It degree with all-on the combination of these two images for our notions of the actual forms and relative distances of things. Take the pen with which you are writing, away from the paper: shield it from the light so that no tell-tale shadow is cast by it-shut one eye, and we defy you to say whether the pen is an inch or a line from the paper. A lamp stands upon a side-table as we write, with a glass globe encircling its chimney. It is an old acquaintance-not exactly a "friend of forty years perhaps but still very familiar, and we have a pretty accurate mental impression of its portly aspect to fall back upon; but as we place it in a light falling upon its surface with tolerable uniformity, and look at it with one eye through a roll of paper just narrow enough to shut out its bounding line, we cannot for the life of us say, upon the view, whether the surface is round or flat: but give us the other eye to help us, and we instantly recognize its true form. Now why is it, that when we want to admire the magnificent perspective and relief of Jones's work, we industriously shut one eye, and confine our gaze to the canvas itself, apart from the frame? is because it suits us voluntarily to abandon one source of information, which would be sure to tell us with too unsparing fidelity that it was only a flat surface after all; so we give the go-by to this inconveniently truthful monitor, and look at the great artist's work with one eye only. The imagination is now free from the trammels of too much knowledge, and the mind transports us, with more or less facility, to the scene the painter has rendered, without the risk of being called back again by the matter-of-fact appeal made to us by the second eye. It is from the same desire to prevent interference with the play of imagination that we so carefully exclude the frame. Though the scene be one of oriental richness, the idea we wish to cherish would be sadly discomposed by our being called upon to fancy also that the sparkling streams and the orange groves, beneath which the enamoured damsel is disconsolately brooding over her soft sorrows, were really set in a gorgeous frame of gilded oak or plaster of Paris. With the stereoscope, on the other hand, we endeavour to reproduce, in some degree, the conditions under which we behold the scenes of nature; and we present to each eye a different picture. The right eye looks at the picture taken with the camera planted at one spot, the left eye at the picture taken with the camera shifted to the left of its former position. Thus an approach to the conditions of the actual vision of the scene represented is secured; the mind is required, as in looking as the actual object, to combine two different pictures, and it has far less difficulty in arriving at the conception of the solidity and of the differing distances of the objects portrayed. Now, when we gain our conceptions of the true character of a distant scene by the use of the eyes merely, it is obvious that the difference between the two pictures formed upon the two eyes respectively must be very small indeed. The parallax of an object two miles off, seen first at one place and then at another, no farther from it than one eye is from its fellow, must be minute indeed. It is a proof of how admirable is the optical mechanism of the eye, that it is in general quite sufficient to give us a very fair notion of the character of the scenery we are looking at; but we are apt, not unfrequently, to be deceivedand deceived very much-as to the amount of relief possessed by a distant object. A recess on a distant glacier, which we imagine to be trifling, often turns out to be a deep inlet in the mountain chain. If we go still further off, we are quite misled by the eyes. The full moon might well be the "glorious silver shield" to which one of our sweetest poets compares it, for all the unaided sight can tell us and this bewilderment becomes the greater in proportion to our want of previous and independent knowledge of the objects we are looking at. We need two sensibly different pictures to give us the true conception of solidity, rotundity, or relative distance. Hence it is that the photographer, in order to render the illusion more perfect, is in the habit of separating the two positions of his camera by a larger space than that between the two eyes. It is another illustration of the share the mind has in forming the conception of the object looked at, that, to a certain limited extent, it corrects this anomaly; and if the two positions of the camera are not more than four or five times the distance of the eyes from one another, no distortion, or no appreciable distortion, ensues, even with near objects. The process, however, is often carried too far: people are fond of seeing the effect of "startling relief;" but the relief is often much greater than in nature. A spherical ball is drawn out into an egg-shaped solid, with the one end pointed towards the spectator, and points of the picture at different distances from the eye are thrown farther apart than they should be. The effect upon the human face and figure is very curious, and very unpleasant, and great bad taste is sometimes shown, in this respect, in stereoscopic portraiture. With distant objects, however, we may often, by the aid of the stereoscope, get a much correcter notion of their true relative positions than we can by the unassisted eyes. Some of the best stereoscopic pictures of distant Alpine scenes are marvelously successful in bringing home to the mind the true amount of relief and indentation of the mountain ranges and recesses they represent; and we may get in this way very accurate information as to the character of scenery we can never hope to reach. In the case of a distant mountain view, the two positions of the camera would be many feet apart; but when we come to multiply by hundreds of thousands the figures which express the distance of the object from us, how are we to get sufficient distance between the two stations of our camera, to give us the requisite sensible difference between the pictures received by the two eyes? If we wish, for instance, to test by stereoscopy the actual figure of the moon, how are we to get sufficient parallax? It is Probably few persons of the thousands who have seen the beautiful stereoscopic portraits of the moon now to be had in London, and who have marveled at her unwonted aspect, appearing no longer as the "silver shield," but in all the full-blown dignity of her true