The stereoscope

However perfect be the stereoscope which we employ, the effect which it produces depends upon the accuracy with which the binocular pictures are prepared. The pictures required for the stereoscope may be arranged in four classes:—

Geometrical Solids.

Representations of geometrical solids, were, as we have already seen, the only objects which for many years were employed in the reflecting stereoscope. The figures thus used are so well known that it is unnecessary to devote much space to their consideration. For ordinary purposes they may be drawn by the hand, and composed of squares, rectangles, and circles, representing quadrangular pyramids, truncated, or terminating in a point, cones, pyramids with polygonal bases, or more complex forms in which raised pyramids or cones rise out of quadrangular or conical hollows. All these figures may be drawn by the hand, and will produce solid forms sufficiently striking to illustrate the properties of the stereoscope, though not accurate representations of any actual solid seen by binocular vision.

If one of the binocular pictures is not equal to the other in its base or summit, and if the lines of the one are made crooked, it is curious to observe how the appearance of the resulting solid is still maintained and varied.

The following method of drawing upon a plane the dissimilar representations of solids, will give results in the stereoscope that are perfectly correct:—

Let L, R, Fig. 43, be the left and right eye, and A the middle point between them. Let MN be the plane on which an object or solid whose height is CB is to be drawn. Through B draw LB, meeting MN in c; then if the object is a solid, with its apex at B, Cc will be the distance of its apex from the centre C of its base, as seen by the left eye. When seen by the right eye R, Cc′ will be its distance, c′ lying on the left side of C. Hence if the figure is a cone, the dissimilar pictures of it will be two circles, in one of which its apex is placed at the distance Cc from its centre, and in the other at the distance Cc′ on the other side of the centre. When these two plane figures are placed in the stereoscope, they will, when combined, represent a raised cone when the points c, c′ are nearer one another than the centres of the circles representing the cone’s base, and a hollow cone when the figures are interchanged.

If we call E the distance between the two eyes, and h the height of the solid, we shall have

which will give us the results in the following table, E being 2½, and AC 8 inches:—

If we now converge the optic axes to a point b, and wish to ascertain the value of Cc, which will give different depths, d, of the hollow solids corresponding to different values of Cb, we shall have

which, making AC = 8 inches, as before, will give the following results:—

The values of h and d when Cc, Cc′ are known, will be found from the formulæ

As Cc is always equal to Cc′ in each pair of figures or dissimilar pictures, the depth of the hollow cone will always appear much greater than the height of the raised one. When Cc = Cc′ = 0.75, h:d = 3:12. When Cc = Cc′ = 0.4166, h:d = 2:4, and when Cc = Cc′ = 0.139, h:d = 0.8:1.0.

When the solids of which we wish to have binocular pictures are symmetrical, the one picture is the reflected image of the other, or its reverse, so that when we have drawn the solid as seen by one eye, we may obtain the other by copying its reflected image, or by simply taking a copy of it as seen through the paper.

When the geometrical solids are not symmetrical, their dissimilar pictures must be taken photographically from models, in the same manner as the dissimilar pictures of other solids.

Portraits of Living Persons or Animals.

Although it is possible for a clever artist to take two portraits, the one as seen by his right, and the other as seen by his left eye, yet, owing to the impossibility of fixing the sitter, it would be a very difficult task. A bust or statue would be more easily taken by fixing two apertures 2½ inches distant, as the two points of sight, but even in this case the result would be imperfect. The photographic camera is the only means by which living persons and statues can be represented by means of two plane pictures to be combined by the stereoscope; and but for the art of photography, this instrument would have had a very limited application.

It is generally supposed that photographic pictures, whether in Daguerreotype or Talbotype, are accurate representations of the human face and form, when the sitter sits steadily, and the artist knows the resources of his art. Quis solem esse falsum dicere audeat? says the photographer, in rapture with his art. Solem esse falsum dicere audeo, replies the man of science, in reference to the hideous representations of humanity which proceed from the studio of the photographer. The sun never errs in the part which he has to perform. The sitter may sometimes contribute his share to the hideousness of his portrait by involuntary nervous motion, but it is upon the artist or his art that the blame must be laid.

