OPTIC

, or Optical, something that relates to vision, or the sense of seeing, or the science of optics.

Optic Angle. See Angle.

Optic Axis. See Axis.

Optic Chamber. See Camera Obscura.

Optic Glasses, are glasses ground either concave or convex; so as either to collect or disperse the rays of light; by which means vision is improved, and the eye strengthened, preserved, &c.

Among these, the principal are spectacles, reading glasses, telescopes, microscopes, magic lanterns, &c.

Optic Inequality, in Astronomy, is an apparent irregularity in the motions of far distant bodies; so called, because it is not really in the moving bodies, but arising from the situation of the observer's eye. For if the eye were in the centre, it would always see the motions as they really are.|

The Optic Inequality may be thus illustrated. Suppose a body revolving with a real uniform motion, in the periphery of a circle ABD &c; and suppose the eye in the plane of the same circle, but at a distance from it, viewing the motion of the body from O. Now when the body goes from A to B; its apparent motion is measured by the angle AOB or the arch or line HL, which it will seem to describe. But while it moves through the arch BD in an equal time, its apparent motion will be determined by the angle BOD, or the arch or line LM, which is less than the former LH. But it spends the same time in describing DE, as it does in AB or BD; during all which time of describing DE it appears stationary in the point M. When it really describes EFGIQ, it will appear to pass over MLHKN; so that it will seem to have gone retrograde. And lastly, from Q to P it will again appear stationary in the point N.

Optic Nerves, the second pair of nerves, springing from the crura of the medulla oblongata, and passing thence to the eye.

These are covered with two coats, which they take from the dura and pia mater; and which, by their expansions, form the two membranes of the eye, called the uvea and cornea. And the retina, which is a third membrane, and the immediate organ of sight, is only an expansion of the fibrous, or inner, and medullary part of these nerves.

Optic Pencil. See Pencil of Rays.

Optic Place, of a star &c, is that point or part of its orbit, which is determined by our sight, when the star is seen there. This is either true or apparent; true, when the observer's eye is supposed to be at the centre of the motion; or apparent, when his eye is at the circumference of the earth. See also Place.

Optic Pyramid, in Perspective, is the pyramid ABCO, whose base is the visible object ABC, and the vertex is in the eye at O; being formed by rays drawn from the several points of the perimeter to the eye.

Hence also may appear what is meant by Optic triangle.

Optic Rays, particularly means those by which an Optic pyramid, or Optic triangle, is terminated. As OA, OB, OC, &c.