PHOTOGRAPHY
120. The Magic of the Sun. Ribbons and dresses washed and hung in
the sun fade; when washed and hung in the shade, they are not so apt
to lose their color. Clothes are laid away in drawers and hung in
closets not only for protection against dust, but also against the
well-known power of light to weaken color.
Many housewives lower the window shades that the wall paper may not
lose its brilliancy, that the beautiful hues of velvet, satin, and
plush tapestry may not be marred by loss in brilliancy and sheen.
Bright carpets and rugs are sometimes bought in preference to more
delicately tinted ones, because the purchaser knows that the latter
will fade quickly if used in a sunny room, and will soon acquire a
dull mellow tone. The bright and gay colors and the dull and somber
colors are all affected by the sun, but why one should be affected
more than another we do not know. Thousands of brilliant and dainty
hues catch our eye in the shop and on the street, but not one of them
is absolutely permanent; some may last for years, but there is always
more or less fading in time.
Sunlight causes many strange, unexplained effects. If the two
substances, chlorine and hydrogen, are mixed in a dark room, nothing
remarkable occurs any more than though water and milk were mixed, but
if a mixture of these substances is exposed to sunlight, a violent
explosion occurs and an entirely new substance is formed, a compound
entirely different in character from either of its components.
By some power not understood by man, the sun is able to form new
substances. In the dark, chlorine and hydrogen are simply chlorine and
hydrogen; in the sunlight they combine as if by magic into a totally
different substance. By the same unexplained power, the sun frequently
does just the opposite work; instead of combining two substances to
make one new product, the sun may separate or break down some
particular substance into its various elements. For example, if the
sun's rays fall upon silver chloride, a chemical action immediately
begins, and as a result we have two separate substances, chlorine and
silver. The sunlight separates silver chloride into its constituents,
silver and chlorine.
121. The Magic Wand in Photography. Suppose we coat one side of a
glass plate with silver chloride, just as we might put a coat of
varnish on a chair. We must be very careful to coat the plate in the
dark room,[B] otherwise the sunlight will separate the silver chloride
and spoil our plan. Then lay a horseshoe on the plate for good luck,
and carry the plate out into the light for a second. The light will
separate the silver chloride into chlorine and silver, the latter of
which will remain on the plate as a thin film. All of the plate was
affected by the sun except the portion protected by the horseshoe
which, because it is opaque, would not allow light to pass through and
reach the plate. If now the plate is carried back to the dark room and
the horseshoe is removed, one would expect to see on the plate an
impression of the horseshoe, because the portion protected by the
horseshoe would be covered by silver chloride and the exposed
unprotected portion would be covered by metallic silver. But we are
much disappointed because the plate, when examined ever so carefully,
shows not the slightest change in appearance. The change is there, but
the unaided eye cannot detect the change. Some chemical, the
so-called "developer," must be used to bring out the hidden change and
to reveal the image to our unseeing eyes. There are many different
developers in use, any one of which will effect the necessary
transformation. When the plate has been in the developer for a few
seconds, the silver coating gradually darkens, and slowly but surely
the image printed by the sun's rays appears. But we must not take this
picture into the light, because the silver chloride which was
protected by the horseshoe is still present, and would be strongly
affected by the first glimmer of light, and, as a result, our entire
plate would become similar in character and there would be no contrast
to give an image of the horseshoe on the plate.
[Footnote B: That is, a room from which ordinary daylight is
excluded.]
But a photograph on glass, which must be carefully shielded from the
light and admired only in the dark room, would be neither pleasurable
nor practical. If there were some way by which the hitherto unaffected
silver chloride could be totally removed, it would be possible to take
the plate into any light without fear. To accomplish this, the
unchanged silver chloride is got rid of by the process technically
called "fixing"; that is, by washing off the unreduced silver chloride
with a solution such as sodium thiosulphite, commonly known as hypo.
After a bath in the hypo the plate is cleansed in clear running water
and left to dry. Such a process gives a clear and permanent picture on
the plate.
[Illustration: FIG. 82.--A camera.]
122. The Camera. A camera (Fig. 82) is a light-tight box containing
a movable convex lens at one end and a screen at the opposite end.
Light from the object to be photographed passes through the lens,
falls upon the screen, and forms an image there. If we substitute for
the ordinary screen a plate or film coated with silver chloride or any
other silver salt, the light which falls upon the sensitive plate and
forms an image there will change the silver chloride and produce a
hidden image. If the plate is then removed from the camera in the
dark, and is treated as described in the preceding Section, the image
becomes visible and permanent. In practice some gelatin is mixed with
the silver salt, and the mixture is then poured over the plate or film
in such a way that a thin, even coating is made. It is the presence of
the gelatin that gives plates a yellowish hue. The sensitive plates
are left to dry in dark rooms, and when the coating has become
absolutely firm and dry, the plates are packed in boxes and sent forth
for sale.
Glass plates are heavy and inconvenient to carry, so that celluloid
films have almost entirely taken their place, at least for outdoor
work.
123. Light and Shade. Let us apply the above process to a real
photograph. Suppose we wish to take the photograph of a man sitting in
a chair in his library. If the man wore a gray coat, a black tie, and
a white collar, these details must be faithfully represented in the
photograph. How can the almost innumerable lights and shades be
produced on the plate?
