How to take accurate Binocular Pictures.

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binocular camera, fitted up with two semi-lenses, obtained by the bisection of a whole lens. These semi-lenses can be placed at any distance we choose, or may be put, when required, into separate cameras, and it is obvious that the pictures which they furnish will be perfectly equal. Mr. Slater of Euston Square has already constructed several of these binocular cameras, which have been sent to America. As the semi-lens will give only half the light of a whole lens, a longer time will be required to take the Daguerreotype or the Talbotype; but these processes have been so greatly accelerated that this disadvantage becomes insignificant when compared with the advantage of obtaining two perfectly equal pictures. The semi-lenses, however, have another advantage, the statement of which will startle the Photographer. They will give better binocular pictures than two whole lenses whose focal length is exactly the same, if two such lenses could be made.

In order to make the reader understand this, we must write a new chapter on optics for the special benefit of the Binocular Photographer, which we trust all Photographers will be. The Daguerreotypist and the Talbotypist justly pride themselves on the size of the achromatic lenses of their cameras, on the perfection of which their superiority as artists must always depend. They will be surprised to learn, however, for the first time, that owing to the size of their lenses, even if absolutely perfect, they never can obtain a picture such as that which is seen by one or by both eyes. To explain this we must make some observations on the nature of monocular and binocular vision, that is, on vision with one or two eyes. Lord Bacon has said that we see more exquisitely with one eye shut than with both. He might have said that we see more exquisitely with one eye than with both, for it is not necessary to shut the other eye in order to obtain better vision. Mr. Wheatstone has given an explanation of this superiority of single vision, which we cannot for a moment admit. Having previously stated, that beyond the distance when the optic axes are parallel, (nearly parallel must be meant,) "we see with both eyes all objects precisely as we see near objects with a single eye," he observes, "that every one must be aware how greatly the perspective effect of a picture is enhanced by looking at it only with one eye, especially when a tube is employed to exclude the vision of adjacent objects, whose presence might disturb the illusion. Seen under such circumstances from the proper point of sight, the picture projects the same lines, shades and colours on the retina, as the more distant scene which it represents would do were it substituted for it. The appearance which would make us certain that it is a picture is excluded from the sight, and the imagination has room to be active." Now, this is really nothing more than guessing at the cause of a distinct physical fact, which demands a distinct and intelligible physical explanation. The tube has nothing whatever to do with the question, because two tubes might be used, as they often are, in binocular vision. Nor has the distance of the scene anything to do with the matter, because the proposition is true, and much more strikingly true of scenes not distant. The "appearance which would make us certain that it is a picture" is not mentioned, and even if it were "excluded from the sight," we should still be certain that it is a picture however "greatly the perspective effect of it is enhanced." We demur too, to the help of the imagination, and we demur the more, the more room that is given it to be active.

The following is, we doubt not, the true physical cause of the very interesting fact, that the perspective effect of a picture, especially of pictures in true perspective, such as those taken photographically, is best seen with one eye. When we direct both eyes to such a picture and survey its parts, the optic axes maintain throughout the same convergency, so as to prove to us that every part of the canvas or the ground of the picture is at the same distance from us. In nature the optic axes would have constantly changed their convergence as they were directed to the nearer and remoter portions of the scene. But when we look at the picture with one eye, we are deprived of the means of ascertaining that the different parts of the picture are at the same distance from the eye; and the artist's skill in aerial perspective, and in characterizing the foreground and middle distance by their proper touches, is left to make its full impression on the mind. But besides this another cause is in operation; the canvas or ground reflects light from its surface, that is the light from the walls or other objects which lie in the direction of the incident rays, which correspond with the rays reflected to each eye; and the sight of the canvas destroys the illusion. When one eye is closed the reflected light is diminished one half, and therefore the illusion is increased. A greater portion of the reflected light may be destroyed by looking through a polarizing prism, or by placing a black cloth so that the reflected picture of it may hide the whole of the canvas.

From the representations of nature on a plane let us pass to the vision of bodies of three dimensions. Here too monocular vision is superior to binocular. With one eye every part of the solid is seen with the degree of light which falls upon it, and with equal distinctness as the eye directs itself to its different parts; but with two eyes there are many parts of the solid that are seen only with one eye, while all the other parts of it are seen with both eyes, and hence the parts of it seen with one Why the Perspective of a Picture is seen best with one eye. 183

eye are only half as luminous as the other parts. If the object viewed is a six-sided pyramid, with its apex more turned to one eye than the other, there are certain positions of it in which the left eye sees only four faces of it, while the right eye sees all the six, 80 that the two sides seen only by one eye must be much less luminous than the other faces. But there is still another imperfection of binocular vision which requires to be mentioned. With two eyes we see more of the round of objects in lines parallel to the visual base than we do in lines perpendicular to that base, that is, more in horizontal than in vertical planes. When we view a sphere, for example, with one eye, its projection on a plane is circular, while with both eyes its projection is an oval, the large axis of the oval being horizontal. Hence we do not see the true forms of objects in binocular vision. Two eyes, however, were necessary to give beauty and symmetry to the human form, and to enable us to form a more correct estimate of distance and position; and they have the still greater advantage of preserving us from blindness when one of them is lost. The superiority of monocular vision would have been a poor compensation to Polyphemus cui lumen ademptum.

