CHAPTER XII

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