Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XXV.

HAVING obtained a sharp transparent positive, it is evident that, by a reverse process, a negative may be reproduced, and of course as many negatives as may be required. It is thus that photographic negatives may be stereotyped. Not only can we thus procure a matrix for the reproduction of a valued negative, (a proviso which ought never to be omitted,) but from such a transparent positive may be obtained enlarged negatives. The enlargement depends upon the capacity of the lens of the camera. The bellows part of the latter admits of greater elongation and correlative lateral expansion than that of the ordinary camera. As soon as we have found the distance of equal conjugate foci, as before directed, then by diminishing the distance between the positive and the lens, we increase the distance between the lens and the new negative. (The transparent positive is placed in the opening in front of the lens, where originally the negative was placed.) But in the same proportion as this distance is increased, in like manner is the new negative enlarged. The amount of enlargement* will depend, as boon as the camera is arranged, upon the perfection of the lens, which, be it ever so good, leas to be stopped down to a small aperture, in order to overcome spherical aberration, which causes distortion; and detracts from the sharpness on the peripheral parts. With the bright light of the sun there is no difficulty in thus obtaining a negative magnified ten times diametrically with such a lens, and in a very reasonable time. Thus a stereoscopic portrait or view maybe enlarged into a cabinet-sired picture or landscape, with bat a small expenditure of time and expense. Nor is a large lens required for this operation. The same lens with which the original negative was taken may be applied to the purposes of enlargement. In making enlarged negatives, however, we require particularly a greater amount or a greater intensity of light, so that with a given light the exposure must be so much the longer. In such cases, then, where the enlargement is as great as before mentioned, it is advisable to construct a. system of reflectors in front of the aperture for the reception of the negative or positive.

Reflectors used as Condensers of Light.

Let the aperture for the negative, etc., be four inches square; then construct a frustum of a pyramid out of four pieces of silvered glass, of the following dimensions: The barrow end of each piece is four inches, the broad end is 14 78/100 inches; the length of either side is 21 56/100 inches. Fix these pieces of glass in a tin frame, with the silvered side inward, and attach the frustum to the aperture for the negative. When the latter or a transparent positive is in its place, turn the camera (which for this purpose must be fixed upon a universal joint) toward the sun; it will be found that the intensity of the light has been greatly increased. Such a condensing reflector is calculated to condense all the rays that fall upon it, either by one or two reflections, so that they all fall upon the negative. But the amount of light that impinges directly upon the larger base of the frustum is at least thirteen times greater than that which falls upon the smaller base; and if there were no loss of actinic power by reflection, the light condensed on the negative would be thirteen times more than would impinge upon it without the aid of the condensers. If then the light be increased by ten times in intensity, and the picture be enlarged by ten times, the time of exposure would remain the same.

* Vide Chapter for the table of distances and magnitudes.