high-lights, halated windows, etc., and this is often very valuable,
especially in the case of under-exposed negatives. A small quantity
of hypo and ferricyanide solution is mixed in a measuring glass or
some other suitable vessel. The plate is immersed in plain hypo
solution in a white dish for a short time and is then raised by one
corner or one edge until the part to be reduced is above the
solution. The mixture of hypo and ferricyanide is carefully applied
with a camel's hair brush to the parts that are too opaque, and
after a few moments the plate is allowed to slip back into the hypo
solution and the dish is rocked. If the reduction is not sufficient,
the same proceeding is gone through as often as necessary. The
reducer should not be allowed to act too long before putting the
plate back into the hypo, otherwise the reduction may spread further
than is desired. Further, the reducer must not be too strong
(_i.e._, contain too much ferricyanide), otherwise it will produce
brownish stains and the action may be too energetic.
The other reducer is known as Belitzski's reducer, and is made up as
follows:--
Ferric potassium oxalate 1 oz. or 5 parts[5]
Sodium sulphite 1 oz. or 4 parts
Oxalic acid 1/4 oz. or 1 part
Hypo solution (25 in 100) 5 oz. or 25 parts
Water 20 oz. or 100 parts
[5] The formula in "parts" does not strictly correspond with
that in ounces, but the difference is immaterial.
The constituents must be dissolved in water in the order given. The
solution can be used at once and it keeps fairly well if protected
from light, in well corked bottles filled up to the neck.
INTENSIFICATION.
Intensification is a process in which the opacity of the image is
increased by adding some fresh matter, metallic or otherwise, to the
reduced silver that constitutes the developed image.
The usual plan is to bleach the image by means of a solution of
mercuric chloride (mercury perchloride or corrosive sublimate),
which converts the dark-coloured silver into a white mixture of
silver chloride and mercurous chloride, and this is subsequently
treated with some re-agent which will reconvert the image into a
dark product of greater opacity than the original.
It is absolutely essential to successful intensification that the
negative be completely fixed and completely washed after fixing, for
any trace of hypo left in the film will give rise to brown stains.
It is also important, in order to prevent stains of another sort and
to secure uniform action, that the mercuric chloride solution be
mixed with a small quantity of hydrochloric acid. Too much acid will
cause frilling. If the negative has been dried it must be immersed
in water for, as a rule, not less than half-an-hour, in order that
it may be thoroughly and uniformly wetted.
MERCURIC CHLORIDE SOLUTION.
Mercuric chloride 1 oz. or 5 parts
Hydrochloric acid 1-1/2 drachms or 1 part
Water to make up to 20 oz. or 100 parts
When uniform intensification is required the negative is allowed to
remain in this solution until it is completely bleached. If,
however, it is desired to intensify the shadows more than the
high-lights, the plate should be removed from the solution as soon
as the shadows have bleached, and should be rapidly washed in order
to stop the action on the more opaque parts of the image.
In either case the negative must be thoroughly washed after
bleaching, and the water used must be soft water. Hard water tends
to produce a precipitate of the mercury salt in the film, which may
subsequently lead to stain or fog.
Perhaps the best plan of all, when constant results are desired, is
to treat the bleached negative with the ferrous oxalate developer,
which will gradually convert the white image into a black one, after
which the plate is thoroughly washed and dried. It is recommended
that the first water used for washing should be slightly acidified
with oxalic acid.
Instead of using ferrous oxalate the bleached plate may be treated
with a weak solution of ortol or metol to which some sodium
carbonate (soda crystals) solution has been added, but _no
sulphite_. After the image has blackened completely the plate is
washed.
With any of these methods if the first intensification is not
sufficient, the plate may be again bleached with the mercury
solution and the process repeated.
An old method, frequently used, is to treat the bleached plate with
dilute ammonia, which converts the white image into a dark brown one
of very considerable printing opacity. The results are often very
good, but are somewhat uncertain, since the precise effect obtained
depends on the strength of the ammonia solution and the time during
which it is allowed to act. With somewhat strong ammonia, allowed to
act for a fairly long time, part of the intensification first
produced is removed. This affects the shadows more strongly than the
lights and the result is to increase the contrast of the negative,
which is very useful for certain purposes.