If the single portrait of an individual is a misrepresentation of his form and expression, the combination of two such pictures into a solid must be more hideous still, not merely because the error in form and expression is retained or doubled, but because the source of error in the single portrait is incompatible with the application of the stereoscopic principle in giving relief to the plane pictures. The art of stereoscopic portraiture is in its infancy, and we shall therefore devote some space to the development of its true principles and practice.

In treating of the images of objects formed by lenses and mirrors with spherical surfaces, optical writers have satisfied themselves by shewing that the images of straight lines so formed are conic sections, elliptical, parabolic, or hyperbolic. I am not aware that any writer has treated of the images of solid bodies, and of their shape as affected by the size of the lenses or mirrors by which they are formed, or has even attempted to shew how a perfect image of any object can be obtained. We shall endeavour to supply this defect.

In a previous chapter we have explained the manner in which images are formed by a small aperture, H, in the side, MN, of a camera, or in the window-shutter of a dark room. The rectangles br, b′r′, and b″r″, are images of the object RB, according as they are received at the same distance from the lens as the object, or at a less or a greater distance, the size of the image being to that of the object as their respective distances from the hole H. Pictures thus taken are accurate representations of the object, whether it be lineal, superficial, or solid, as seen from or through the hole H; and if we could throw sufficient light upon the object, or make the material which receives the image very sensitive, we should require no other camera for giving us photographs of all sizes. The only source of error which we can conceive, is that which may arise from the inflexion of light, but we believe that it would exercise a small influence, if any, and it is only by experiment that its effect can be ascertained.

The Rev. Mr. Egerton and I have obtained photographs of a bust, in the course of ten minutes, with a very faint sun, and through an aperture less than the hundredth of an inch; and I have no doubt that when chemistry has furnished us with a material more sensitive to light, a camera without lenses, and with only a pin-hole, will be the favourite instrument of the photographer. At present, no sitter could preserve his composure and expression during the number of minutes which are required to complete the picture.

But though we cannot use this theoretical camera, we may make some approximation to it. If we make the hole H a quarter of an inch, the pictures br, &c., will be faint and indistinct; but by placing a thin lens a quarter of an inch in diameter in the hole H, the distinctness of the picture will be restored, and, from the introduction of so much light, the photograph may be completed in a sufficiently short time. The lens should be made of rock crystal, which has a small dispersive power, and the ratio of curvature of its surfaces should be as six to one, the flattest side being turned to the picture. In this way there will be very little colour and spherical aberration, and no error produced by any striæ or want of homogeneity in the glass.

As the hole H is nearly the same as the greatest opening of the pupil, the picture which is formed by the enclosed lens will be almost identical with the one we see in monocular vision, which is always the most perfect representation of figures in relief.

With this approximately perfect camera, let us now compare the expensive and magnificent instruments with which the photographer practises his art. We shall suppose his camera to have its lens or lenses with an aperture of only three inches, as shewn at LR in Fig. 45. If we cover the whole lens, or reduce its aperture to a quarter of an inch, as shewn at a, we shall have a correct picture of the sitter. Let us now take other four pictures of the same person, by removing the aperture successively to b, c, d, and e: It is obvious that these pictures will all differ very perceptibly from each other. In the picture obtained through d, we shall see parts on the left side of the head which are not seen in the picture through c, and in the one through c, parts on the right side of the head not seen through d. In short, the pictures obtained through c and d are accurate dissimilar pictures, such as we have in binocular vision, (the distance cd being 2½ inches,) and fitted for the stereoscope. In like manner, the pictures through b and e will be different from the preceding, and different from one another. In the one through b, we shall see parts below the eyebrows, below the nose, below the upper lip, and below the chin, which are not visible in the picture through e, nor in those through c and d; while in the picture through e, we shall see parts above the brow, and above the upper lip, &c., which are not seen in the pictures through b, c, and d. In whatever part of the lens, LR, we place the aperture, we obtain a picture different from that through any other part, and therefore it follows, that with a lens whose aperture is three inches, the photographic picture is a combination of about one hundred and thirty dissimilar pictures of the sitter, the similar parts of which are not coincident; or to express it in the language of perspective, the picture is a combination of about one hundred and thirty pictures of the sitter, taken from one hundred and thirty different points of sight! If such is the picture formed by a three-inch lens, what must be the amount of the anamorphism, or distortion of form, which is produced by photographic lenses of diameters from three to twelve inches, actually used in photography?[46]

