camera obscura

camera obscura defined in 1909 year

camera obscura - camera obscura;
camera obscura - This simply consists of a dark chamber having a small aperture in one of its walls into which is fitted a convex lens. This simple arrangement provides for an image - a picture in natural colours, with beautiful effects - being projected on the wall of the chamber, this picture being an exact resemblance of the scenery or objects outside the chamber (on the side that the lens faces); and if suitably disposed very delightful landscape views, with moving objects (if there are any) may be obtained. The image can be received on the wall of the chamber, as mentioned, but it is more convenient to receive it on a screen (dull white in colour) as this can be placed to precisely agree with the focal length of the lens and so get the best possible definition (as with focusing in a photographic camera). For very simple purposes a small plain hole in the wall is sufficient, but does not produce the effect that a lens does, and the picture is inverted. By using a double convex lens, the picture is received correctly, and is brighter and definite. The least expensive suitable lens is one of 3 in. diameter, double convex, of 8 in. focal length, but the best results will be obtained with a photographic objective of 9 in. focus. For artistic purposes, perhaps the image thrown on a vertical screen is sufficient, but the camera obscura, as generally exhibited, has the picture appearing on a horizontal table, and this is effected by arranging for the lens to come at a higher point and throwing its rays on a mirror placed at an angle of 45 ° to receive and deflect them, as Fig. The mirror would be placed at precisely 45 °, if the optical axis of the lens is perfectly horizontal, but otherwise the mirror and lens are adjusted to one another. The distance between the lens and the mirror is the focal length of the former. Thus with a lens of 8 in. focal length this distance would separate the two.

As usually arranged, the chamber is circular with a dome or pointed top, and in this top is placed the lens and mirror. If the top is made to revolve, then, of course, a great variety of views will be obtained. The chamber should be constructed of material as dark as possible so that the picture may be the more bright and real, but for special purposes a canvas bell-tent can be made to do. As regards the height, if the focus of the lens is 8 in., then the distance from the mirror to the table should be 8 ft., and this will give a picture of nearly 3 ft. diameter.

A difficulty was experienced with the early examples of this appliance in that the definition of the picture on a flat table was not regular, the centre appearing to be a little out of focus, with the edges well denned, or vice versa. To overcome this the table or other surface on which the picture was received, was made a little concave - i.e. saucer-shaped or hollowed out, but later experience showed that the fault was due to cheap lenses being used, for a flat table gives quite good results with a good lens.

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