Photographical Pictures combined by the Stereoscope. 191

ancient and of modern warfare will unfold themselves to the soldier's eye in faithful perspective and unerring outline; while in his fancy reanimated squadrons will again form on the plains of Marathon, and occupy the gorge of Thermopylae.

"But it is not merely the rigid forms of art and of external nature-the mere outlines and sub-divisions of space, that are thus fixed and recorded. The self-delineated landscape is seized at one epoch of time, and is embalmed amid all the co-existing events of the social and physical world. If the sun shines, his rays throw their gilding over the scene. If the gentle shower descends, the earth and the trees glisten with its varnish. If the wind blows, we see in the partially obliterated foliage the amount of its agitation. If the air is nearly at rest, the indistinctness of the aspen leaf measures the zephyr's breath. The objects of still life, too, give animation to the scene. The streets display their stationary chariots, the esplanade its military array, and the market place its colloquial groups, while the fields and the woodlands are studded with the various forms and attitudes of life. In this manner are the incidents of time and the forms of space simultaneously recorded. Every picture becomes an authentic chapter in the history of the world, and the direction and the length of the shadow of the spire marks the season, while the shadow of the dial's gnomon points to the hour when nature has been caught in her charms.

"In considering the relations of photography to the art of portraiture, we are disposed to give it a still higher rank. Could we now see in photogenic light and shade, Demosthenes launching his thunder against Macedon, or Brutus at Pompey's statue, bending over the bleeding Cæsar, or Paul preaching at Athens, or Him whom we must not name, in godlike attitude. and celestial beauty, proclaiming good will to man-with what rapture would we gaze on impersonations so exciting and divine! The heroes and sages of ancient times, mortal though they were, would thus have been embalmed with more than Egyptian skill, and the forms of life and beauty, and the lineaments of glowing affections and intellectual power, the real incarnations of immortal man, would have replaced the hideous fragments of royal mortality scarcely saved from corruption.

"But even within the narrower though not less hallowed sphere of the affections, where the magic names of kindred and home are inscribed, what a thrilling interest do the realities of photography excite! In the transition forms of their offspring which link infancy with manhood, the parent will observe the traces of his own mortality, and in the successive phases which mark the sunset of life, the child in his turn will read the lesson that his pilgrimage too is destined to close.

"Nor are these delineations interesting only from their minute

accuracy, or their moral influence. They are instinct with associations at once vivid and endearing :-Sensibilities peculiarly touching connect the picture with its original :-It was the very light which radiated from the hallowed brow, the identical gleam which lighted up the speaking eye, the pallid hue which hung upon the marble cheek, that pencilled the cherished image, and fixed themselves for ever there.

The subject of binocular vision is by no means restricted to the recombination of dissimilar plane pictures into the original solids which they represent. The union of similar pictures forms an interesting branch of binocular optics, and has been treated of with great fulness by Sir David Brewster in the article "On the Knowledge of Distance given by Binocular Vision." This class of phenomena are best seen by using a numerous series of plane figures, such as those of flowers, or geometrical patterns upon paper hangings or carpets. These figures being always at equal distances from one another, and almost perfectly equal and similar, the coalescence of any pair of them, effected by directing the optic axis to a point between the paper-hanging and the eye, is accompanied by the coalescence of every other pair. If we therefore look at a papered wall without pictures, or doors, or windows, at the distance of three feet, and unite two of the figures-flowers for example-at the distance of twelve inches from each other, the whole wall will appear covered with flowers as before, but as each flower is composed of two flowers united at the point of convergence of the optical axes, the whole papered wall, with all its flowers, (in place of being seen, as in ordinary vision, at the distance of three feet,) is seen suspended in the air at the distance of six inches from the observer. At first the observer does not decide upon the distance of the suspended wall from himself. It generally advances from the wall to its new position, and when it has taken its place it has a very singular character. The surface of it seems slightly curved. It has a silvery transparent aspect. It is more beautiful than the real paper, and it moves with the slightest motion of the head. If the observer, who is now three feet from the wall, retires farther from it, the suspended wall of flowers will follow him, moving farther and farther from the real wall, and also, but very slightly, farther and farther from the observer. When the observer stands still, and the picture is suspended before him, he may stretch out his hand and place it on the other side of the picture or wall, and even hold a candle on the other side of it, so as to satisfy himself that the suspended papered wall stands between his hand and himself.* This is a true pseudoscopic

• Errors in the construction of complex geometrical diagrams may be detected by this process. These errors, or rather unavoidable imperfections, arise partly

Effects produced by the Union of Similar Pictures. 193

phenomenon, in which the nearest of two objects appears the most distant.

In looking attentively at this picture some of the flowers have the appearance of real flowers. In some the stalk retires from the plane of the picture; in others it rises above it; one leaf will come farther out than another; one coloured portion, red for example, will be more in relief than the blue, and the flower will then appear thicker and more solid, resembling a real flower compressed, and deviating considerably from the plane representation of it as seen by one eye. All this arises from slight and accidental differences of distance in similar parts of the united figures. If the distance, for example, between two corresponding leaves is greater than the distance between other two corresponding leaves, then the two first, when united, will appear nearer the eye than the other two, and hence the appearance of a solid flower is partially given to the combination.