But it is not merely by the size of the lenses that hideous portraits are produced. In cameras with two achromatic lenses, the rays which form the picture pass through a large thickness of glass, which may not be altogether homogeneous,—through eight surfaces which may not be truly spherical, and which certainly scatter light in all directions,—and through an optical combination in which straight lines in the object must be conic sections in the picture!

Photography, therefore, cannot even approximate to perfection till the artist works with a camera furnished with a single quarter of an inch lens of rock crystal, having its radii of curvature as six to one, or what experience may find better, with an achromatic lens of the same aperture. And we may state with equal confidence, that the photographer who has the sagacity to perceive the defects of his instruments, the honesty to avow it, and the skill to remedy them by the applications of modern science, will take a place as high in photographic portraiture as a Reynolds or a Lawrence in the sister art.

Such being the nature of single portraits, we may form some notion of the effect produced by combining dissimilar ones in the stereoscope, so as to represent the original in relief. The single pictures themselves, including binocular and multocular representations of the individual, must, when combined, exhibit a very imperfect portrait in relief,—so imperfect, indeed, that the artist is obliged to take his two pictures from points of sight different from the correct points, in order to produce the least disagreeable result. This will appear after we have explained the correct method of taking binocular portraits for the stereoscope.

No person but a painter, or one who has the eye and the taste of a painter, is qualified to be a photographer either in single or binocular portraiture. The first step in taking a portrait or copying a statue, is to ascertain in what aspect and at what distance from the eye it ought to be taken.

In order to understand this subject, we shall first consider the vision, with one eye, of objects of three dimensions, when of different magnitudes and placed at different distances. When we thus view a building, or a full-length or colossal statue, at a short distance, a picture of all its visible parts is formed on the retina. If we view it at a greater distance, certain parts cease to be seen, and other parts come into view; and this change in the picture will go on, but will become less and less perceptible as we retire from the original. If we now look at the building or statue from a distance through a telescope, so as to present it to us with the same distinctness, and of the same apparent magnitude as we saw it at our first position, the two pictures will be essentially different; all the parts which ceased to be visible as we retired will still be invisible, and all the parts which were not seen at our first position, but became visible by retiring, will be seen in the telescopic picture. Hence the parts seen by the near eye, and not by the distant telescope, will be those towards the middle of the building or statue, whose surfaces converge, as it were, towards the eye; while those seen by the telescope, and not by the eye, will be the external parts of the object, whose surfaces converge less, or approach to parallelism. It will depend on the nature of the building or the statue which of these pictures gives us the most favourable representation of it.

If we now suppose the building or statue to be reduced in the most perfect manner,—to half its size, for example,—then it is obvious that these two perfectly similar solids will afford a different picture, whether viewed by the eye or by the telescope. In the reduced copy, the inner surfaces visible in the original will disappear, and the outer surfaces become visible; and, as formerly, it will depend on the nature of the building or the statue whether the reduced or the original copy gives the best picture.

If we repeat the preceding experiments with two eyes in place of one, the building or statue will have a different appearance; surfaces and parts, formerly invisible, will become visible, and the body will be better seen because we see more of it; but then the parts thus brought into view being seen, generally speaking, with one eye, will have less brightness than the rest of the picture. But though we see more of the body in binocular vision, it is only parts of vertical surfaces perpendicular to the line joining the eyes that are thus brought into view, the parts of similar horizontal surfaces remaining invisible as with one eye. It would require a pair of eyes placed vertically, that is, with the line joining them in a vertical direction, to enable us to see the horizontal as well as the vertical surfaces; and it would require a pair of eyes inclined at all possible angles, that is, a ring of eyes 2½ inches in diameter, to enable us to have a perfectly symmetrical view of the statue.

