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Direct Enlargements in Platinum Paper.

The platinum process has, as a rule, been devoted to the production of prints by contact,
but some years ago, in America, and also at the present time, enlargements have been made
direct on the paper by means of the electric light. In a paper read before the Camera Club,
one of us showed examples of enlargements made by himself, with an arc electric light of
about 1100 candle power. This electric light is of but small power compared with that which
can be obtained, as it consumed only about 1100 watts of electricity. The apparatus
required for this light is very limited, where the enlargement required is not more than nine
times the area, and every photographer who has a direct current of electricity at his
command, is in a position to avail himself of it. It is proposed in this chapter to describe the
apparatus and the methods of adjustment when using the electric light of this dimension, and
also with solar light.
We will suppose that we are limiting ourselves to the enlargement of a quarter-plate of twice
linear.
In the first place we must have two condensers, preferably plano-convex, sufficient in diameter
for our purpose, and it is convenient that they should be about nine inches focal length each.
They can be obtained very cheaply, of cast glass, and there is no great precaution necessary
to take in selecting them, beyond seeing that they are fairly free from bubbles; if many
bubbles are present the print is apt to show images of them, though, by keeping the
negative a little way from the lens, they can be made to disappear. We can find the necessary
diameter of the condensers by taking the length of the diagonal of the quarter-plate to the
diameter of the condenser. It is a little over five inches, but it is convenient to allow an extra
inch, which will make them six inches in diameter. This allows the negative to be in the
cone of rays even when some distance from either one of them, a very necessary point, as
we shall see when focussing. A convenient way of setting up the apparatus is as follows: The
condensers are mounted in two firm woodenframed stands, with a device enabling their
heights to be altered at pleasure. Their centres are so adjusted (Fig. 4). One of the condensers,
C2, is placed in front of the electric light, E, with its plane side towards it in such a position
that its axis passes through the electric arc. The distance of this lens is guided by its focal
length. The arc should be placed just within its focal distance, so that it throws a circle of light,
a little bit larger than the diameter of the lens, on a screen placed about twelve inches from it.
In this disc of light the second condenser, C1, is placed, and it should throw a cone of rays
coming to an approximate focus of about seven inches from it. In this case the plane side of the
condenser is away from the light. A photographic lens of some seven and a half
inches focus, and which has a fairly flat field, is then, without any stop being inserted, held in a
perfectly rigid camera front, or in a clamp, and so placed that the focus of the rays, coming
from the condenser, falls within its front combination. The negative, N, to be enlarged
is then inserted in a proper holder in front of the second condenser in such a manner that the
image, when sharp, is of the required size on a board, S, to which a sheet of white paper has been
pinned. It frequently happens that a circle of orange and blue light is seen cutting the picture,
this shows that the adjustments are not accurate. If the coloured circle cuts the corners of the picture
uniformly, it must be got rid of by moving the lens backwards till a clear circular white disc of
the necessary size is obtained, and the negative must be moved backwards or forwards from
the condenser, till a sharp image is again secured. Patches of blue are due to the light
from the arc in contradistinction to that of the poles. The positive pole of the electric light
should be the top pole, and be thrown a little behind the bottom or negative pole, and the
current should be sufficiently strong to allow the white-hot crater of the positive pole to be
clear of the latter, for it is this crater which creates the most photographically active light.
When patches of blue light are seen they indicate that this position of the crater has not
been secured. That the most photographically active light proceeds from the crater is easily
proved by throwing an image of the poles on a piece of chloride paper by means of a lens,
when it will be found that the image of the crater blackens the paper much more rapidly
than that of any other portion of the luminous image.
When the focus of the image is correct, platinum paper is placed on the focussing
paper, and the exposure is given. With an electric light of about 1100 candles, the time
for exposure for a thinnish negative will be from about 15 to 25 minutes. With a denser
negative it will, of course, be longer; but it rarely happens that any negative will take more
than an hour to print. For the purposes of enlargement the deepest shadows of the
negative should be as nearly bare glass as possible, and any yellowness should be
avoided. It should be remarked that the density of the negative, to give a good enlargement,
may be less than that required for a contact print; in fact, a rather feeble negative, if very bright, will
give the best results. It is a good precaution to place a sheet of glass between the electric light
and C2.
