The body is represented as consisting of two portions which slide within each other; but the same object of lengthening or shortening the focal distance may be attained by making the Lens itself movable. A luminous image of any object placed in front of the Camera is formed by means of the Lens, and received upon a surface of ground glass at the back part of the instrument. When the Camera is required for use, the object is focussed upon the ground glass, which is then removed, and a slide containing the sensitive layer inserted in its place.
The luminous image, as formed upon the ground glass, is termed the "Field" of the Camera; it is spoken of as being flat or curved, sharp or indistinct, etc. These and other peculiarities which depend upon the construction of the Lens will now be explained.
Chromatic Aberration of Lenses.—The outside of a biconvex lens is strictly comparable with the sharp edge of a prism, and therefore necessarily produces decomposition in the white light which passes through it.
The action of a prism in separating white light into its constituent rays may be simply explained;—all the coloured rays are refrangible, but not to the same extent. The indigo and violet are more so than the yellow and red, and consequently they are separated from them, and occupy a higher position in the Spectrum. (See the diagram at [p. 47].)
A little reflection will show that in consequence of this unequal refrangibility of the coloured rays, white light must invariably be decomposed on entering any dense medium. This is indeed the case; but if the surfaces of the medium are parallel to each other the effect is not seen, because the rays recombine on their emergence, being bent to the same extent in the opposite direction. Hence light is transmitted colourless through an ordinary pane of glass, but yields the tints of the Spectrum in its passage through a prism or a lens, where the two surfaces are inclined to each other at an acute angle.
Chromatic aberration is corrected by combining two lenses cut from varieties of glass which differ in their power of separating the coloured rays. These are the dense flint-glass containing Oxide of Lead, and the light crown-glass. Of the two lenses, the one is biconvex, and the other biconcave; so that when fitted together they produce a compound Achromatic lens of a meniscus form, thus:—
The first Lens in this figure is the flint- and the second the crown-glass. Of the two the biconvex is the most powerful, so as to overcome the other, and produce a total of refraction to the required extent. Each of the Lenses produces a spectrum of a different length; and the effect of passing the rays through both, is, by overlapping the coloured spaces, to unite the complementary tints, and to form again white light.
Spherical Aberration of Lenses.—The field of a Camera is not often equally sharp and distinct at every part. If the centre be rendered clear and well defined, the outside is misty; whilst, by slightly altering the position of the ground glass, so as to define the outside portion sharply, the centre is thrown out of focus. Opticians express this by saying that there is a want of proper flatness of field; two causes may be mentioned as concurring to produce it.