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.
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.
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.