When sunlight is used, very much the same procedure is carried out; but the condenser C2,
next the electric light, need not always be used,
if the photographic lens employed is exactly the proper focus. A large looking-glass, M, is
employed to reflect the light on to the condenser next to the lens, and care is taken
that the focus of the beam of the condenser falls within the lens; so long as it does not
travel outside, it does not matter if it is not always quite central. Supposing the lens to be
of slightly too long or too short a focal length for the image (Fig. 5) of the necessary size, the
focus of the condenser lying either too near or too far from the photographic lens, L, to give a
proper illumination of the field where the size is correct, then the use of the second
condenser, C2, becomes necessary. In Fig. 5 it will be seen that the position of F1, the focus
of C2, can be caused to recede or approach C1. The further F1 is from it, the shorter will be
the focus of C1, and the closer it is (of course, within limits), the longer will be the
focus. Hence C1 is placed at such a distance from the condenser (behind which is the
negative, N), the focus of the latter being altered to the necessary extent, that it falls
exactly on the photographic objective. The great point to observe is that the centres of the
condensers be on the same axis. To secure this the objective is removed, and the mirror turned
so that light coming through the aperture cut for the lens, occupies the centre of the
focussing screen. The first lens is then approximately focussed, the small patch of light
formed being also in the centre of the screen.
The second condenser is then placed in position to five the same result, after which the
photographic objective is replaced. The negative is then put in position, the image
focussed to the proper size, and the back condenser moved parallel to its position till a
uniformly lighted field is secured.
The printing with a bright sun is quicker than with the electric light, and a good print may
often be obtained in ten minutes, or even less.This will be understood when it is
considered that the light which falls on the negative is simply spread out to fall on the area
of the enlargement. There is no loss of light by reflection and absorption, and it may be
concluded that the exposure is about three times that which calculation would give.
Suppose a negative has to be enlarged twice linear, and that a contact print can be obtained
in one minute, then it will take twelve minutes to make the enlargement. There is nothing
peculiar in manipulating the paper; the great point is that it should be flat, and it is well to
leave it ten minutes lying on a table in the room where the enlargement is to be made,
before pinning it in position, so that it may posses the same moisture as the air in which it
has to be exposed. Arrangements should be made to prevent diffused light reaching it
during exposure, and very obvious precautions should be taken to effect this. The prints
produced are, as a rule, more even than the contact prints, for the margins of the print
receive slightly less exposure than the centres, and, as of necessity, the margins of a negative
are slightly less dense than its centre, on account of the want of perfectly equal
illumination of the field, a compensation is made in the camera prints.
A small calculation will show the relative values of the printing powers of sunlight and the
electric light. Sunlight has the same value, optically, as 5000 candles when placed one foot
off the screen. The electric light used has the value of 1100 candles. Hence, the value of
sunlight is about 4 times that of the electric light, when placed one foot off. All we have to
do is to compare the values of the two sources when falling on the condenser, which is next
the photographic lens. If the light be six inches from the first condenser, evidently four times
as much light will fall on it as would be the case if it were one foot off, but the area of the
disc of light in which the second condenser is placed is at least eight inches in diameter. As
the first condenser is six inches in diameter, the light on the second condenser is, therefore,
only or (say half) of that falling on the first. Had it all fallen on it, it would have been equal
to 4400 candles, but owing to the increased disc, it is only equal to 2200 candles, and as the
photographically active rays in sunlight are about 1.3 times greater than that of the electric
light, it follows that the 2200 must be divided by 1.3, or the value of the light formed. The
intensity of the electric light compared with that of the sun is as 1700 to 5000, that is, the
latter will print three times more quickly than the former, always supposing the sunlight is
that near midday and from an unclouded sun with a dark sky. It may be of interest to note
the fact that the different thicknesses of glass cut off approximately the same amount of the
photographically active rays. Roughly, it may be said that of these rays reachthe plate, the
different amounts of surface reflection due to the varying curvature of the glass, probably
making up for the different absorptions.