Crown-glass is the hardest and most colourless kind of window-glass, made almost entirely of sand and alkali and a little lime, and used in connection with flint-glass for optical instruments in order to destroy the disagreeable effect of the aberration of colours. Research Crown-Glass
James Bradley was an English astronomer. He was born in 1692 at Sherborne, and died in 1762. He studied theology at Oxford, and took orders; but devoting himself to astronomy he was appointed, in 1721, professor of that science at Oxford. Six years afterwards he made known his discovery of the aberration of light, and his researches for many years were chiefly directed towards finding out methods for determining precisely that aberration. It is largely owing to Bradley's discoveries that astronomers have since been able to make up astronomical tables with the necessary accuracy. In 1741 he was made Astronomer Royal and removed to Greenwich. Research James Bradley
In optics an aberration is a defect in the image formed by a lens or curved mirror. In chromatic
aberration the image formed by a lens, but not a mirror, has coloured fringes as a result of the different extent to which light of different colours is refracted by glass. It is corrected by using an achromaticlens. In spherical aberration, the rays from the object come to a focus in slightly different positions as a result of the curvature of the lens or mirror. For a mirror receiving light strictly parallel with its axis, this can be corrected by using a parabolic surface rather than a spherical surface. Spherical aberration in lenses is minimized by making both surfaces contribute equally to the ray deviations, and can (though with reduced image brightness) be reduced by the use of diaphragms to let light pass only through the centre part of the lens. In astronomy an aberration is the apparentdisplacement in the position of a star as a result of the earth's motion round the sun. Light appears to come from a point that is slightly displaced in the direction of the earth's motion. The angular displacement a = v/c, where v is the earth's orbitalvelocity and c is the speed of light. Aberration was discovered in 1728 by the English astronomer James Bradley and was the first observational proof that the Earth orbits the Sun.
In astronomy, aberration is a small periodical change of the position in the stars and other heavenly bodies, due to the combined effect of the motion of light and the motion of the observer; this is called annual aberration, when the observer's motion is that of the earth in its orbit, and daily or diurnal aberration, when of the earth on its axis; amounting when greatest, in the former case, to 20.4', and in the latter, to 0.3'. Planetary aberration is that due to the motion of light and the motion of the planet relative to the earth. In optics, aberration is the convergence to different foci, by a lens or mirror, of rays of light emanating from one and the same point, or the deviation of such rays from a single focus; called spherical aberration, when due to the spherical form of the lens or mirror, such form giving different foci for central and marginal rays; and chromatic aberration, when due to different refrangibilities of the coloured rays of the spectrum, those of each colour having a distinct focus. Research Aberration
In optics, achromatic refers to transmitting colourless light, that is, not decomposed into the primary colours, though having passed through a refracting medium. A single convex lens does not give an image free from the prismatic colours, because the rays of different colour making up white light are not equally refrangible, and thus do not all come to a focus together, the violet, for instance, being nearest the lens, the red farthest off. If such a lens of crown-glass, however, is combined with a concave lens of flint-glass - the curvatures of both being properly adjusted - as the two materials have somewhat different optical properties, the latter will neutralize the chromaticaberration of the former, and a satisfactory image will be produced. Telescopes, microscopes, etc, in which the glasses are thus composed are called achromatic. Research Achromatic
In optics, aplanatic is a term specifically applied to reflectors, lenses, and combinations of them, capable of transmitting light without spherical aberration. An aplanatic lens is a lens constructed of different media to correct the effects of the unequal refrangibility of the different rays. Research Aplanatic
Chromatic aberration is the presence of prismatic colours at the edges of an optical image due to the refractive index of the lens material being different for light of different frequencies. Research Chromatic Aberration
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Aberration