We are now prepared for the consideration of the influence of size in lenses over the images which they form. The object of photographic art is to obtain an accurate representation of nature, as it appears when seen either with one or with two eyes. We have already proved that with one eye a sphere is truly seen, that is, it is seen perfectly spherical, its roundness being equally well developed in all sections of it by a plane passing through its centre and the eye. The same is true of all irregular solids. The appearance of the sphere, however, and of all such solids, changes as the pupil expands. Rays from parts of the sphere, and other solids that do not enter the smaller pupil, enter the larger one, and exhibit portions of the solid not previously seen. This change of form, however, is too small to be noticed, and we may assume that we have a perfect representation of an object of three dimensions when we view it with one eye. It is such a picture as this that the artist paints, and all photographic delineations should resemble a picture thus painted. In order to produce such a picture the lens of the camera should have the same diameter as that of the pupil of the eye. Every addition to the area of the lens introduces parts of the object which have nothing to do with the picture, and when we use lenses of two, four, or six inches in diameter, we obtain, though a common eye may not discover it, monstrous representations of humanity, which no eye and no pair of eyes ever saw or can see. Every portion of the lens exterior to the central portion, the size of the human pupil, introduces parts of the picture which ought not to be seen. The face is enlarged all round, so as to be larger and broader than it ought. The ears are expanded, the head, nose, and chin elongated, hollows enlarged, prominences elevated, and different degrees of illumination which do not exist in the original mark the photographic picture. The likeness is therefore imperfect, and we now see the cause of defects which have hitherto been ascribed to want of steadiness in the sitter. The only remedy for such defects is to diminish the aperture of the lens to the smallest size, and the photographic art will not be perfect till we can take portraits in the shortest possible time, and with the smallest possible lenses! In order to explain this result, let us suppose that we look at a cone with its base turned to the eye, the true picture of the cone is a circle, as we see only its circular base; but if we place the cone before a camera with a large lens, and if the cone is small, so that rays from its conical surface fall upon the marginal parts of the lens, its picture will be a circle as before, surrounded with a luminous ring representing the surface of the cone. In like manner every solid will be incorrectly represented, and the resultant picture will be a combination of pictures, some as seen by binocular vision, with eyes at various distances, and some as seen by one or both eyes in different positions.

The most important application of these views is to binocular portraits as taken at the same time with the usual cameras, in order to be combined by the stereoscope. The portraits as now taken in London by professional photographers, are gross misrepresentations of the human form, because taken with large lenses, in order that they may be taken quickly. Each picture, which ought to be strictly monocular, is really binocular and much worse, so that the stereoscope unites into a monstrous solid, two binocular or worse than binocular portraits. If the sitter, for example, is placed in such a position, that his left ear is hidden from both eyes of the artist, the portrait taken by the camera on his right hand, will exhibit this ear, while the portrait taken by the camera on his left, will exhibit a portion of the head behind his

In like manner, the broad margin of the lens on his right hand will shew the right ear much more expanded than it should be, and also a considerable portion of the head beyond that ear. For the same reason, the upper and the under margins, and all the other margins of the lenses, will exhibit portions of the figure in vertical and all other planes, which ought not to be seen at all. When these two pictures, therefore, are united by the stereoscope, the resultant portrait is anything but an accurate or an agreeable representation of the individual. Hence, it is obvi

Monocular and Binocular Portraits incorrect.

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ous, that semi-lenses, such as those in the binocular camera, will give better binocular portraits than whole lenses of the same diameter, because the effects which we have described will be reduced in the proportion of 2 to 1 in all horizontal sections of the solid, the most influential sections in binocular vision. But, though semi-lenses have this advantage, their area is still too large, and photographic portraiture can be perfected only by a very great reduction in the area of the lenses employed. For statues, busts, and buildings, where quickness in the photographic process is not necessary, we should use exceedingly small lenses, and thus obtain very accurate representations of that class of objects.

Having obtained a camera fitted to give good monocular pictures, the next step is to apply it to take binocular ones. After determining the best aspect for the portrait, that is the distance and line of direction at which it should be taken, the camera must be placed in succession on each side of the line of direction, and at equal angular distances from it; or if we use two cameras, this line of direction must bisect the line of direction of the two cameras. If we call D the distance of the object from the camera, d the distance between the eyes, and A the angle subtended by the distance d, at the distance D or at the ob

D

ject, we shall have tang A= and making d=21⁄2 inches, we shall obtain the following values of A, or the inclination of the two lines of direction of the camera, in order to obtain pictures of the solid as seen by each eye.

Having obtained the binocular portraits or pictures, the next step is to unite them, so as to reproduce the original solid. This may be done either by the eyes alone, or by a stereoscope. When we do it by the eyes alone, we must place the picture as seen by the left eye on the right side, and the picture as seen by the right eye on the left side; and if we have not the power of doubling each of the pictures by squinting, so as to cause the two middle