These observations will enable us to answer the question, whether or not a reduced copy of a statue, of precisely the same form in all its parts, will give us, either by monocular or binocular vision, a better view of it as a work of art. As it is the outer parts or surfaces of a large statue that are invisible, its great outline and largest parts must be best seen in the reduced copy; and consequently its relief, or third dimension in space, must be much greater in the reduced copy. This will be better understood if we suppose a sphere to be substituted for the statue. If the sphere exceeds in diameter the distance between the pupils of the right and left eye, or 2½ inches, we shall not see a complete hemisphere, unless from an infinite distance. If the sphere is very much larger, we shall see only a segment, whose relief, in place of being equal to the radius of the sphere, is equal only to the versed sine of half the visible segment. Hence it is obvious that a reduced copy of a statue is not only better seen from more of its parts being visible, but is also seen in stronger relief.

On the Proper Position of the Sitter.

With these observations we are now prepared to explain the proper method of taking binocular portraits for the stereoscope.

The first and most important step is to fix upon the position of the sitter,—to select the best aspect of the face, and, what is of more importance than is generally supposed, to determine the best distance from the camera at which he should be placed. At a short distance certain parts of one face and figure which should be seen are concealed, and certain parts of other faces are concealed which should be seen. Prominent ears may be either hid or made less prominent by diminishing the distance, and if the sight of both ears is desirable the distance should be increased. Prominent features become less prominent by distance, and their influence in the picture is also diminished by the increased vision which distance gives of the round of the head. The outline of the face and head varies essentially with the distance, and hence it is of great importance to choose the best. A long and narrow face requires to be viewed at a different distance from one that is short and round. Articles of dress even may have a better or a worse appearance according to the distance at which we see them.

Let us now suppose the proper distance to be six feet, and since it is impossible to give any rules for taking binocular portraits with large lenses we must assume a standard camera with a lens a quarter of an inch in diameter, as the only one which can give a correct picture as seen with one eye. If the portrait is wanted for a ring, a locket, or a binocular slide, its size is determined by its purpose, and the photographer must have a camera (which he has not) to produce these different pictures. His own camera will, no doubt, take a picture for a ring, a locket, or a binocular slide, but he does this by placing the sitter at different distances,—at a very great distance for the ring picture, at a considerable distance for the locket picture, and at a shorter distance for the binocular one; but none of these distances are the distance which has been selected as the proper one. With a single lens camera, however, he requires only several quarter-inch lenses of different focal lengths to obtain the portrait of the sitter when placed at the proper distance from the camera.

In order to take binocular portraits for the stereoscope a binocular camera is required, having its lenses of such a focal length as to produce two equal pictures of the same object and of the proper size. Those in general use for the lenticular stereoscope vary from 2.1 inches to 2.3 in breadth, and from 2.5 inches to 2.8 in height, the distance between similar points in the two pictures varying from 2.30 inches to 2.57, according to the different distances of the foreground and the remotest object in the picture.

Having fixed upon the proper distance of the sitter, which we shall suppose to be six feet,—a distance very suitable for examining a bust or a picture, we have now to take two portraits of him, which, when placed in the stereoscope, shall have the same relief and the same appearance as the sitter when viewed from the distance of six feet. This will be best done by a binocular camera, which we shall now describe.

The Binocular Camera.

