A Manual Of Photography

308 PRACTICE OF PHOTOGRAPHY. second image. « may represent the object to be copied, 6 being the distance at which the camera c is placed, which is, say 100 feet from the point a: the picture from this point being taken, the camera is moved round, still preserving the same distance from the object by means of a cord attached to the camera obscura, or by measuring the space. Two cameras with lenses of the same focal length may be employed, and are indeed employed, by M. Claudet and others for the purposes of obtaining stereoscopic portraits, and it is found that with lenses of the same focus the figures are sufficiently exact for all practical purposes, and produce the most perfect stereoscopic pictures. Sir David Brewster con- tends that it is not practicable to obtain sufficient exactness by either of these methods. He therefore proposes the use of a binocular camera, which he thus describes :—" In order to obtain photographic pictures mathematically exact, we must construct a binocular camera, which will take the pictures simultaneously, and of the same size ; that is, a camera with two lenses of the same aperture and focal length, placed at the same distance as the two eyes. As it is impossible to grind and polish two lenses, whether single or achromatic, of exactly the same focal lengths, even if we had the very same glass for each, I propose to bisect the lenses, and construct the instru- ment with semi-lenses, which will give us pictures of precisely the same size and definition. These lenses should be placed with their diameters of bisection parallel to one another, and at a distance of 2| inches, which is the average distance of the eyes in man ; and when fixed in a box of sufficient size, will form a binocular camera, which will give us, at the same instant, with the same lights and shadows, and of the same size, such dissimilar pictures of statues, buildings, landscapes, and living objects, as will reproduce them in relief in the stereoscope.” There appears but one objection to the binocular camera of Sir David Brewster, and that is one arising from the circum- stance of employing really the very worst portion of the lens; i.e., the two sides. This, however, in practice, is not found to be of any disadvantage; the images are sufficiently perfect, although not so absolutely correct as those formed by the centre