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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 2 J 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 2£ 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 = aL& = 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', E« being equal to aL, and e'a' to Ol.
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 6t6 a la fois surpris et tres-satisfait de retrouver dans le Quinetoscope la chambre binoculaire de notre ami Sir David Brewster, telle que nous 1'avons decrite apres lui il y a dix-huit mois dans notre brochure intitulée Stereoscope." 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, évidemment, il ne donnera pas bien l'effet stêrêoscopique voulu, quand on voudra rappliquer à de très-grands objets, on a des vues ou paysages pris d'une très-grande distance; mais il est de la nature des œuvres humaines dêtre essentiellement bornées."i 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 veryrepresentations 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,"2 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."^ No question of science can be a matter of taste, and no
i See Comet, vol. U. pp. 622, 624. • Id. Tol vii. p. 494.
a Id. Tol iii. p. 658.