Page:The New International Encyclopædia 1st ed. v. 01.djvu/63

have chromatic aberration, as there is no refraction of the rays. Moreover, it is possible, by combining two or more prisms or lenses, to diminish greatly the aberration. (See Achromatism.) Tηe colors which are not thus brought to the same focus form the "secondary spectrum."

Reference to the diagrams will possibly serve to explain the matter more fully. Fig. 1 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.

Fig 1.

Fig 2.

In fig. 2 let MN represent a convex lens, which may be considered as consisting of a number 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 eye-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 correction (see Achromatism) should aim to bring them to the desired focus. For a visual telescope or microscope the yellow rays must be considered, and such a combination of lenses made that they are brought to the same focal plane. The chapters on optics in Müller-Pouillet's Lehrhurh der Physik (Brunswick, 1897) treat the subject most fully, as does Glazebrook's Physical Optics (London, 1898). The correction of this evil in photographic lenses is extensively treated from the theoretical standpoint in S. P. Thompson's translation of Lummer's Photographic Optics (London, 1900).

ABERRATION, Spherical. A term used in geometrical optics (see Light) to express the difference in path and effect of rays of light incident perpendicularly and obliquely upon a mirror or upon a surface separating two portions of transparent matter, e.g., upon a surface of

water. If a source of light is very small, it can be called a "point-source," and can be considered as sending out "rays of light" in all directions, like the radii of a sphere. If one of these rays is perpendicular to the surface of the mirror or to the surface of separation of the two media, the rays near this will form a small cone or "pencil of rays;" and in optics it is shown that such a perpendicular pencil of rays always gives rise by reflection or refraction to another pencil of rays which meet in a point called the "image" or "focus" of the point-source. If, however, a small cone or pencil of rays be chosen around a ray which falls obliquely on the mirror or sep- arating surface, it will give rise by reflection or refraction to rays which do not form a cone and therefore do not have a point as a focus, except in the case of a plain mirror, such as an ordinary looking-glass. If the incident pencil is narrow, the reflected or refracted rays will have two foci, in the form of two short, straight lines, some distance apart and perpendicular to each other. These are called "focal lines;" and in between them the rays come the closest to forming a point focus, producing what is called the "circle of least confusion." If instead of considering a narrow pencil of rays, we study the whole bundle of rays falling on the entire reflect- ing or refracting surface, it is evident that the rays are brought to a focus on a surface which can be thought of as due to the combined effect of the short focal lines produced by the indi- vidual pencils of which the bundle of rays is composed, and which has a cusp or projecting point ending at the point-focus due to the per- pendicular pencil. A section of this "caustic surface" is often seen on looking down on a cup of coffee or a glass of milk, if there is a lighted lamp near: because the projecting sides of the cup or glass act as a curved mirror. An imme- diate consequence of spherical aberration is that the image formed of any object by a curved mirror or by a lens or prism is not "sharp," but blurred, unless care be taken to exclude the oblique rays. This is done ordinarily by the use of diaphragms, such as are seen in opera- glasses, photographic lenses, etc. The smaller the opening in the diaphragm, so much the sharper is the image. See Caustic.

Fig 1.

The accompanying diagrams will show the effect of spherical aberration in the case of spherical and parabolic mirrors and convex lenses. In fig. 1 parallel rays are incident on a spherical mirror. Those falling perpendicularly or near the centre of the mirror are reflected to the point Q, which is termed the principal focus of the mirror. The rays which strike the surface more obliquely do not meet at Q after reflection, but at points which lie on the caustic surface whose section is represented by the