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A commissure; especially, the optic commissure, or crucial union of the optic nerves. -- Chi*as"mal (, a..

An instrument for showing the optical effects of color.

The science of colors; that part of optics which treats of the properties of colors.

A device in a magic lantern or stereopticon to produce kaleidoscopic effects.

telescope of an astronomical or geodetic instrument, or the line which passes through the optical center of the object glass and the intersection of the cross wires at its focus.

ed by the action of cold water or air. See Illust. of Steam engine. Achromatic condenser (Optics), an achromatic lens used as a condenser. -- Bull's-eye condenser, or Bull's-eye (Optics), a lens of short focal distance used for concentrating rays of light. -- Injection condenser, a vessel in which steam is condensed…

parallel to the other. -- Conjugate focus (Opt.) See under Focus. -- Conjugate mirrors (Optics), two mirrors so placed that rays from the focus of one are received at the focus of the other, especially two concave mirrors so placed that rays proceeding from the principal focus of one and reflected in a parallel beam…

onancy of our persecuted church to the doctrines of Scripture and antiquity. Hammond. The optic nerve responds to the waves with which it is in consonance. Tyndall.

nt, the critical minute, on which every good work so much depends. South. Critical angle (Optics), that angle of incidence of a luminous ray at which it is wholly reflected, and no portion of it transmitted. The sine of this angle is the reciprocal of the refractive index of the medium. -- Critical philosophy, the met…

of calcite, or crystallized calcium carbonate, brought from Iceland, and used in certain optical instruments, as the polariscope. -- Rock crystal, or Mountain crystal, any transparent crystal of quartz, particularly of limpid or colorless quartz.

mark out with distinctness; to ascertain or exhibit clearly; as, the defining power of an optical instrument. Rings . . . very distinct and well defined. Sir I. Newton.

Distinctness or clearness, as of an image formed by an optical instrument; precision in detail.

by him to a property of light which he called diffraction. Whewell. Diffraction grating. (Optics) See under Grating. -- Diffraction spectrum. (Optics) See under Spectrum.

An optical instrument, invented by Hipparchus, for taking altitudes, leveling, etc.

The science of the refraction of light; that part of geometrical optics which treats of the laws of the refraction of light in passing from one medium into another, or through different mediums, as air, water, or glass, and esp. through different lenses; -- distinguished from catoptrics, which refers to reflected light…

different colored rays, arising from their different refrangibilities. Dispersion of the optic axes (Crystallog.), the separation of the optic axes in biaxial crystals, due to the fact that the axial angle has different values for the different colors of the spectrum.

to each other as to be seen separate only by means of a telescope. Such stars may be only optically near to each other, or may be physically connected so that they revolve round their common center of gravity, and in the latter case are called also binary stars. -- Double time (Mil.). Same as Double-quick. -- Double…

An arrangement of a fleet in a wedge or Encyc. Dict. Echelon lens (Optics), a large lens constructed in several parts or layers, extending in a succession of annular rings beyond the central lens; - - used in lighthouses.

image (Elec.), a collection of electrical points regarded as forming, by an analogy with optical phenomena, an image of certain other electrical points, and used in the solution of electrical problems. Sir W. Thomson. -- Electrical light, the light produced by a current of electricity which in passing through a resis…

An optical toy; a card on which confused or imperfect figures are drawn, but which form to the eye regular figures when the card is rapidly revolved. See Thaumatrope.