The negatives intensified with mercury solution followed by ammonia
are more liable to spontaneous change and deterioration than those
intensified with mercury solution followed by one of the developers.
The latter, in fact, if properly washed, may safely be regarded as
permanent.
URANIUM INTENSIFIER.--A very considerable degree of intensification
can be obtained by the use of the uranium intensifier, which is very
different in its mode of action, and is a little uncertain in its
results. A solution containing potassium ferricyanide and a uranium
salt, generally the nitrate, is applied to the negative, and a
deposit of a deep orange-red colour is formed upon the silver image
and very greatly increases its printing opacity. The great
difficulty is to prevent this deposit forming on the whole of the
film, and it is absolutely necessary that every trace of hypo should
be washed out of the film. The addition of acetic acid to the
solution not only promotes uniformity of action, but also helps to
keep the shadows of the image clear.
FERRICYANIDE SOLUTION.
The same as for the ferricyanide reducer.
URANIUM SOLUTION.
Uranium nitrate 1 oz. or 10 parts
Water to make up to 10 oz. or 100 parts
THE INTENSIFIER.
Uranium solution (1:10) 1 drachm or 5 parts
Ferricyanide solution (1:10) 1 drachm or 5 parts
Acetic acid (glacial) 2 drachms or 10 parts
Water to make up to 2-½ oz. or 100 parts
The negative is placed in this solution and allowed to remain with
occasional rocking until the degree of intensification is sufficient,
which can only be learnt by experience. If it is seen that the
deposit is beginning to form on the clear parts of the negative, the
plate should be at once removed. After intensification the plates are
well washed. If the water is "hard" the intensification will be
slightly reduced during washing, and this is often useful in removing
a slight stain over the whole of the plate. Treatment with water
containing a small quantity of ammonia or sodium carbonate removes
the whole of the deposit, but leaves the original image slightly
reduced and also partially altered in composition.
VARNISHING.
A negative after been thoroughly dried may be used for printing
without any further treatment, especially if only a few prints are
required and the ordinary ready sensitized papers or emulsion
papers are used. It is, however, better to protect the negative from
mechanical as well as chemical injury by means of a film of hard
varnish or collodion.
Many excellent negative varnishes can now be purchased, and the
general mode of application is the same. The negative must be
thoroughly dry, and in order to secure this and to make the varnish
flow more easily, the negative is very carefully heated in front of
a fire or over a small stove until it is just warm, but not hot. The
negative is best supported by means of a pneumatic holder held in
the left hand, and a fairly large pool of varnish (the exact amount
can only be learnt by experience) is poured on the plate somewhat
towards the right-hand top corner, and by carefully tilting the
plate it is made to run first to the nearest corner, then along the
edge to the further left-hand corner down to the nearer left-hand
corner, and back to the right-hand bottom corner, from which it is
poured into a bottle. The plate is gently rocked whilst it drains
into the bottle, and as soon as the varnish ceases to drop the plate
is again carefully warmed until the back of it is just too hot for
the back of the hand to bear, after which it is placed in a rack to
cool.
It is necessary that the varnish should be quite clear and free from
any solid particles, and if necessary it must be filtered through a
plug of cotton wool moistened with alcohol and placed in the apex of
a glass funnel which is resting in the neck of a clean and dry
bottle. Since dust may fall into the varnish whilst it is on the
negative, it is the best plan to pour the excess of varnish off the
negative into a second bottle instead of back into the first, out of
which it was poured. To put it in another way, one bottle should be
kept for the clear varnish, and a second bottle for the varnish
poured off the plate. When the second bottle is full, its contents
are filtered into the first bottle for use again.
Instead of varnish, a film of collodion, toughened by the addition
of a few drops of castor oil, and known as "leather" collodion, may
be used. The collodion is applied to the plate in the same way as
varnish except that the plate is not warmed.
