This view of the polarising structure of Apophyllite, affords a complete explanation of the singular tints which surround its resultant axis. Each order of colours is as it were a residual spectrum †, arising from the opposite actions of the negative and the positive axis, and the tints of which these orders are composed, will consequently vary, according to the locality of the ray of compensation."

This partial equilibrium of polarising forces is analogous to the paradoxical phenomena of a compound lens, which, as I have elsewhere shewn, may be constructed so as to converge the Blue rays, diverge the Red rays, and exercise no action at all upon the Yellow ones. That is, the same compound lens is a Plane lens in yellow light, a convex one in blue light, and a concave one in red light.

+ Among the various residual or secondary spectra which I have examined in the course of my experiments "On the Action of Transparent Bodies upon the differently coloured Rays of Light," there are many among the polarised rings which have exactly the same tint; and there are some which resemble as nearly as possible those in Apophyllite.-See Edinburgh Transactions, vol. viii. p. 1.

In all the crystals which I have examined, including Apophyllites from Fassa, Utoë, Faroe, and Iceland, the doubly refracting force as exhibited by a distinct separation of the images, was Positive or Attractive, whether the ray was red, blue, or yellow. I therefore consider the property discovered by Mr Herschel as belonging only to some particular crystals, or parts of crys tals, in which one of the three positive axes has had its intensity reduced. Specimens will, I have no doubt, yet be found, in which the Apophyllite exerts a Negative action on all the rays of the spectrum; for I have shewn in the paper already quoted, that the intensity of the principal axis varies in the same crystal according to a regular law, and that all its variations are symmetrical, in relation to the centre or the summits of a complete crystal. Hence, it is extremely probable, that the points or nodes at which the negative passes into the positive structure, may be found in the same crystal, and near the points a, b in Fig. 13. of my paper on Apophyllite; for in several crystals the transition at these points appeared to be very sudden.

ART. XXII.-Notice respecting the House, &c. of Copernicus. In a Letter from GEORGE HARVEY, Esq. Member of the Astronomical Society.

DEAR SIR,

IN a former number of your valuable Journal, you introduced a very interesting fac simile of one of the letters of the celebrated Copernicus. I am a lover of such fragments, because they awaken associations of a pleasing kind; and in moments of difficulty and depression, bring accessions to human happiness, more consolatory than all the pleasures which power can command.

By the kindness of a scientific friend, I have been favoured with the inclosed sketch of the house in which this illustrious astronomer resided *; and also of the chair in which he was accustomed to sit. The pyramidal figure near the house, is the well which supplied him with water. They have afforded me

See Plate II. Figs. 5. and 6.

gratification, and with the view of communicating pleasure to your readers, they are now offered to your attention.

The astronomer, I am informed, resided in the highest garret," a true nestling place of genius, which seems to delight in hatching its offspring in bye corners." What mingled recollections crowd on the mind, when contemplating a sketch of this kind! In this room was nursed the genius of those deep and profound speculations, which, in succeeding years, was to produce so mighty a revolution in the world. It was in this chamber that Copernicus investigated the systems of ancient times, and resolved to break through the theories which had been sanctioned by the authority of ages;-which had grown up with a long race of centuries, and become identified, as it were, with the very habits and institutions of time. It was here the native independence of his mind was displayed in its full vigour and power; and which enabled him to abandon systems that had been embellished by the purest offspring of fancy, and illustrated by talents of the most splendid order ; and which seemed even in the more advanced periods of their career, to gain new accessions of strength, by becoming more intimately blended with the increasing sophistries of the schools. In this house Copernicus annihilated the labours of centuries, and prepared the elements of that mighty system, which, in an after age, was to be perfected, by the commanding genius of a Newton. This house might have often been to him an abode of uncertainty and of sorrow. Within its walls he was no doubt alternately elevated and depressed, as hope cheered him to advance, or error laid low his sanguine anticipations. ring the long period on which he meditated on his noble system, his great mind must have been the frequent theatre of conflicting doubts. Fame seldom encouraged him in his career, and the vistas of hope which the vantage ground of modern science now unfolds to the strong conception of the astronomer, but seldom soothed his anxious soul.

