A Manual Of Photography

180 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. the rays b b, and the ray c 0, are refracted, and meet at D d' ; the line 6 b represents the focal image produced of the body from which the light proceeds. In the last figure the image produced by the lens is repre- sented as curved: a little consideration will show that it is not possible that such a curved surface as that represented could produce an image of equal distinctness over every part of a plane surface: the rays cannot meet, as they are refracted from curved surfaces along any straight line, as F F F ; and supposing we receive on the surface of a lens a bright circular image, it will be brilliant and well defined around the centre, the light becoming fainter towards the edge, and at length passing into a cloudy halo, exhibiting the prismatic colours. This is called spherical aberration, and to it is due that want of distinctness which commonly is found around the edges of pictures taken in the camera obscura. It is therefore important, in the selection of lenses, that we look for sharpness of definition over the whole of a perfectly flat field. To manufacture a lens which shall effect this, is a task of some difficulty ; but by attention to the two facts, that a lens, one surface of which is a section of an ellipse, and the other of a circle struck from the farthest of the two foci of that ellipse, as in Tig. 34, produces no aberration, much may be effected. A meniscus lens, therefore, with a convex surface, part of an ellipsoid, the focal distance of which coincides with its far- ther focus, and a con- cave surface, part of a sphere, whose centre is that focus, will meet all our require- ments. The mecha- nical difficulties of producing such lenses are great, but they may, by cautious manipulation, be to a great extent over- come. There are other methods by which the aberration of sphericity may be corrected, but for a description of these the reader is referred to Sir John Herschel’s Treatise on Light, in the Encyclopædia Metropolitana. If we take such a lens as we have been describing, and stop its centre with a blackened disc, leaving only a small portion of the edge for the light to pass through, and throw its image on a screen, we shall find it will be bordered with fringes of