_C. H. Bothamley._
[Illustration]
_Lenses._
[Illustration]
Photographs of flat objects such as leaves, lace, drawings, etc.,
can be made by simply putting the object on the sensitive surface
and exposing the arrangement to light. But this method will not
serve if the photograph is wanted of any other size than the
original, nor with solid objects of any size, except perhaps in the
production of full-size profiles of faces. It is therefore quite the
exception in photography to "print" directly from the object itself,
and the only alternative is to produce an image on the sensitive
surface.
All illuminated objects reflect light and so become for practical
purposes sources of light, just as the moon shines, as we say,
although it only shines because it is shone upon by the sun. The
simplest source of light to consider is a point of light, and if we
can get a dot of light on a white surface from a point of light we
have at once an image of that point of light. The smaller the dot
the sharper or more perfect is the image, the larger the dot the
more diffused or fuzzy is the image. It is impossible by any known
means to get the dot so small that it is an actual point, that would
be absolute perfection, and on the other hand there is no size of
the dot at which it can be definitely said that it ceases to be an
image. Every point of an illuminated object is a point of light, and
fine definition consists in keeping these points separate in the
image. So far as the dots overlap they are confused. Confusion, or
diffusion, or fuzziness is sometimes desirable, as for example in a
portrait, which may be excellent although it is impossible to
distinguish in the picture the individual hairs on the person's
head.
[Illustration: Fig. 1.]
The simplest means for getting an image is a small hole in an opaque
screen. In fig. 1, two points of light, A and B, shine through the
hole in the screen S and produce two dots of light, _a_ and _b_, on
the surface T. The two pencils of light do not practically interfere
with each other although they pass through the same small hole, nor
would any greater number; so that an illuminated object, which may
be regarded as consisting of an infinite number of points of light,
would give an image on the surface T. The disadvantages of a small
hole, or "pinhole," for the production of images are (1) it must be
so small that it lets very little light through and therefore gives
a very feeble image, (2) that it can never give a sharp image. The
first disadvantage is obvious. With regard to the second, a little
consideration will show that the image of a point must be larger
than the hole itself, it is always larger though it may have a
central brighter part that is smaller. If the hole is reduced in
size beyond a certain limit, it gives an increased spreading of
light on the surface, so that a sharp image can never be produced.
[Illustration: Fig. 2.]
Now the function of a lens is to obviate these drawbacks as far as
possible; namely, to let more light through and form a brighter
image, and to give sharper definition. In figure 2, the lens L
collects all the light that falls upon it from the point B, and
condenses it to the point _b_ on the surface T. The light from the
point A that falls on the lens is also condensed and would be
brought to a point or "focus" at _a_ beyond the surface T, but on
the surface the light forms a patch of considerable size. Suppose
that the lens is thirty times the diameter of the pinhole its area
is 900 times as large, and the light that falls upon it is 900 times
as much as the light that passes through the hole. Such an enormous
gain of light is worth so much that photographers willingly put up
with the very many imperfections of lenses for the sake of it, and
if to this gain there is added the superior definition that is
possible, it will be seen that lenses are indispensable to the
photographer. To take a Daguerreotype portrait with a pinhole might
have required several days if not weeks exposure of the plate and
therefore would have been impossible, so that the gain in brightness
of image is a great deal more than a mere convenience.
It will be observed in figure 1 that both points of light, A and B
produce images on the surface T, although they are at different
distances from it, but in fig. 2, although the effect of the lens is
to concentrate the light from both points to two other points, one
of these is beyond the surface T. This is a disadvantage inherent in
lenses. They have so many other imperfections or "aberrations" that
it is desirable to consider these separately. The reader should bear
in mind that the one aim of opticians in perfecting lenses is to
concentrate as much light as possible from each point in the object
to a corresponding point, or as small as possible a dot, in the
image, and the image should be flat because the plates used in
photography are flat.
[Illustration: Fig. 3.]