Du

Such men as Copernicus demand a solemn and thoughtful observation. The world but rarely sympathises with the man of close and studious retirement. How often, indeed, is he entirely neglected, till, from his lowly and unpretending soliVOL. VII. NO. 13. JULY 1822.

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tude, there issues forth some splendid discovery to interest and instruct mankind. Yet if we trace the histories of these neglected individuals, and contemplate the influence which, in after ages, they exercise on human affairs, we shall see new reason to admire the splendid privileges of the mind, and that unbounded influence which the silent progression of time, slowly, but permanently creates, for the triumphant productions of its powers. I remain, Dear Sir, yours, &c.

PLYMOUTH, 1 May 4. 1822.

GEORGE HARVEY.

ART. XXIII-Account of a singular Experiment depending on the Polarisation of Light by Reflexion. By DAVID BREWSTER, L. L. D., F. R S. Lond., & Sec. R. S. Ed.

IN the course of some experiments on the polarisation of light

by reflexion, I was led to observe, that a film of aqueous vapour might be breathed upon a plate of glass, without altering the distinctness of the image reflected from it surface. This effect arises from the extreme minuteness of the particles of water which compose the aqueous film; for when they are too large, which sometimes happens, the reflected image is more or less obliterated. By a little practice, however, it is easy to transfer a sufficiently attenuated film to a small plate of glass.

Let two glass reflectors A, B, Plate II. Fig. 7., be arranged in the usual manner, so that a ray of light R is incident on A, at an angle of about 56°, while the reflected ray AB is incident at the same angle upon B, so placed that the plane of reflexion from B may be at right angles to the plane of re flexion from A. When this adjustment is well made, scarcely a ray of light will be reflected from B, and the candle from which the ray R proceeds will be nearly invisible. Close to A, place another reflector of glass C, and let the ray R from the same candle be incident upon C at an angle of about 53°, while the reflected ray CD is received upon a second reflector D, (placed near B,) at an angle of 56°, and in such a manner that the plane of reflection from D is at right angles to the plane of reflection from C. As the ray R is incident upon C at an

*angle less than 56°, the polarising angle for glass, the reflected ray CD will not be polarised, and consequently, an image of that candle, not very bright, will be seen by reflection from D.

When an observer, therefore, looks into the two mirrors B, D, he will see no image of the candle in B, and a distinct image in D. While he is looking at these reflectors, let another person breath gently upon the glass reflectors A, C, and the image of the candle will instantly appear in B, where there was none before, while the image of the candle that was seen in D will be extinguished. The effect, therefore, of the same film of vapour has been to extinguish the image of the candle in D, and to create a new image in C. When the aqueous film has evaporated, which takes place very rapidly, the new image will be extinguished, and the extinguished one will reappear.

The cause of this paradoxical effect may be easily understood. When a film of aqueous vapour is laid upon the reflector A, the surface of the glass is, as it were, converted into water, and the ray RA is no longer polarised, because it is reflected from an aqueous surface, whose polarising angle is about 53°. Hence, as the ray AB is not polarised, a portion of it will be reflected from B, and will cause the candle from which it proceeds to be visible.

In the reflector C, where the ray CB is not polarised, in consequence of its being incident at an angle less than the polarising angle for glass, the substitution of a film of aqueous vapour will polarise the ray CD comp etely, as the ray RC is incident. upon the surface of it at an angle of 53°, which is the polarising angle for water. Hence, since the reflector D is placed in the position which will not reflect a polarised ray, no part of the ray CD will now be reflected from D, and, consequently, the image of the candle, which was formerly seen, will be extinguished by throwing a film of vapour upon the reflector C.

This experiment, independent of its paradoxical character, affords a very simple and striking illustration of the relation between the refractive power of bodies, and the angle at which they polarise the incident light.

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