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 = _a_L_b_ = _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_a_ being equal to _a_L, and E′_a′_ to _a_L. [Illustration: FIG. 46.]