"If A B C is an object placed before a convex lens, L L, every point of it will send forth rays in all directions; but, for the sake of simplicity, suppose only three points to give out rays, one at the top, one at the middle, and one at the bottom; the whole of the rays then that proceed from the point A, and fall on the lens L L, will be refracted and form an image somewhere on the line A G E, which is drawn direct through the centre of the lens; consequently the focus E, produced by the convergence of the rays proceeding from A, must form an image of A, only in a different relative position; the middle point of C, being in a direct line with the axis of the lens, will have its image formed on the axis F, and the rays proceeding from the point B will form an image at D; so that by imagining luminous objects to be made up of an infinite number of radiating points, and the rays from each individual point, although falling on the whole surface of the lens, to converge again and form a focus or representation of that point from which the rays first emerged, it will be very easy to comprehend how images are formed, and the cause of those images being reversed. "It must also be evident, that in the two triangles A G B and D G E, that E D, the length of the image, must be to A B, the length of the object, as G D, the distance of the image, is to G B, the distance of the object from the lens." [Sidenote: SPHERICAL ABERRATION.] It will be observed, that in the last cut the image produced by the lens is curved. Now, it would be impossible to produce a well-defined image from the centre to the edge upon a _plain_ surface; the outer edges would be misty, indistinct, or crayon-like. The centre of the image might be represented clear and sharp on the ground glass, yet this would be far from the case in regard to the outer portions. This is called _spherical aberration_, and to it is due the want of distinctness which is frequently noticed around the edges of pictures taken in the camera. To secure a camera with a _flat, sharp field_, should be the object of every Operator; and, in a measure, this constitutes the great difference in cameras manufactured in this country. Spherical aberration is overcome by proper care in the formation of the lens:--"It can be shown upon mathematical data that a lens similar to that given in the following diagram--one surface of which is a section of an ellipse, and the other of a circle struck from the furthest of the two foci of that ellipse--produces no aberration. "At the earliest period of the employment of the camera obscura, a _double-convex_ lens was used to produce the image; but this form was soon abandoned, on account of the spherical aberration so caused. Lenses for the photographic camera are now always ground of a concavo-convex form, or meniscus, which corresponds more nearly to the accompanying diagram." [Illustration: Fig. 11.] [Sidenote: CHROMATIC ABERRATION.]