_Spherical Aberration._--The surfaces of lenses are always ground to
spherical curves, and this fact makes it impossible for a single
lens, such as that shown in figure 2, to bring to a point all the
light that falls upon it from a point. If a pencil of light passes
through a piece of glass with sloping sides it is bent or
"refracted" towards the thicker part of the glass, and the greater
the angle of inclination of the two sides the more is it refracted
from its original path. In figure 3 it is clear that the two sides
of the lens shown in section are inclined to each other at a
continually increasing angle as they approach each other at the
edges of the lens. The refracting effect of the lens increases from
the centre outwards, and it increases to a greater extent than is
necessary to bring the incident light to a point. The focus of the
pencils of light that pass through the edges of the lens is nearer
to the lens than the focus of the pencils that pass through its
central part. In the figure two foci are shown, _a_ and _b_, but of
course, in fact, intermediate parts of the lens produce intermediate
foci, and what should be a point in the image, is spread out into a
line on the axis of the lens, and all along this line is surrounded
with the light that either is coming to a focus or that has come to
a focus and has spread out again. On a screen placed at _b_ there
would be a point of light surrounded by a halo, while at _a_, nearer
the lens, the central focus or point is surrounded by a brighter or
more condensed light, and the appearance is of a circular patch of
light with a brighter boundary. This is positive spherical
aberration. Negative spherical aberration is due to over correction,
the focus of the light passing through the margins being furthest
from the lens, and the appearances on a screen are of course
reversed.
_Chromatic Aberration._--When light is refracted, that is bent out
of its original path by a single piece of glass, it is not refracted
as a whole, but each constituent behaves as if none other were
present. Ordinary white light or daylight is a mixture of many
coloured lights as seen in the rainbow, and when refracted, the blue
is bent more than the green, the green more than the yellow, and the
yellow more than the red. So that using a single lens the focus of
the blue light is nearer the lens than the focus of the red light
and the others come in between. In figure 4 this is represented in
an exaggerated degree to make it more distinct. It will be observed
that a screen placed at the focus of the blue light will show a
reddish margin and if removed further from the lens the margin or
halo will be bluish.
[Illustration: Fig. 4.]
These two aberrations, spherical and chromatic _were_ the principal
faults that opticians had to deal with, because they affect the
whole of the image, even the very central parts. But in photography
it is necessary to get an image of a very large size as compared
with the focal length of the lens, and there are some faults that
only begin to show themselves at a little distance from the centre
of the image and increase as the distance from the centre is
greater. These aberrations were, practically speaking, incurable
until a few years ago, but as recent optical advances have provided
kinds of glass by the use of which they may be eliminated, or nearly
so, they have become of practical importance. They are astigmatism
and curvature of field.
_Astigmatism and Curvature of Field._--If a diagram of suitable size
is made with a series of concentric circles and radial lines upon
it, and the centre of it is arranged exactly opposite the centre of
the lens, and in a line with the centre of the focussing screen, the
screen and diagram being parallel, then if the lens suffers from
astigmatism it will be found impossible to get the outer circles and
the radial lines where they cross them simultaneously focussed.
Where this difficulty begins the astigmatism begins, and the greater
the difference there is between the focal planes of the radial lines
and the circles, the greater is the astigmatism. It will probably
be found with any of the older types of lenses that neither is in
focus at the same time that the centre of the diagram is, but that
the screen must be racked in; this is due to curvature of field, and
the difference between the curvature of field for the circles and
the radial lines is due to astigmatism. In the older lenses a
flatter field could only be obtained by the introduction of
astigmatism, but now by the employment of the new glasses made at
Jena, it is possible to practically eliminate astigmatism, and still
keep the field flat.
[Illustration: Fig. 5.]