In surveying the suspended image another remarkable phenomenon often presents itself;-a part of one of the pieces of paper, and sometimes a whole stripe, from the roof to the floor, will retire behind the general plane of the image, as if there were a recess in the wall, or rise above it as if there were a projection, thus displaying on a large scale an imperfection in the workmanship which it would otherwise have been difficult to discover. This defect arises from the paper-hanger having cut off too much of the white margin of one or more of the adjoining stripes or pieces, or leaving too much of it, so that in the first case, when the two halves of a flower are joined together, part of the middle of the flower is left out, and hence when this defective flower is united binocularly with the one on the right hand of it, and the one on the left hand united with the defective one, the united or corresponding portion being at a less distance, will appear farther from the eye than those parts of the suspended image composed of complete flowers. The opposite effect will be produced when the two portions of the flowers are not brought together, but separated by a small space. We have, therefore, by means of this result, an accurate method of discovering defects in the workmanship of paper-hangers, carpet-makers, painters, and all artists whose profession it is to combine a series of similar patterns, in order to form a uniformly ornamented surface. The smallest defect in the similarity and equality of the figures or lines which compose a pattern, and any difference

from the points of the compasses sinking into different depths in the paper, and from the difficulty of making a number of lines pass through the same point. This effect is finely seen in the diagram of the homogeneous curve which forms plate IX. of Mr. Hay's work "On the Harmony of Form."

VOL. XVII. NO. XXXIII.

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in the distance of single figures, is instantly detected, and, what is very remarkable, a small inequality of distance in a line perpendicular to the axis of vision, or in one dimension of space, is exhibited in a magnified form, as a distance coincident with the axis of vision, and in an opposite dimension of space.

A little practice will enable the observer to realize, and to maintain the singular binocular picture which replaces the real one. The occasional retention of the picture after one eye is closed, and even after both have been closed and reopened, shews the influence of time over the dissolution, as well as over the creation of this class of phenomena. On some occasions a singular effect is produced, which is thus described by Sir David Brewster:-"When the flowers on the paper are distant six inches, we may either unite two six inches distant, or two twelve inches distant. In the latter case, when the eyes have been accustomed to survey the suspended picture, I have found, that after shutting and opening them, I neither saw the picture formed by the two flowers, twelve inches distant, nor the papered wall itself, but a picture formed, by uniting all the flowers sie inches distant! The binocular centre (the point to which the optic axes converged, and consequently the locality of the picture) had shifted its place, and instead of advancing to the wall, as is generally the case, and giving an ordinary vision of the wall, it advanced exactly as much as to unite the nearest flowers, just as in a ratchet wheel the detent stops one tooth at a time; or, to speak more correctly the binocular centre advanced in order to relieve the eyes from their strain, and when the eyes were opened, it had just reached that point which corresponded with the union of the flowers six inches distant."

The phenomenon of a suspended paper wall removed beyond the real wall, would be exhibited, could we fix the binocular centre on a point beyond the wall, so as to unite the flowers as before. The opacity of the wall does not permit this, but we may make the same experiment by looking through transparent patterns cut out of paper, or metal, or a particular kind of trellis work, or windows with small lozenges; but the readiest pattern is the cane bottom of a chair placed upon a table, the height of the eye, with the cane bottom in a vertical plane. If the observer, pressing his two hands against the cane bottom, directs his optic axes to a point beyond the chair, or doubles the picture of the cane bottom till he unites the open patterns, as he formerly did the flowers, he will then see the cane bottom suspended in front of the real cane bottom upon which his hands press, and which is absolutely invisible. He actually feels what he does not see, and sees what he does not feel. If he feels the real cane bottom all over, with the palms of his hands, the result will be the same.

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No knowledge derived from touch, no measurement of real distances, no actual demonstration from previous or subsequent vision, that there is a real body which his hands touch, and nothing at all where he sees it, can remove or even shake the infallible conviction of the sense of sight, that the cane bottom is where he sees it, and at the distance at which he sees it.

In the body of his paper, Sir David Brewster states it as a remarkable circumstance, that no examples have been recorded of false estimates of the distance of near objects, in consequence of the accidental binocular union of similar images; but after his paper had been read, the two following interesting cases, given in an appendix, were communicated to him :

"A gentleman who had taken too much wine saw, when in a papered room, the wall suspended near him in the air." "Some years ago," says Dr. Christison, in a letter to the author, "when I resided in a house, where several rooms are papered, with rather formally recurring patterns, and one, in particular, with stars only, I used, occasionally, to be much plagued with the wall suddenly standing out upon me, and waving, as you describe, with the movements of the head. I was sensible that the cause was an error as to the point of union of the visual axes of the two eyes; but I remember it sometimes cost me a considerable effort to rectify the error; and I found that the best way was to increase still more the deviation in the first instance. As this accident occurred most frequently while I was recovering from a severe attack of fever, I thought my nearsighted eyes were threatened with some new mischief; and this opinion was justified in finding, that after removal to my present house, —where, however, the papers have no very formal patterns-no such occurrence has ever taken place. The reason is now easily understood from your researches."-Edin. Trans. vol. xv. p. 675.

From this department of binocular vision some practical results may be deduced. In the decoration of apartments, both private and public, and in the dresses both of males and females, patterns, consisting of the regular recurrence of small figures, or of narrow stripes, at short distances, should be carefully avoided; and when it is deemed necessary to adopt some regularly recurring figure, they should be placed at such a distance that the two nearest could not be readily united by the convergency of the optical axes. When the patterns consist of small squares, as in the plaid dresses now so common, the observer cannot avoid uniting some of these squares, and thus causing a portion of the dress, and consequently of the part which it covers, to bulge out beyond its proper place. In such cases the eyes are distracted by the sight, and actually suffer pain in the vision of surfaces so unnecessarily subdivided.

Hitherto we have studied the union of dissimilar and similar