20

NEGATIVE PHOTOGRAPHS.

b. On using the Photographic paper in the Camera Obscura.

The most important object of the photographic art, is the securingthe shadowy images of the camera obscura—the fixing of a shade,and multiplying its likeness. As this is the most important, sois it the department requiring the greatest care. It need scarcelybe mentioned, that a camera obscura is a darkened chamber or box, towhich light is admitted through a small hole in which a lens is fixed.In the ordinary cameras used by artists for sketching, a mirror is con-stantly used, which throws the image on a semitransparent table.

Fig. 7 is a section of such an instrument, a a represents the box, in

one end of which is fixed the lens b. The lenticular image falls on themirror c, placed at such an anglo that it is reflected on the plate of groundglass d. e is a screen to prevent the overpowering influence of daylight,which would render tlio picturo almost invisible. This form of the ap-paratus, though very interesting as a philosophical toy, and extremelyuseful to tho artist, is by no means fitted for photographic purposes.The light radiated from external objects, suffers considerable diminu-tion of chemical power in penetrating the lens, and the reflection fromthe mirror so far reduces its intensity, that its action on photographicagents is infinitely slow. To obviate tho objection of the reflectedimage, it is only necessary to place the photographic paper in the placeof the mirror, but not in an angular position.

Fig. 8, represents tho photographic camera of tho common form ;

Fig. 8.

a a is the outer box, in which is fixed the lens b, and c c another boxsliding within it; at the inner end of which is placed the preparedpaper d; by sliding this box forth and back, we are enabled to adjustthe paper to the correct focus of the lens, which can be observed througha small hole at e.

The most sensitive pieces of paper being selected, by testing the