A Manual Of Photography

APPARATUS NECESSARY FOR PHOTOGRAPHY ON PAPER. 199 (Fig. 44), the same effect will be seen. Suppose, in the first place, the box to be without the lens, the rays would pass from the external arrow in nearly right lines througli the opening, refracted only in passing the solid edges of the hole, and form an image on the back of the dark box. The lens refracts the rays, and a smaller but a more perfectly defined picture is the result. As in the phenomena of vision, so in the camera obscura, the image is produced by the radiations proceeding from the external object ; and as these radiations progress from various parts, more or less illuminated, so are the high lights, the middle tints and shadows, most beautifully preserved in the spectral appearance. The colours, also, being in the first instance the effect of some physical modification of the primary cause, are repeated under the same influence ; and the definition, the colour, and soft gradation of light and shadow, are so perfect, that few more beautiful optical effects can be produced than those of the camera obscura. Now as every ray of light producing the coloured image is accompanied by the chemical principle actinism, and as this is regulated in action by the luminous intensity of the rays, the most luminous (yellow) producing the least chemical effect, while it increases with the diminishing illuminating power of the coloured rays of the radiating source, we have the impression made in accordance witli the colour of the object we would copy, and not correct as to light and shadow. By referring to the frontispiece to the present volume, the effects produced by copying a coloured image will be seen. The yellows, reds, and those colours usually regarded as lights, are copied as shadows: hence the importance of attention to the colours of the dress, when a portrait is to be taken by any photographic process. In the ordinary cameras used by artists for sketching, a mirror is introduced, which throws the image on a semitrans- parent table. Fig. 45 is a section of one form of such an instrument: a a represents the box, in one end of which is fixed the lens 5.