A Manual Of Photography

116 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY, ae-ain - it is a common complaint with amateurs that failures beset them at every stage of the process, and frequently they have abandoned the practice of photography in despair. To pursue photography with success, it is essentially nece - sary that, by practice, the hand should be accustomed to the numerous manipulatory detail»; that, by repeated experiments the causes leading to failure should be ascertained ;. knowledge of the conditions under which the chemical changes take place should be obtained. This study, without which there will be no real success, is most favourably pursued by experi- meats on paper ; and such are therefore recommended to the amateur when first he enters upon this interesting pursuit, proceeding only to the more delicate processes when he has mastered the rudimentary details of the more simple forms of actino-chemistry. . , Previously, however, to explaining the practice of photography to which a separate division is given, it appears important that the physical conditions of the elements with which we have to work should be understood. . , ., The sun-beam is our pencil, and certain delicate chemical preparations form our drawing-board. Every beam of light which flows from its solar source is a bundle of rays, having each a very distinct character' as to colour and its chemical functions. ' These rays are easily shown by allowing a pencil of sunlight to fall on one angle of a prism : it is bent out of its path, or refracted, and an elongated image is obtained, present- Violet. J Indigo. Blue. Green. Yellow. Orange. I Red. ing the various colours of which Light app qi^ tuted—red, orange, yellow, green, blue, indig » ', coloured image is called the solar or the pr ■ p* The red ray, being the least refracted, is found at the lower edge, and the violet, being the most so, at the other extremi V of this chromatic series. Below the ordinarily visible red another ray of a deeper red, distinguished as the extreme red, or