第29章

"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 proceding 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 all 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 Band 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.

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,[amdg_11.gif]

or meniscus, which corresponds more nearly to the accompanying diagram."Chromatic Aberration is another difficulty that opticians have to contend with in the manufacturing of lenses. It will be remembered, that in a former page (133) a beam of light is decomposed by passing through a glass prism giving seven distinct colors--red, orange, yellow, green, blue, indigo and violet.

Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light passing through a double-convex lens without its being fringed with color.

Its effect will be readily understood by reference to the accompanying cut.

If L L be a double convex-lens, and R R R parallel rays of white light, composed of the seven colored rays,[amdg_8b.gif]

each having a different index of refraction, they cannot be refracted to one and the same point; the red rays, being the least refrangible, will be bent to r, and the violet rays, being the most refrangible, to v:

the distance v r constitutes the chromatic aberration, and the circle, of which the diameter is a l, the place or point of mean refraction, and is called the circle of least aberration. If the rays of the sun are refracted by means of a lens, and the image received on a screen placed between C and o, so as to cut the cone L a l L, a luminous circle will be formed on the paper, only surrounded by a red border, because it is produced by a section of the cone L a l L, of which the external rays L a L l, are red; if the screen be moved to the other side of o, the luminous circle will be bordered with violet, because it will be a section of the cone M a M l, of which the exterior rays are violet.

To avoid the influence of spherical aberration, and to render the phenomena of coloration more evident, let an opaque disc be placed over the central portion of the lens, so as to allow the rays only to pass which are at the edge of the glass; a violet image of the sun will then be seen at v, red at r, and, finally, images of all the colors of the spectrum in the intermediate space; consequently, the general image will not only be confused, but clothed with prismatic colors."To overcome the difficulty arising from the chromatic aberration, the optician has only to employ a combination of lenses of opposite focal length, and cut from glass possessing different refrangible powers, so that the rays of light passing through the one are strongly refracted, and in the other are bent asunder again, reproducing white light.

To the photographer one of the most important features, requiring his particular attention. is, that he be provided with a good lens.

By the remarks given in the preceding pages, he will be enabled, in a measure, to judge of some of the difficulties to which he is occasionally subjected.

We have in this country but two or three individuals who are giving their attention to the manufacture of lenses, and their construction is such, that they are quite free from the spherical or chromatic aberration.