ABERRATION, in Optics, a certain deviation in the rays of light, from the true or geometrical focus of reflection or refraction in curved specula or lenses, arising from two causes, viz. 1st, the figure of the speculum or lens, giving rise to what is called the spherical aberration; and, 2d, the unequal refrangibility of the rays of light giving rise, in lenses only however, to a far more material, and in other respects inconvenient aberration, termed the chromatic, or the aberration of colour, or of refrangibility. The object of all specula or lenses, is to collect the rays of light proceeding from any object into a single point, so as to form there a distinct image of the object, either enlarged or diminished, according as our purposes may require: and on this principle depends the whole operation of the telescope, the microscope, and other optical instruments. The more completely the rays can be collected into a focus, so much the more distinctly, in every case, does the image of the object appear at that point, and so much the more perfect is the operation of the instrument. But there are certain curves or figures in the speculum or lens, which are necessary to produce this effect. Parallel rays, for example, can only be collected into one focus by a reflecting speculum of a parabolic form, or by a refracting lens of parabolic or hyperbolic, combined with spherical curves: all other forms cause more or less a dispersion or aberration of the rays from the focus. In practice, however, it is extremely difficult to form the lenses into these complex curves; and as the
spherical form is much more easily constructed, and as the aberration from it is not generally attended with serious inconvenience, this form has been universally adopted. The amount of the aberration is measured either by the distance longitudinally at which the rays meet from the true focus, or by the distance laterally by which they are dispersed from it. In all double convex lenses of equal spheres, the longitudinal aberration of the extreme ray is 1/4 of the thickness of the lens. The smallest aberration takes place when the radii of the spheres are as 1 to 6, the more convex surface being exposed to the rays; in that case, it is only 1/12th of the thickness of the lens. See OPTICS.
The aberration of refrangibility is of far more importance. It arises from this circumstance, that in a homogeneous lens of glass the violet rays are greatly more refracted than the red. The latter are therefore thrown to a greater distance, and the others in proportion almost all deviating from the true focus: hence arises that confusion of images, and that fringe of extraneous colour with which objects are surrounded when seen through glasses of this description, which has ever formed the great obstacle to the perfection of the refracting telescope: so much so, that Sir Isaac Newton, misled at the time by a partial view of the subject, and others after him, were led to despair of success in correcting this defect, and thus directed their chief attention to those of the reflecting kind. Subsequent discoveries, however, led to the invention of achromatic glasses, by which the refracting telescope has been wonderfully improved; and some important experiments, we understand, are now going on at the Royal Institution in London, by M. Faraday, under the direction of the Board of Longitude, in the manufacture of a more perfect glass than has hitherto been used, from which we may hope to see these instruments carried to a yet higher degree of perfection. See ACHROMATIC GLASSES; also Phil. Trans. vols. xxxv. xviii., and from I. to lv.; Mem. Acad. Par. from 1737 to 1770; Mem. Acad. Berlin, from 1746 to 1798; Nov. Comment. Petrop. 1762; Mem. Irish Academy, vol. iv.; Edinb. Trans. vol. iii.; Comment. Gottingen, vol. xiii.; Huygenii Dioptrica; Boscovichii Opera; Klingensteinia de Aberrationibus Luminis, &c. (G.B.)