sphericity, have any idea of the amount of mechanical contrivance and astronomical knowledge involved in this remarkable performance. To get a photograph of the moon at all is not so simple a matter as might be supposed. The photographic action of moonlight is so feeble, that despite all the improvements brought to bear by recent research on the preparation of films sensitive to light, an exposure of hours would be needed, under ordinary circumstances, to produce any appreciable result. But the moon is moving quickly through the skies, and anything beyond a momentary exposure, if it produced any effect at all, would give a blurred outline and a hazy picture. necessary, therefore, to call in aid the complicated machinery already existing in observatories, by which the telescope is made to follow the motions of the heavenly bodies, so that the moon or other heavenly body being once got into the centre of its field, it remains in the centre as long as may be necessary. A very large telescope will, of course, concentrate and condense very much the light of the moon, and it has been found possible, with the powerful instruments of our best observatories, actually to obtain a photograph of the moon on a prepared glass plate placed in the focus of the eye-piece in eighteen or nineteen seconds! But the stereoscopic difficulty still remains-viz: how to obtain two sensibly different pictures of the moon, which, as everyone knows, has the same side always turned to us. Here the astronomer calls in aid the facts that the rotation of the moon on its axis is not absolutely uniform; so that it is not absolutely the same. portion of the moon that is always turned to us. If a line were drawn from the centre of our globe to the centre of the moon, the combination of the irregularities of the moon's motion and our motion gives the appearance of a slight oscillatory movement towards one side or the other side of this line; thus bringing under our eyes a little more now of the eastern, now of the western hemisphere. Another motion, in virtue of which the axis of the moon sways a little from side to side, causes a similar slight change in the portions of the northern and southern hemispheres visible from time to time. Now all these little eccentricities are perfectly well known to the astronomer, and are calculated and laid down beforehand with the utmost nicety and precision. All that is necessary, therefore, is to select two periods when the moon is equally near to the full or to the new, and when there is a sufficient amount of difference between the portions exposed to view, and to take one half of the compound stereoscopic picture on each occasion. The result has proved as remarkable as the steps which lead to it are complicated and delicate. Not merely has the rotundity of the moon been brought out as a matter of ocular demonstration, but the irregular character of its surface; its lofty mountains and deep volcanic craters have been so graphically rendered, as probably to dispose of all future controversy as to their nature. It is to the ingenuity and perseverance of Professor Wheatstone that this beautiful and remarkable discovery is due. The convenient instrument now in common use is not the original application of the principle, but even the ordinary stereoscope was, we believe, first constructed by the same distinguished man. In the first instance, the experiments were almost all made with diagrams, consisting of the outlines of solid figures as seen from different points of view; and some results were obtained illustrating curiously the great part taken by the mind, as distinguished from the mere faculty of vision, in producing the effects in question. Some of the most recent investigations in stereoscopy supply additional evidence in support of the same proposition. It has been shown that if a page of print be taken, and the type be slightly altered, so as to throw a letter or a word here and there out of place, by moving it a little to the right or to the left of its former position, and a page be struck off from the altered type, and the two then placed side by side in an ordinary stereoscope, the words or letters which have been displaced in the second page start instantly into high relief. It is obvious here that the effect is a mental one, and that the simple fact of the letters occupying places slightly different in the two pages suggests to the mind the idea of relief, for which there is no foundation in fact. It has been suggested that this phenomenon will supply a means of detecting forged bank-notes. If a forged note and a genuine one of the same date be placed side by side in the stereoscope, the eye will instantly detect the misplacement of a single stroke, or a single water-mark, by the relief into which it will be thrown. It is obvious that, if methods now available be used, and the plate be engraved by photography, the test may fail, so far as the engraved matter of the note is concerned; but as the water-marks cannot be introduced by photo graphic agency into the substance of the paper, any deviation in this respect from the pattern of the genuine note will probably be apparent. What wonders photography in general, or stereoscopy in particular, may yet be destined to achieve, it would be rash to predict; but it is not too much to hope that, considering the short period over which the history of the art as yet extends, and the number of skilful and ingenious minds now engaged in photographic researches, the best productions of the present day will be completely thrown into the shade by those of future years. If photography should ever succeed in the difficult task of copying the colours, as well as of recording the differing amounts of chemical light and shade, of nature, the fancy may safely run riot without the slightest danger of exaggerating the beauty of the results we may look for. Nor will the prospect appear hopeless, when we remember that one French chemist has already succeeded in obtaining, by photographic means, all the colours of the spectrum, though he has failed in rendering them permanent; and that another,† if he has not actually done that by which the satirist meant to suggest the highest pitch of chimerical insanity, and extracted sunbeams from cucumbers, has at all events shown us how to bottle up light in any convenient receptacle,—and, for aught we know, it might perfectly well be stored in a hollow cucumberand bring it out fresh and fit for use whenever occasion may require. *M. Becquerel. |