The white collar would send through the lens the most light to the
sensitive plate; hence the silver chloride on the plate would be most
changed at the place where the lens formed an image of the collar. The
gray coat would not send to the lens so much light as the white
collar, hence the silver chloride would be less affected by the light
from the coat than by that from the collar, and at the place where the
lens produced an image of the coat the silver chloride would not be
changed so much as where the collar image is. The light from the face
would produce a still different effect, since the light from the face
is stronger than the light from the gray coat, but less than that from
a white collar. The face in the image would show less changed silver
chloride than the collar, but more than the coat, because the face is
lighter than the coat, but not so light as the collar. Finally, the
silver chloride would be least affected by the dark tie. The wall
paper in the background would affect the plate according to the
brightness of the light which fell directly upon it and which
reflected to the camera. When such a plate has been developed and
fixed, as described in Section 121, we have the so-called negative
(Fig. 83). The collar is very dark, the black tie and gray coat white,
and the white tidy very dark.
[Illustration: FIG. 83.--A negative.]
The lighter the object, such as tidy or collar, the more salt is
changed, or, in other words, the greater the portion of the silver
salt that is affected, and hence the darker the stain on the plate at
that particular spot. The plate shows all gradations of intensity--the
tidy is dark, the black tie is light. The photograph is true as far as
position, form, and expression are concerned, but the actual
intensities are just reversed. How this plate can be transformed into
a photograph true in every detail will be seen in the following
Section.
124. The Perfect Photograph. Bright objects, such as the sky or a
white waist, change much of the silver chloride, and hence appear
dark on the negative. Dark objects, such as furniture or a black coat,
change little of the chloride, and hence appear light on the negative.
To obtain a true photograph, the negative is placed on a piece of
sensitive photographic paper, or paper coated with a silver salt in
the same manner as the plate and films. The combination is exposed to
the light. The dark portions of the negative will act as obstructions
to the passage of light, and but little light will pass through that
part of the negative to the photographic paper, and consequently but
little of the silver salt on the paper will be changed. On the other
hand, the light portion of the negative will allow free and easy
passage of the light rays, which will fall upon the photographic paper
and will change much more of the silver. Thus it is that dark places
in the negative produce light places in the positive or real
photograph (Fig. 84), and that light places in the negative produce
dark places in the positive; all intermediate grades are likewise
represented with their proper gradations of intensity.
[Illustration: FIG. 84.--A positive or true photograph.]
If properly treated, a negative remains good for years, and will serve
for an indefinite number of positives or true photographs.
125. Light and Disease. The far-reaching effect which light has upon
some inanimate objects, such as photographic films and clothes, leads
us to inquire into the relation which exists between light and living
things. We know from daily observation that plants must have light in
order to thrive and grow. A healthy plant brought into a dark room
soon loses its vigor and freshness, and becomes yellow and drooping.
Plants do not all agree as to the amount of light they require, for
some, like the violet and the arbutus, grow best in moderate light,
while others, like the willows, need the strong, full beams of the
sun. But nearly all common plants, whatever they are, sicken and die
if deprived of sunlight for a long time. This is likewise true in the
animal world. During long transportation, animals are sometimes
necessarily confined in dark cars, with the result that many deaths
occur, even though the car is well aired and ventilated and the food
supply good. Light and fresh air put color into pale cheeks, just as
light and air transform sickly, yellowish plants into hardy green
ones. Plenty of fresh air, light, and pure water are the watchwords
against disease.
[Illustration: FIG. 85--Stems and leaves of oxalis growing toward the
light.]
In addition to the plants and animals which we see, there are many
strange unseen ones floating in the atmosphere around us, lying in the
dust of corner and closet, growing in the water we drink, and
thronging decayed vegetable and animal matter. Everyone knows that
mildew and vermin do damage in the home and in the field, but very few
understand that, in addition to these visible enemies of man, there
are swarms of invisible plants and animals some of which do far more
damage, both directly and indirectly, than the seen and familiar
enemies. All such very small plants and animals are known as
_microorganisms_.
Not all microörganisms are harmful; some are our friends and are as
helpful to us as are cultivated plants and domesticated animals. Among
the most important of the microörganisms are bacteria, which include
among their number both friend and foe. In the household, bacteria are
a fruitful source of trouble, but some of them are distinctly friends.
The delicate flavor of butter and the sharp but pleasing taste of
cheese are produced by bacteria. On the other hand, bacteria are the
cause of many of the most dangerous diseases, such as typhoid fever,
tuberculosis, influenza, and la grippe.
By careful observation and experimentation it has been shown
conclusively that sunlight rapidly kills bacteria, and that it is only
in dampness and darkness that bacteria thrive and multiply. Although
sunlight is essential to the growth of most plants and animals, it
retards and prevents the growth of bacteria. Dirt and dust exposed to
the sunlight lose their living bacteria, while in damp cellars and
dark corners the bacteria thrive, increasing steadily in number. For
this reason our houses should be kept light and airy; blinds should be
raised, even if carpets do fade; it is better that carpets and
furniture should fade than that disease-producing bacteria should find
a permanent abode within our dwellings. Kitchens and pantries in
particular should be thoroughly lighted. Bedclothes, rugs, and
clothing should be exposed to the sunlight as frequently as possible;
there is no better safeguard against bacterial disease than light. In
a sick room sunlight is especially valuable, because it not only kills
bacteria, but keeps the air dry, and new bacteria cannot get a start
in a dry atmosphere.