[CHAPTER IX.]
On a mode of Printing enlarged and reduced Positives, etc., from Collodion Negatives.
To explain the manner in which a photograph may be enlarged or reduced in the process of printing, it will be necessary to refer to the remarks made at [page 20], on the conjugate foci of lenses.
If a collodion negative be placed at a certain distance in front of a camera, and (by using a tube of black cloth) the light be admitted into the dark chamber only through the negative, a reduced image will be formed upon the ground glass; but if the negative be advanced nearer, the image will increase in size, until it becomes first equal to, and then larger than, the original negative; the focus becoming more and more distant from the lens, or receding, as the negative is brought nearer.
Again, if a negative portrait be placed in the camera slide, and if the instrument be carried into a dark room, a hole be cut in the window-shutter so as to admit light through the negative, the luminous rays, after refraction by the lens, will form an image of the exact size of life upon a white screen placed in the position originally occupied by the sitter. These two planes, in fact, that of the object and of the image, are strictly conjugate foci, and, as regards the result, it is immaterial from which of the two, anterior or posterior, the rays of light proceed.
Therefore in order to obtain a reduced or enlarged copy of a negative, it is necessary only to form an image of the size required, and to project the image upon a sensitive surface either of collodion or paper.
A good arrangement for this purpose may be made by taking an ordinary portrait camera, and prolonging it in front by a deal box blackened inside and with a double body, to admit of being lengthened out as required; or, more simply, by adding a framework of wood covered in with black cloth. A groove in front carries the negative, or receives the slide containing the sensitive layer, as the case may be.
In reducing photographs, the negative is placed in front of the lens, in the position ordinarily occupied by the object; but in making an enlarged copy it must be fixed behind the lens, or, which is equivalent, the lens must be turned round so that the rays of light, transmitted by the negative, enter the back glass of the combination, and pass out at the front. This point should be attended to in order to avoid indistinctness of image from spherical aberration.
A portrait combination of lenses of 2½ or 3¼ inches is the best form to use, and the actinic and luminous foci should accurately correspond, as any difference between them would be increased by enlarging. A stop of an inch or an inch and a half aperture placed between the lenses obviates to some extent the loss of sharp outline usually following enlargement of the image.