THE PRINCIPLE OF PHOTOGRAPHY Photography, as its name implies, con sists of drawing by the aid of light, and is based upon the fact that various sub stances undergo such changes in their con dition as to exhibit new properties under the action of light. This new property, generally speaking, consists of their chang ing colour, usually to a darker shade, when exposed to light. Precisely how these The progress of photography during recent years has been rapid and phenomenal. New fields of usefulness have been discovered, in volving fresh and novel applications of the art-science to the increasingly exacting demands of modern life. The great bulk of magazine and book illustrations, the pictorial chronicle of current events, the beautiful photogravures and tricolour reproductions which have made the pictures of the world's famous artists familiar to us all — these, and nu merous other advantages, are the out come of the perfected camera. The
physician and the surgeon have gath ered wider knowledge of the complicated human system by means of photomicro graphy and the Röntgen rays. means the astron omer has discovered, by eans of the sensitive plate, stellar systems otherwise imperceivable, even with the aid of a telescope. Aided by the optical lantern and cinematograph the crowded assembly at places of popular entertainment may witness all the incidents and movements of a royal progress, a steeplechase, or yacht race within a few hours of the actual event. These are only a few of the many applica tions of photography ; there is scarcely an art or an industry that is not indebted to it, directly or indirectly.