AB'ERRA'TION, CHROMATIC ( from Lat. ah, away + crave, to wander, and Gk. XpEnia, chroma, color, literally colored deviation). A phenomenon observed when images of an emitting white light are formed by a lens or a prism, it being observed that there is then not one white image, but many colored ones, which do not occupy the same position, and which are of different sizes, thus producing a blurred image with a colored border. It is explained in the article LIGHT that the sensations of different colors are due to waves in the ether of different wave-number or wave-length, and that these waves, in passing through portions of trans parent matter, such as glass, travel with different velocities, depending upon their wave-number. As a consequence of this, in passing through lenses or prisms, waves of different wave-num- • ber have different paths. White light is shown to be due to the reception by the eye of waves of different wave-number ; or, in other words, from a "white object," or an object 'emitting white light," waves of different wave-numbers proceed outward. These waves are such that each train of waves of a definite wave-number would produce in the eye a definite eolor-sen sation, e.g., blue, green, etc. In this sense we may speak of "blue-waves." "green-waves," etc.; and in general white light is due to the recep tion by the eye of waves which correspond to the "colors of the spectrum"—violet, blue, green, yellow, orange, red, and all the intermediate Shades. Therefore. owing to this difference in path in a lens or prism of waves of different color, if an image of a white object is formed there will be a series of images corresponding to the different colors, these images differing in position and size, as well as in color. This re sult is said to he due to the "chromatic aberra tion" of the lens or prism. (There are, of course, ether-waves which do not affect the sense of sight; and any prism or lens which is trans parent to them will in general deviate waves of different wave-number differently, and so have this same kind of aberration, as ordinary glass lenses have for visible waves.) Mirrors do not
have chromatic aberration, as there is no re fraction of the nip. MOITOVUT, it is possible. by combining two or more priS1114; or lenses, to diminish greatly the aberration. (See ACHROMA TISM.) The colors which are not thus brought to the same focus form the "secondary spec trum." Reference to the diagrams will possibly serve to explain the matter more fully. Fig. I shows the dispersion (q.v.) of a beam of white light on passing through a prism, or, in other words, its separation into its constituent colors.
In fig. 2 let MN represent a convex lens, which may be considered as consisting of a num ber of prisms and having the same dispersive effect. Let A represent a source of white light. Considering a pencil which falls on the lens at c, where it is refracted, it is found that dispersion takes place, and the red rays after being deviated proceed to D. where an image of the object A is formed, while the violet rays which undergo greater refraction proceed to C, and there form an image of the object. A. Consequently, if the image at C is examined with an eve-piece, or allowed to fall on a screen, it will be found to have a red border, while that at D will be seen surrounded by violet. When correction is made for chromatic aberration, the purpose for which the lens is designed must be considered. (See TELESCOPE.) For photographic work the violet rays are required, and any correc tion (see ACIII:)NIATIsm) should aim to bring them to the desired focus. For a visual telescope or microscope the yellow rays must, be considered, and such a of lenses made that they are brought to the same focal plane. The chap ters on optics in 31iiller-Pouillet's Lchrburh der Physik (Brunswick. 1897) treat the subject most fully, as does ellazebrook's Physical Opt irs (Lon don, 1898). The correction of this evil in photo graphic lenses is extensively treated from the theoretical standpoint in S. P. Thompson's trans lation of Lummer's Photographic Optics (London, 1900).