This instrument differs from the common camera in having two lenses with the same aperture and focal length, for taking at the same instant the picture of the sitter as seen at the distance of six feet, or any other distance. As it is impossible to grind and polish two lenses, whether single or achromatic, of exactly the same focal length, even when we have the same glass for both, we must bisect a good lens, and use the two semi-lenses, ground into a circular form, in order to obtain pictures of exactly the same size and definition. These lenses should be placed with their diameters of bisection parallel to one another, and perpendicular to the horizon, at the distance of 2½ inches, as shewn in Fig. 45, where MN is the camera, L, L′ the two lenses, placed in two short tubes, so that by the usual mechanical means they can be directed to the sitter, or have their axes converged upon him, as shewn in the Figure, where AB is the sitter, ab his image as given by the lens L, and a′b′ as given by the lens L′. These pictures are obviously the very same that would be seen by the artist with his two eyes at L and L′, and as

ALB = aLb = a′L′b′,

the pictures will have the same apparent magnitude as the original, and will in no respect differ from it as seen by each eye from E, E′, Ea being equal to aL, and E′a′ to aL.

Since the publication in 1849 of my description of the binocular camera, a similar instrument was proposed in Paris by a photographer, M. Quinet, who gave it the name of Quinetoscope, which, as the Abbé Moigno observes, means an instrument for seeing M. Quinet! I have not seen this camera, but, from the following notice of it by the Abbé Moigno, it does not appear to be different from mine:—“Nous avons été à la fois surpris et très-satisfait de retrouver dans le Quinetoscope la chambre binoculaire de notre ami Sir David Brewster, telle que nous l’avons décrite après lui il y a dix-huit mois dans notre brochure intitulée Stéréoscope.” Continuing to speak of M. Quinet’s camera, the Abbé is led to criticise unjustly what he calls the limitation of the instrument:—“En un mot, ce charmant appareil est aussi bien construit qu’il peut être, et nous désirons ardemment qu’il se répand assez pour récompenser M. Quinet de son habileté et de ses peines. Employé dans les limites fixées à l’avance par son véritable inventeur, Sir David Brewster; c’est-à-dire, employé à reproduire des objets de petite et moyenne grandeur, il donnera assez beaux résultats. Il ne pourra pas servir, evidemment, il ne donnera pas bien l’effet stéréoscopique voulu, quand on voudra l’appliquer à de très-grands objets, on a des vues ou pay sages pris d’une très-grande distance; mais il est de la nature des œuvres humaines d’être essentiellement bornées.”[47] This criticism on the limitation of the camera is wholly incorrect; and it will be made apparent, in a future part of the Chapter, that for objects of all sizes and at all distances the binocular camera gives the very representations which we see, and that other methods, referred to as superior, give unreal and untruthful pictures, for the purpose of producing a startling relief.

In stating, as he subsequently does, that the angles at which the pictures should be taken “are too vaguely indicated by theory,”[48] the Abbé cannot have appealed to his own optical knowledge, but must have trusted to the practice of Mr. Claudet, who asserts “that there cannot be any rule for fixing the binocular angle of camera obscuras. It is a matter of taste and artistic illusion.”[49] No question of science can be a matter of taste, and no illusion can be artistic which is a misrepresentation of nature.

When the artist has not a binocular camera he must place his single camera successively in such positions that the axis of his lens may have the directions EL, EL′ making an angle equal to LCL′, the angle which the distance between the eyes subtends at the distance of the sitter from the lenses. This angle is found by the following formula:—

d being the distance between the eyes, D the distance of the sitter, and A the angle which the distance between the eyes, = 2.5, subtends at the distance of the sitter. These angles for different distances are given in the following table:—

The numbers given in the greater part of the preceding table can be of use only when we wish to take binocular pictures of small objects placed at short distances from cameras of a diminutive size. In photographic portraiture they are of no use. The correct angle for a distance of six feet must not exceed two degrees,—for a distance of eight feet, one and a half degrees, and for a distance of ten feet, one and a fifth degree. Mr. Wheatstone has given quite a different rule. He makes the angle to depend, not on the distance of the sitter from the camera, but on the distance of the binocular picture in the stereoscope from the eyes of the observer! According to the rule which I have demonstrated, the angle of convergency for a distance of six feet must be 1° 59′, whereas in a stereoscope of any kind, with the pictures six inches from the eyes, Mr. Wheatstone makes it 23° 32′! As such a difference is a scandal to science, we must endeavour to place the subject in its true light, and it will be interesting to observe how the problem has been dealt with by the professional photographer. The following is Mr. Wheatstone’s explanation of his own rule, or rather his mode of stating it:—

“With respect,” says he, “to the means of preparing the binocular photographs, (and in this term I include both Talbotypes and Daguerreotypes,) little requires to be said beyond a few directions as to the proper positions in which it is necessary to place the camera in order to obtain the two required projections.

