In ordinary photographic work, pencils having an obliquity of from to are introduced, and the lens has to be so con structed as to meet this difficulty ; but the form of lens best calcu. lated for giving a flat field when pencils of great obliquity occur, is not that which at the same time gives the greatest distinctness of focus of the central pencils ; that is to say, if the central pencils alone had to be corrected in the best possible way for spherical aberration, the form of lens which this condition would impose would not be such as to satisfy at the same time the conditions necessary for flatness of field when very oblique pencils are introduced. When, therefore, the necessity for obviating the defects due to obliquity is aome measure removed, the form of the lens may be so modified as to remedy more perfectly the defects of central pencils. In the lens of a copying camera, therefore, the posterior lens need not be much larger than the anterior; in fact, a single achromatic lens, like the olaject-glass of a telescope, may be employed. But this subject has not yet received the attention which its importance deserves, and the best possible form of lens for the copying camera remains yet to be investigated.
In copying a small photograph or print on a larger scale, the projecting front of the camera should be continued until it nearly touches the picture, and this should be illuminate,d as strongly as possible, either by the sun, or strong diffused daylight. NVIen artificial light is employed (the oxycalchun light, for instance), it may be brought very near to the end of the camera, and its light C01.1• centrated on the picture by means of a convex lens, as shown in Fig. 6.
Whenever the light is sufficiently intense to allow of a stop beano. used, it should certainly be introduced as a remedy for the unavoig able defects of the lens. When an achromatic convex lens is used, like that in Fig. 6, the stop should be placed immediately in contact with it, either in front or behind.
Small bas-reliefs may be copied in this way very successfully, by throwing the light obliquely on them, and using a reflector to diminish the intensity of the shadows. It is generally found difficult in practice to place the plane of the sensitive surface accurately parallel to the plane of the picture to be copied, and unless this is done, the lines of the copy are distorted. Where the original is small, it naay be placed on a holder connected with the bottom of the camera, as shown in Pig. 6 ; but, if this is not possible, the end of the sliding part of the camera should be fitted with the arrange ment employed in the ordinary camera, for allowing the plane of the picture to be inclined at any small angle to the axis of the lens. By means of these adjustments, any distortion of the image on the ground glass may be easily remedied.
When the picture to be copied is transparent, it should, if pos sible, be inserted within the camera, the front of which should be sufficiently lengthened to receive it, and the light which is transmitted through it should either proceed from the sky, or a large white illuminated disc, or an artificial light placed behind a condenser. When practicable, the sky should always be used as a luminous background, in preference to any other kind of light. This will be fully explained in the article on " Micro-Photography." When the sky is used as the source of transmitted light, the instrument may be mounted on a stand, in such a way as to turn on an axis, to permit of its being directed to any part of the heavens. When the copying camera is placed with its axis vertical, the free nitrate on a sensitive plate does not drain towards the lower part, and the picture is con sequently more uniform in density. Whenever it is practicable, therefore, to point the instrument towards the zenith, this should certainly be done.
Having now explained the principle of the construction of a copying camera, it will be unnecessary to enter further into detail, as the operator will find no difficulty in modifying his arrangements to suit any particular case.
Solar Camera.—This is a form of copying camera, in which the sun's 'rays are transmitted through a transparent negative. It only differs from the copying camera in which artificial light is employed, in the form of the condenser necessary for modifying the course of parallel rays, and preventing the formation of an image of the sun between the lens and the sensitive surface. This form of condenser will be described in the article on " Micro-Photography." Stereoscopic Camera.—The stereoscopic camera is employed for taking duplicate pictures of objects, from different stations, to be viewed in the stereoscope.
There are two forms of stereoscopic camera. The first arrange ment merely relates to the camera stand, which is so contrived as to allow the camera to be shifted from one end of it to the other, and at the same time have its axis always directed towards the same point. In this way the two pictures are taken separately, in an ordinary single camera, and the contrivance, which merely affects the stand, should be called by some appropriate name.
Since the above mode of taking stereoscopic pictures involves an error of principle, which will be pointed out, it becomes unnecessary to describe the various modes by which an operation that is errone ous in principle is rendered more or less convenient in practice. It will be sufficient to point out clearly in what this error consists, and then to pass on to the description of the other form of stereoscopic camera which is scientifically conrect.