CHROMATIC ABERRATION (Pr., Aber ration chromatique ; Ger., Chromatische Abweichung) To arrive at a proper understanding of the cause of chromatic aberration, it is necessary to remember that a lens is practically a prism with the power of refracting or altering the direction of rays of light and, in an uncorrected form, of refracting rays of different colours to a different extent. Diagram A indicates the effect of passing a ray of white light through a prism ; the bending of the rays will be noticed. The most active in their chemical action are the blue and violet, and these are diverted from their original path more than on the others ; B shows two prisms placed base to base so that in the case of each colour the rays are directed to a common point. The effect of this arrangement more nearly approximates to that of a simple lens as in C, where only three colours are in cluded for the sake of simplicity. Such a lens is in fact a circular prism with the power of bending the rays of one colour so that after passing through it they meet at one point. This point is called the focus, and it will be seen that the focus for the blue-violet rays to which ordinary photographic plates are most sensitive is much nearer to the lens than the luminous rays, green-yellow, which are those that form the visual image upon the focusing screen. This is chromatic aberration in its simplest form. If the image produced by such a lens upon the double concave flint one (see D). In ordinary photographic lenses the optician combines the most visually powerful region of the spectrum, namely the green and yellow near the D line, and the most chemically active, namely the blue and violet near G. It will be seen that the red
rays are neglected, bat in ordinary photography this is of little moment. In three-colour work, in which one of the images is made through a red screen, a higher degree of correction is necessary, and by the use of a third variety of glass it is possible to bring the red rays to a focus in the same plane as the green and blue, ground-glass screen be examined with a magnifier, it will be found that the outline of any bright object, such as that of a white china knob, is surrounded with a fringe of colour, either blue or orange. If a photographic plate be substituted for the focusing screen there will be obtained a blurred outline, the image being " out of focus." On moving the plate nearer the lens by one-thirtieth to one-fortieth of the total distance between the lens and the visual focus, there will be obtained on a plate an image which is prac tically sharp. When using ordinary spectacle lenses, a course that is possible where extreme definition is not required, the precaution above mentioned must be observed.
The avoidance of chromatic aberration in a photographic lens is effected by the use of at so that images taken through screens of these colours are identical in sharpness and size. A lens of this description is called apochromatic (which see). Recent advances in glass manu facture have rendered the old terms " crown " and " flint " somewhat meaningless, as the dispersive elements are now frequently made of special forms of crown glass.