“We will suppose that the binocular pictures are required to be seen in the stereoscope at a distance of eight inches before the eyes, in which case the convergence of the optic axes is about 18°. To obtain the proper projections for this distance, the camera must be placed with its lens accurately directed towards the object successively in two points of the circumference of a circle, of which the object is the centre, and the points at which the camera is so placed must have the angular distance of 18° from each other, exactly that of the optic axes in the stereoscope. The distance of the camera from the object may be taken arbitrarily, for so long as the same angle is employed, whatever that distance may be, the picture will exhibit in the stereoscope the same relief, and be seen at the same distance of eight inches, only the magnitude of the picture will appear different. Miniature stereoscopic representations of buildings and full-sized statues are, therefore, obtained merely by taking the two projections of the object from a considerable distance, but at the same time as if the object were only eight inches distant, that is, at an angle of 18°.”[50]

Such is Mr. Wheatstone’s rule, for which he has assigned no reason whatever. In describing the binocular camera, in which the lenses must be only 2½ inches distant for portraits, I have shewn that the pictures which it gives are perfect representations of the original, and therefore pictures taken with lenses or cameras at any other distance, must be different from those which are seen by the artist looking at the sitter from his camera. They are, doubtless, both pictures of the sitter, but the picture taken by Mr. Wheatstone’s rule is one which no man ever saw or can see, until he can place his eyes at the distance of twenty inches! It is, in short, the picture of a living doll, in which parts are seen which are never seen in society, and parts hid which are always seen.

In order to throw some light upon his views, Mr. Wheatstone got “a number of Daguerreotypes of the same bust taken at a variety of different angles, so that he was enabled to place in the stereoscope two pictures taken at any angular distance from 2° to 18°, the former corresponding to a distance of about six feet, and the latter to a distance of about eight inches.” In those taken at 2°, (the proper angle,) there is “an undue elongation of lines joining two unequally distant points, so that all the features of a bust appear to be exaggerated in depth;” while in those taken at 18°, “there is an undue shortening of the same lines, so that the appearance of a bas-relief is obtained from the two projections of the bust, the apparent dimensions in breadth and height remaining in both cases the same.”

Although Mr. Wheatstone speaks thus decidedly of the relative effect produced by combining pictures taken at 2½° and 18°, yet in the very next paragraph he makes statements entirely incompatible with his previous observations. “When the optic axes,” he says, “are parallel, in strictness there should be no difference between the pictures presented to each eye, and in this case there would be no binocular relief, but I find that an excellent effect is produced when the axes are nearly parallel, by pictures taken at an inclination of 7° or 8°, and even a difference of 16° or 17° has no decidedly bad effect!”

That Mr. Wheatstone observed all these contradictory facts we do not doubt, but why he observed them, and what was their cause, is a question of scientific as well as of practical importance. Mr. Wheatstone was not aware[51] that the Daguerreotype pictures which he was combining, taken with large lenses, were not pictures as seen with two human eyes, but were actually binocular and multocular monstrosities, entirely unfit for the experiments he was carrying on, and therefore incapable of testing the only true method of taking binocular pictures which we have already explained.

