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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.
ENLARGEMENT OF NEGATIVES BY THE ORDINARY CAMERA.
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.
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