Page:The American Cyclopædia (1879) Volume I.djvu/44

 ABERRATION ABEYANCE son appear to stand in T'. If the velocity of our earth was so much slower as to be for our most delicate instruments incompara- ble to the velocity of light, no apparent in- fluence would be exerted on the apparent direction, and there would be no appreciable aberration; but the relation happens to be within the pale of actual measurement. Tak- ing the length of the earth's yearly orbit in round numbers at 600,000,000 miles and the length of the year at 31,556,931 seconds, the velocity of our earth is nearly 19'2 miles per second ; and light being transmitted at the rate of 192,000 miles per second, it is clear that it travels about 10,000 times faster than the earth. If now we consider that an equal ve- locity would change the direction of the per- pendicular, or 90, into its half, or 45, we see that a velocity of only Tff .V olf would deviate the angle approximately y^.V^ of 45, or about 16 seconds. This, however, is a rough esti- mate; trigonometrical ly calculated, we obtain more, namely, 20 seconds. This now must be the maximum aberration produced by the yearly motion of the earth on the position of all stars observed at right angles to the direc- tion of that motion. They must all appear displaced to an amount of 20" forward, and this is in fact observed in all heavenly bodies at right angles to the plane of the ecliptic. As after six months the earth moves in an oppo- site direction at the other side of the sun, this displacement must be observed in an opposite direction after the lapse of every half year, making a total displacement of 40" in the position of all the stars situated near the poles of the ecliptic ; therefore they appear to have a yearly movement in small ellipses of 40" mean diameter, or about one fortieth the di- ameter of the moon. II. Aberration In Optical Instrument*. As white light is composed of colored rays of different refrangibility, any kind of refraction must split it up into rays of different colors. This is called dispersion. As the convex lenses used in telescopes, micro- scopes, and other optical instruments refract the light to focal points, this dispersion causes an infinite number of foci. Those consisting of the most refrangible rays, the violet, are the nearest to the lens, and they follow in the order of their refrangibility blue, green, yel- low, orange, and red ; the focus of the last is the furthest distant from the lens. This grand defect, called chromatic aberration, is correct- ed by the construction of achromatic lenses. sented in the adjoined figure, in which A B is the lens, V the focus of the most refrangible or violet, and R that of the least refrangible or red rays. Another defect, called spherical aberra- tion, arises from the nature of the curve used in making lenses and reflectors. Geometry prnvi-s that parallel rays can only be refracted and re- flected to a single focus by a parabolic curve ; however, lenses and reflectors are ordinarily ground as parts of a sphere, which differs from a parabola in the fact that in the latter the amount of curvature increases toward the cen- tre or axis. The consequence is that a sec- tion of a sphere, not having curvature enough toward this point, has an infinite number of foci at different distances; those formed by parts nearest to the axis will be the furthest off, while those formed by the refractions or reflections near the circumference of the lens or mirror will be the nearest. The two figures Chromatic Aberration. (See ACHROMATIC LENS.) The course of the rays producing chromatic aberration is rcpiv- Sphfrical Aberration. given here represent the case of this aberration by refraction and reflection : C D is the lens, of which the rays passing near the centre P are united in F, while the rays passing near the circumference D C unite nearer to the lens in E. G H is the curved mirror or reflector which reflects the rays U I and W K falling on it near its centre in N, while the rays S G and T II, falling on it near the circumference, are brought together much nearer in M. When the aper- ture of the lens or mirror is small, for instance only 5 or about ^ part of the circumference, these differences are practically inappreciable ; but when the aperture must be large, as is the case with astronomical telescopes, peculiar ar- rangements are contrived, so that in making the lenses or reflectors a curve is obtained as nearly as possible of the parabolic form. ABERYSTWITH, a seaport town of Cardigan- shire, Wales, near the outlet of the Ystwith and Rheidiol, 39 m. N. E. of Cardigan ; pop. in 1871, 6,896. It is a bathing place, and has considerable commerce and extensive fisheries. In the vicinity are many lead mines. ABEYANCE (la w Fr - Mayer, to expect, wait for ; Fr. bayer, to gape), a law term im- plying expectation, suspense, though by the signification preferred by the best authors t lie- thing in abeyance is conceived to be in the remembrance or consideration of the law. The title to a ship captured in war is said to