Had Mr. Wheatstone combined pictures, each of which was a correct monocular picture, as seen with each eye, and as taken with a small aperture or a small lens, he would have found no discrepancy between the results of observation and of science. From the same cause, we presume, namely, the use of multocular pictures, Mr. Alfred Smee[52] has been led to a singular method of taking binocular ones. In one place he implicitly adopts Mr. Wheatstone’s erroneous rule. “The pictures for the stereoscope,” he says, “are taken at two stations, at a greater or less distance apart, according to the distance at which they are to be viewed. For a distance of 8 inches the two pictures are taken at angles of 18°, for 13 inches 10°, for 18 inches 8°, and for 4 feet 4°.” But when he comes to describe his own method he seems to know and to follow the true method, if we rightly understand his meaning. “To obtain a binocular picture of anybody,” he says, “the camera must be employed to take half the impression, and then it must be moved in the arc of a circle of which the distance from the camera to the point of sight[53] is the radius for about 2½ inches when a second picture is taken, and the two impressions conjointly form one binocular picture. There are many ways by which this result may be obtained. A spot may be placed on the ground-glass on which the point of sight should be made exactly to fall. The camera may then be moved 2½ inches, and adjusted till the point of sight falls again upon the same spot on the ground-glass, when, if the camera has been moved in a true horizontal plane the effect of the double picture will be perfect.” This is precisely the true method of taking binocular pictures which we had given long before, but it is true only when small lenses are used. In order to obtain this motion in the true arc of a circle the camera was moved on two cones which converged to the point of sight, and Mr. Smee thus obtained pictures of the usual character. But in making these experiments he was led to take pictures when the camera was in continual motion backwards and forwards for 2½ inches, and he remarks that “in this case the picture was even more beautiful than when the two images were superimposed!” “This experiment,” he adds, “is very remarkable, for who would have thought formerly that a picture could possibly have been made with a camera in continual motion? Nevertheless we accomplish it every day with ease, and the character of the likeness is wonderfully improved by it.” We have now left the regions of science, and have to adjudicate on a matter of opinion and taste. Mr. Smee has been so kind as to send me a picture thus taken. It is a good photograph with features enlarged in all azimuths, but it has no other relief than that which we have described as monocular.

A singular effect of combining pictures taken at extreme angles has been noticed by Admiral Lageol. Having taken the portrait of one of his friends when his eyes were directed to the object-glass of the camera, the Admiral made him look at an object 45°! to the right, and took a second picture. When these pictures were placed in the stereoscope, and viewed “without ceasing, turning first to the right and then to the left, the eyes of the portrait follow this motion as if they were animated.”[54] This fact must have been noticed in common stereoscopic portraits by every person who has viewed them alternately with each eye, but it is not merely the eyes which move. The nose, and indeed every feature, changes its place, or, to speak more correctly, the whole figure leaps from the one binocular position into the other. As it is unpleasant to open and shut the eyes alternately, the same effect may be more agreeably produced in ordinary portraits by merely intercepting the light which falls upon each picture, or by making an opaque screen pass quickly between the eyes and the lens, or immediately below the lens, so as to give successive vision of the pictures with each eye, and with both. The motion of the light reflected from the round eyeball has often a striking effect.

From these discussions, our readers will observe that the science, as well as the art of binocular portraiture for the stereoscope, is in a transition state in which it cannot long remain. The photographer who works with a very large lens chooses an angle which gives the least unfavourable results; his rival, with a lens of less size, chooses, on the same principle, a different angle; and the public, who are no judges of the result, are delighted with their pictures in relief, and when their noses are either pulled from their face, or flattened upon their cheek, or when an arm or a limb threatens to escape from their articulation, they are assured that nature and not art is to blame.

We come now to consider under what circumstances the photographer may place the lenses of his binocular camera at a greater distance than 2½ inches, or his two cameras at a greater angle than that which we have fixed.

1. In taking family portraits for the stereoscope, the cameras must be placed at an angle of 2° for 6 feet, when the binocular camera is not used.

2. In taking binocular pictures of any object whatever, when we wish to see them exactly as we do with our two eyes, we must adopt the same method.

3. If a portrait is wanted to assist a sculptor in modelling a statue, a great angle might be adopted, in order to shew more of the head. But in this case the best way would be to take the correct social likeness, and then take photographs of the head in different azimuths.

