Page:The New International Encyclopædia 1st ed. v. 01.djvu/64

heavy line with a cusp at Q. In fig. 2 the elimina- tion of spherical aberration by the use of a para- bolic mirror is shown, as here, by the peculiar property of a parabola (q.v. ), all rays parallel to the axis are brought to a point at F, called the focus. For this reason the parabolic mirror is theoretically the most available for telescopes (q.v.), but in practice the construction of such mirrors presents great difficulties, which are but rarely effectually surmounted. The effect of spherical aberration in the case of a lens is indicated in fig. 3, where the tays passing through the lens near its circumference are brought to a focus at C, while those lying nearer the axis AB meet at or near F. The foci for intermediate rays lie between that point >and C. From these diagrams the advantages obtained by the use of diaphragms will be seen. The oblique rays, or those which strike the mirror or

Fig. 3. lens at a distance from its centre, and which do not come to a focus at the same point as those passing through the central portion, are accordingly cut off and the image rendered more distinct. The spherical aberration of lenses can be reduced by using two or more lenses in com- bination, as is done in the case of most photo- graphic objectives. Two lenses with equal focal lengths can be combined, and their effect is the same as a lens with one-half the focal length, while the spherical aberration is greatly dimin- ished. The books of reference mentioned under Aberration, Chromatic, will also supply ample information on this subject.

ABERRATION OF LIGHT. An expression used to describe the phenomena that arise from the fact that light requires appreciable time for its transmission through space. The motion of light traveling from a star or a planet toward the earth, combined with the earth's own motion, causes an apparent displacement of the stars on the sky: they all appear to occupy positions a little different from their true ones. In explaining this phenomenon, we often use the analogy of a man running in a rain-storm. Though the raindrops may be falling straight down, they will seem to the running man to descend on his face slantingly. Light, too, may be coming down, as it were, vertically, but as the earth, with the observer on it, is hurrying through space, there will be produced a similar apparent slant of the light, and we shall see the stars displaced on the sky in the direction of the terrestrial motion. But since the motion of our planet takes place in a closed, oval curve, the apparent displacement of the stars is now in one direction, and now in another, corresponding to the earth's position in one or the other half of its oval path. The result is that the stars themselves seem to move each year through a small curve; and this is a sort of miniature reproduction of the earth's orbit around the sun. When the celestial body under observation is itself in motion with respect to our earth, as is the case with the other planets of the solar system, a further somewhat analogous displacement is produced. Astronomers therefore need to correct all their observations by a process of calculation, so as to reduce them to what they would be if no such thing as aberration existed. Aberration was discovered by James Bradley, and was announced to the Royal Society of England in 1729.

The Constant of Aberration. From what has been said above it may be seen that the quantity of apparent displacement depends on the velocities both of light and of the earth. The nature of that dependence is quite simple: the velocity of light is known in miles per second from laboratory experiments; the amount of possible aberration, while inversely proportional to the velocity of light, is large in proportion to the earth's speed. If, therefore, we could determine by direct observation of the stars just how much they are displaced, it would be possible to calculate the earth's orbital velocity from the size of the aberration. The aberration may be determined by the simple method of observing a star at intervals during the year and noting how much its position changes. If we select a star most favorably situated for this purpose, we find that its position throughout the year will vary from the average by a little more than twenty circular seconds. This number (more exactly 20".47) is called the constant of aberration. To measure this constant with the utmost possible precision has long been the object of very earnest efforts: and few other astronomical problems have received so much attention in recent years. Its particular importance, as we have seen, is due to the computations rendered possible by a knowledge of the constant. Combined with the known velocity of light, it gives us the earth's orbital velocity in miles per second. From this we get the length of the annual terrestrial orbit in miles, and then by a simple calculation we find its semi-diameter, or the distance from the earth to the sun. This last is the fundamental unit for astronomical measures of distance, and its exact evaluation is considered the most important of all astronomical problems. See Parallax, section Solar Parallax; Sun.

ABERSYCHAN. ib'f rsik'an. A town in Monmouthshire, England, about 10 miles north of Newport, in the coal district (Map: England,