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Chapter III THE NATURE OE LIGHT
AS the whole structure of photography rests upon the application of the science of physics and chemistry, the student of photography or of cinematography can never be too well informed upon these subjects. While we shall endeavor to merely touch upon the fnore important principles of physics and chemistry which are most intimately concerned in their relation to photography, it would be well for the reader, who is earnestly in search of information, to dig up his highschool text-books and study the subjects of the physics of light and the chemistry of the salts of silver. If he has no such books, he will find a mine of interesting information in the public libraries, which are so numerous over the country that there are very few who do not have access to them. He who has considered these subjects dull and uninteresting will find they contain an unsuspected interest when he comes to trace their relation to and use in photography. It is not necessary to go deep into these subjects to get the simple facts upon which photography is based. When one has a clear conception of these facts, they will form a firm foundation upon which to build a sound structure of photographic knowledge. New facts acquired will then fit upon this foundation like bricks into a wall. If the student is uncertain as to what books to consult to acquire the knowledge which he wishes, he may find some assistance in consulting the bibliography or list of suitable text-books given in another place in this volume.
It is hardly two hundred years ago since people first had any adequate idea that our atmosphere exists and that we live and move about at the bottom of a sea of air — the weight of which presses upon us and all other objects about us with a pressure of approximately fourteen pounds to the square inch. With our present day knowledge gained from barometers, air-ships and balloons floating in the air, and from hundreds of other common facts, we accept the presence of the atmosphere as a matter of course.
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