A little while before the daguerreotyping process was announced, Fox Talbot, a British investigator, discovered a method of making pictures by means of the action of light on chemically prepared paper instead of metal, as in the case of Daguerre. Talbot originated the terms negative and positive which are still used in photography. Daguerre in France and Talbot in Great Britain had independently achieved success in producing pictures, but neither had discovered a way to make photographs permanent. In the course of time the pictures faded. In 1839 Sir John Herschel of England found a chemical process for making photographs permanent, by removing the cause for their fading. The first sunlight photograph of a human face was that of Miss Dorothy Catherine Draper, made by her brother, Prof. John William Draper, of the University of the City of New York, early in 1840.
Various chemical discoveries for improving photographs have been made by different persons from time to time, until the art of photography has now reached a high state of development. An important improvement is in the lessening of the time of exposure to light necessary for producing a photograph. Formerly hours were required, but under improved conditions only the shortest instant of time is requisite.
In 1906 a photographic paper for producing prints in color from an ordinary negative was placed on the market. This paper is coated with three layers of pigmented gelatin, colored respectively red, yellow, and blue. After being exposed to the daylight in the usual way, the paper is placed in hot water, where the image is developed. The grays and blacks of the negative are translated into the colors they represent in the object.
The brothers Lumière of Paris have found a method of producing a photograph on a sensitive plate which, viewed as a transparency, shows the object in its original colors. No prints can be taken from this plate, and the picture cannot be viewed by reflected light, but the colors are true and brilliant.
The cinematograph is an instrument by which about fifteen photographs per second can be received on a film, each representing the photographed group at a different instant from the others. The advantages of this mode of photographing and of throwing pictures on a screen over the older methods are obvious. By controlling the rate at which the pictures are represented on the screen, movements too rapid to be analyzed by the eye may be made slow enough to permit observation; and, similarly, movements too slow for comprehension or rapid observation may often be quickened. The busy life of a city street, the progress of races or other competitions, many scenes in nature, and even the growth of a plant from seed to maturity, may be shown by means of a "moving picture."
Photography is a noble servant of mind and soul. It brings to us likenesses of eminent persons and objects of nature and art which perhaps we should never be able to see otherwise. It has been used in measuring the velocity of bullets and in showing the true positions of animals in motion. Photography has created the "new astronomy." Immediately after its discovery, photography was applied to the science of the stars, and it has been ever since of incalculable service in this field of inquiry. Photographs of the moon were made as early as 1840, and much that is known to-day of the sun has been revealed by photography. So sensitive is the modern photographic plate to the influence of light, that photography has discovered and located stars which are invisible through a strong telescope. Astronomers are now engaged in making a photographic chart of the sky.
CHAPTER XV
CLOCKS
The matters of every-day life, much less the affairs of a complex civilization, could scarcely be carried on without some accurate and uniform system of measuring time. Nature herself furnishes measurements for certain divisions of time. The "two great lights" that God made, as the Bible tells us, were designed "for signs, and for seasons, and for days and for years." The revolution of the earth around the sun marks the year; the revolution of the moon around the earth determines the month; the rotation of the earth on its axis causes and measures day and night. But no object of nature distinguishes the hours of the day or the divisions of the hour.