Chromatic Stereoscope-Relief from Colour.

201 colour alone. The same relief takes place in unassisted vision, but in a much less degree. The effect, however, is such as to hold out an important lesson to the artist in the management of his colours, as he must take care to make them correspond with the distance of the parts of the picture to which they are applied. We expect to see binocular pictures for the lenticular stereoscope executed by Mr. Baxter's beautiful process of oil painting, as well as other pictures for exhibiting the curious effects of the chromatic stereoscope.

The laws of binocular vision, when studied in all their generality, throw much light upon many physical and metaphysical questions of high importance. They enable us to place in its true light the celebrated theory of vision on which Bishop Berkeley reared the ideal philosophy of which he was the founder; and they give us powerful aid in explaining many physical phenomena which have baffled the ingenuity of philosophers. It would be out of place to give any account of these at present, but there is one so remarkable, and at the same time so instructive as to merit special notice. In order to exhibit solids in relief, and hollow at the same time, Sir David Brewster effected this by three drawings in the same straight line, so executed that the left hand one and the middle one gave the hollow cone, and the middle one and the right hand one the raised cone. The cones having their summits truncated exhibit circles either at the bottom of the hollow, or at the top of the raised cone, and when these are placed in an open lenticular stereoscope, so that we can see not only the hollow and the raised cones, but the flat drawing on each side of them, we are enabled to give an ocular and experimental proof of the cause of the large size of the horizontal moon, of her small size when in the meridian at a high altitude, and of her intermediate apparent magnitude at an intermediate altitude.

As the summit of the raised cone appears to be nearest the eye of the observer, the summit of the hollow cone farthest off, and that of the flat drawing on each side at an intermediate distance, these distances will represent the apparent distance of the moon in the zenith of the elliptical celestial vault, in the horizon, and at an altitude of 45°. The circular summits thus seen are in reality exactly of the same size, and at the same distance from the eye, and are therefore precisely in the same circumstances as the moon in the three positions already mentioned. If we now contemplate them in the lenticular stereoscope, we shall see the circular summit of the hollow cone the largest, like the horizontal moon, because it seems at the greatest distance from the eye; the circular summit of the raised cone the smallest, because it appears at the least distance, like the zenith moon; and the circular summit of the cones on each of an intermediate size, like the

moon at an altitude of 45°, because their distance from the eye is intermediate. This effect will be distinctly seen, by placing three small wafers of the same size and colour on the square summits of the drawings of the cones or four-sided pyramids. No change is produced in the apparent magnitude of these circles by making one or more of them less bright than the rest, and hence we see the incorrectness of the explanation of the size of the horizontal moon, as given by Dr. Berkeley.

The only other topic which the subject of this article requires us to notice is what has been called Binocular Perspective, in an ingenious article under that name, by James Hall, Esq., in the Art Journal. Mr. Hall is well acquainted with the laws of distinct vision, and of single vision with two eyes, but in his application of them to painting and perspective he has not succeeded in giving "the true theory of a picture, which he is convinced has never yet been expounded." Before we can determine its true theory, we must first decide what a picture is—a landscape, for example. It is a portion of natural scenery which we see and survey in all its parts. We might see it by directing both eyes to a feature in its centre, without moving them, or the point of convergence of their axes, from a fixed position. In such a case the theory of the picture would be, that this central feature would alone be seen singly and distinctly, while every other point would be seen double and indistinct,-indistinct from the duplication of the lines, and indistinct from the different distances of its parts from the eye. The lines of the painting, though known to be double, are never seen double, and therefore can be expressed only by a particular kind of indistinctness which we have tried in vain to observe, and which we believe no artist can convey to his canvas. But supposing that he could, his foreground would be exceedingly blurred and indistinct; curious optical phenomena, arising from the intersection of the curved and rectilineal branches of trunks of trees, would characterize the right and left sides of the foreground, and something quite ridiculous would be the result of such a combination. The middle and the background would display similar combinations of light and shadow, and the picture, when completed, could be tolerated only when the spectator placed himself before it, and looked at the canvas in the very way in which the artist had viewed and painted it. But when we look at nature in her grandeur and beauty, the eyes range with the rapidity of lightning over all its parts, converging its optical axes upon every point, readjusting their focus to each point in succession, now admiring the cloud-capt tower, now the gorgeous palace, now the picturesque hamlet, now the cattle grazing in the meadow, now the stream and the waterfall, now the impending boughs and the gigantic trunks which almost

Binocular Perspective-Theory of a Picture.

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overhang and touch him, now starting to the far distance, and taking cognizance of the evanescent outline which mingles with the sky. By an act of the memory, and the rapid return of the eye to renew the impression, he surveys and sees, in one field of view, the various parts of the scene with the same distinctness. Setting aside, therefore, all consideration of aerial perspective, it is the duty of the painter to delineate every part of the picture with the same distinctness with which he sees it, whether it be foreground, or middleground, or distance; and when such a picture is hung up, and the object of admiration, the observer runs his eye over all its parts, and obtains the very representation of the scene which was drawn by the artist.