If we wish to have a greater degree of relief than we have with our two eyes, either in viewing colossal statues, or buildings, or landscapes, where the deviation from nature does not, as in the human face, affect the expression, or injure the effect, we must increase the distance of the lenses in the binocular camera, or the angle of direction of the common camera. Let us take the case of a colossal statue 10 feet wide, and suppose that dissimilar drawings of it about three inches wide are required for the stereoscope. These drawings are forty times narrower than the statue, and must be taken at such a distance, that with the binocular camera the relief would be almost evanescent. We must therefore suppose the statue to be reduced n times, and place the semi-lenses at the distance n × 2½ inches. If n = 10, the statue 10 feet wide will be reduced to ¹⁰/₁₀, or to 1 foot, and n × 2½, or the distance of the semi-lenses will be 25 inches. With the lenses at this distance, the dissimilar pictures of the statue will reproduce, when combined, a statue one foot wide, which will have exactly the same appearance and relief as if we had viewed the colossal statue with eyes 25 inches distant. But the reproduced statue will have also the same appearance and relief as a statue a foot wide reduced from the colossal one with mathematical precision, and it will therefore be a better or more relieved representation of the work of art than if we had viewed the colossal original with our own eyes, either under a greater, an equal, or a less angle of apparent magnitude.

We have supposed that a statue a foot broad will be seen in proper relief by binocular vision; but it remains to be decided whether or not it would be more advantageously seen if reduced with mathematical precision to a breadth of 2½ inches, the width of the eyes, which gives the vision of a hemisphere 2½ inches in diameter with the most perfect relief.[55] If we adopt this principle, and call B the breadth of the statue of which we require dissimilar pictures, we must make n = B/2½, and n × 2½ = B, that is, the distance of the semi-lenses in the binocular camera, or of the lenses in two cameras, must be made equal to the breadth of the statue.

In concluding this chapter, it may be proper to remark, that unless we require an increased relief for some special purpose, landscapes and buildings should be taken with the normal binocular camera, that is, with its lenses 2½ inches distant. Scenery of every kind, whether of the picturesque, or of the sublime, cannot be made more beautiful or grand than it is when seen by the traveller himself. To add an artificial relief is but a trick which may startle the vulgar, but cannot gratify the lover of what is true in nature and in art.

The Single Lens Binocular Camera.

As every photographer possesses a camera with a lens between 2½ and 3 inches in diameter, it may be useful to him to know how he may convert it into a binocular instrument.

In a cover for the lens take two points equidistant from each other, and make two apertures, c, d, Fig. 43, ²/₁₀ths of an inch in diameter, or of any larger size that may be thought proper, though ²/₁₀ is the proper size. Place the cover on the end of the tube, and bring the line joining the apertures into a horizontal position. Closing one aperture, take the picture of the sitter, or of the statue, through the other, and when the picture is shifted aside by the usual contrivances for this purpose, take the picture through the other aperture. These will be good binocular portraits, fitted for any stereoscope, but particularly for the Achromatic Reading Glass Stereoscope. If greater relief is wanted, it may be obtained in larger lenses by placing the two apertures at the greatest distance which the diameter of the lens will permit.

The Binocular Camera made the Stereoscope.

If the lenses of the binocular camera, when they are whole lenses, be made to separate a little, so that the distance between the centres of their inner halves may be equal to 2½ inches, they become a lenticular stereoscope, in which we may view the pictures which they themselves create. The binocular pictures are placed in the camera in the very place where their negatives were formed, and the observer, looking through the halves of his camera lenses, will see the pictures united and in relief. If the binocular camera is made of semi-lenses, we have only to place them with their thin edges facing each other to obtain the same result. It will appear, from the discussions in the following chapter, that such a stereoscope, independently of its being achromatic, if the camera is achromatic, will be the most perfect of stereoscopic instruments.

The preceding methods are equally applicable to landscapes, machines, and instruments, and to solid constructions of every kind, whether they be the production of nature or of art.[56]