_The Development of Photographic Lenses._--When photography was
first practised the best lenses available were those made for use as
telescope objectives, and they had to be used with a small diaphragm
to get good definition over a sufficient field. With the slow
processes then in vogue a more rapid lens was much desired, and
Voigtlander introduced a "portrait" lens constructed according to
the results of the calculations of Professor Petzval. This portrait
lens is still very largely used, and figure 5 will serve to show its
general character and will be a guide to the putting of one together
correctly if it has been taken to pieces for cleaning. A rapid lens
such as this could not cover a sufficiently large field for
landscape work, so that single lenses were still used for work in
which rapidity was not of very great importance. Single lenses were
improved, and other kinds of lenses were introduced from time to
time, but it was not till 1866 that the "rapid rectilinears" or
"rapid aplanats," called later "rapid symmetricals," and by
innumerable other names according to the fancies of the makers, were
introduced. Probably no lens has been made in such large numbers as
this.
At about the same time, Dallmeyer introduced his portrait lens in
which the position of the convex and concave elements of the back
combination is reversed, the concave lens being outside, and this
gives the photographer the opportunity of screwing it back a little,
and so introducing a measurable amount of spherical aberration which
has the effect of modifying the otherwise exceedingly fine
definition at the centre of the field, and giving a greater depth of
definition.
In 1881, Messrs. Abbe & Schott began a series of experiments in the
manufacture of optical glasses, and they were so successful in making
new and useful varieties, that an optical glass factory was
eventually established at Jena, by Schott & Co. By the use of these
newer glasses the limitations that had previously restricted
opticians were removed, and it became possible to correct astigmatism
and secure a flat field at the same time. Zeiss of Jena, towards the
end of 1890, introduced his first series of "anastigmats." The
"concentric" lens of Ross was introduced in 1892, a lens which
probably remains unsurpassed for flatness of field and freedom from
astigmatism; but as spherical aberration is present to a notable
degree, an aperture of about _f_/22 is the largest that gives sharp
definition. The "double anastigmat" of Goerz of Berlin was put on the
market in 1893. It is a symmetrical lens, and in this different from
the Zeiss anastigmats that preceded it. It consists of two similar
combinations, each of three lenses cemented together. The unsurpassed
qualities of this lens stimulated other opticians to seek to rival
it, and there appeared similar lenses with four and even five lenses
in each combination, besides other lenses that are more or less a
copy of the double anastigmat. One of the most notable of these is
the "satz-anastigmat" of Zeiss, each combination consisting of four
lenses cemented together and forming an excellent single lens. These
combinations are interchangeable in the same mount so that with, for
example, one mount and three lenses, six different focal lengths can
be obtained, as the lenses may be used singly or any two together as
a doublet.
The "Cooke" lens is remarkable for the simple means by which the
various corrections are made, consisting as it does of only three
single lenses separated from each other. Obviously it must be used
entire. These lenses do not cover so large a plate in proportion to
their focal lengths as most of the other anastigmats, but perform
excellently over the plates for which they are constructed.
The "stigmatic" of Dallmeyer is the latest lens of general utility.
It gives good definition to the margin of the circle of light that
it transmits, reduction of aperture being necessary, when its full
field is employed, to get equality of illumination rather than to
improve the marginal definition. Its two combinations are different,
and either may be used alone as a single lens, giving focal lengths
of approximately one-and-a-half and twice the focal length of the
whole lens.
The "planar" of Zeiss introduced just as we write, is a symmetrical
doublet characterized by a very large aperture, from _f_/3.6 to
_f_/4 up to 10 inches in focal length, and a little smaller above
that. It is therefore comparable with portrait lenses. Although it
is symmetrical, a single combination cannot with advantage be used
alone as a single lens. Telephotographic lenses are subsequently
referred to.
The one aim of opticians in improving photographic lenses has been
to get good definition all over a comparatively large flat surface
without having to use small apertures. A defining power on the axis
of the lens, that is, at the centre of the field, far exceeding what
can be taken practical advantage of in ordinary photography, has
long been possible. But until recently, the defining power always
rapidly deteriorated as the distance from the centre was increased.
But to judge of the quality of a lens, or to compare one lens with
another, there are other matters that must be understood, and these
we shall proceed to consider. Focal length, aperture and image angle
are the chief details concerning lenses, granting that the
aberrations referred to above are satisfactorily corrected.