So long ago as 1828, Mr. Hall drew up a paper on this subject for the Royal Society of Edinburgh, but never communicated it. The following is his own abstract of it; and such of our readers as wish to see copious extracts from the paper itself will find them in the Art Journal for March 1852:

"The true theory of a picture I believe to be as follows: Having fixed upon a particular view of an object, at a distance calculated to show it off to the greatest advantage, let us imagine a vertical plane to pass through the principal part of the object chosen; a plane right opposite the spectator, and parallel to the line which joins the centres of his two eyes.

"All work, whether portrait, history, landscape, or miniature, ought, I conceive, to be first constructed of the full size of life or nature on this imaginary vertical plane passing through the principal part of the principal object, and so as to take into account the spectator's two eyes, which eyes are, of course, supposed to be adjusted for the principal object.*

"All due allowance being thus made for the two eyes, the next step, for either portrait or landscape, is to reduce the whole to a miniature, retaining all the duplications and regulated obscurities' in strictly the same proportions as in the large-scale picture.

"We have been taught heretofore that a picture is produced by intercepting the rays from an object to one of the spectator's eyes, upon a vertical plane interposed between the spectator and the object; which theory of perspective, though strictly demonstrable as any proposition in Euclid, for the circumstances supposed, has yet two capital defects. First, that its results are always necessarily less than the size of nature; and, secondly, that no account is taken of the spectator's two eyes, which is, however, one of the most important provisions in our economy for enabling us to judge of the relative distance and magnitude of near objects.

"The law of distinct and single vision with two eyes, by the concourse of the optic axes at any given point, has long been perfectly

* Mr. Hall has forgotten that when the eyes are directed to the principal object in the landscape, objects seen laterally will vanish and reappear in succession.

known; but its application to painting and perspective appears to have been hitherto entirely overlooked or evaded.

"The operation of the law to painting is chiefly upon the background and retiring portions in portrait and history, and chiefly upon the foreground in landscape; the foreground in landscape and the background in portrait being, respectively, amongst the greatest of all the difficulties and perplexities that embarrass the student, and even the practised master.

"The production of roundness and relief, in place of hardness and flatness, is chiefly the result of our using both our eyes in painting; which is likewise, I am persuaded, the key to the due subordination of parts, or what the painters call breadth' and 'keeping,' and is one of the main secrets for the production of a whole.”

In perusing this passage, and the extracts from the original paper, it seems to us very manifest that Mr. Hall was well acquainted with the fact that the pictures in each eye of the artist were dissimilar, and that roundness and solidity were, as Harris remarks, the result of the union of the two dissimilar pictures.

Memoirs of Dr. Chalmers.

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ART. VIII.-Memoirs of the Life and Writings of Thomas Chalmers, D.D., LL.D. By his Son-in-law, the REV. WILLIAM HANNA, LL.D. Four volumes 8vo, 1850-52.

Ir may well be supposed that the readers of the NORTH BRITISH REVIEW have already perused the volumes before us, or even if there should be a few exceptions, we shall not think ourselves called upon to turn out of our path on the account of such, and should feel it to be an impertinence to occupy the space allotted to this article in that manner which may be quite proper in ordinary instances, when volumes of memoirs come under review. Our readers, in this present instance, will not ask us either to furnish a condensed memoir of him who is the subject of these volumes, or to bring forward copious extracts from them, as samples of the matter which they contain. Such extracts from Dr. Hanna's pages as we may introduce will be selected with a view to a purpose of a more general kind.

Very little ambiguity can attach to those cases, rare as they must be, in which the man of an epoch comes to take his place in the company of historic persons: there can be little risk of mistake on any such occasion. Let it be supposed either that we were intending to compile the history of the country that boasts of him, during the period of his public life-such a history must bring forward his name on almost every page; or, on the other hand, if we were composing a memoir of the man-this biography could differ little from the history of his times; at least in relation to those of its interests with which principally he had to do.

Can we be wrong in saying that THOMAS CHALMERS was one of these EPOCH PERSONS-that he was the man whose mind and soul, whose energies and opinions, and whose public conduct, so impressed his personal image upon the religious and moral aspects of his country, as that his likeness can never be thence effaced, nor himself forgotten? A century or two hence it will not be that the name of CHALMERS has gone to its final resting-place in forgotten books. The youth of Scotland, some hundred years on, will not be putting any such question as this to their sires"Thomas Chalmers !-who was he?" The religious cottager of Scotland's remotest glens, after a generation or two has passed away, will not need to be told that he owes an endless debt of love and reverence to the memory of Thomas Chalmers. None of these things will happen; or not unless social catastrophes shall in the interval sweep Scotland clean of its true heart, its fervent mind, and its retentive memory! Scotland, we think,