[f. EYE sb.1 + PIECE.]
1. Optics. The lens or combination of lenses, usually two in number, known respectively as the field-glass and eye-glass, at the eye-end of a telescope, or other optical instrument, by which the image, formed by the mirror or object-glass, is viewed and magnified.
The principal kinds of eye-pieces are (a) the Huyghenian, or so-called negative from the fact of its forming the image between the lenses; (b) the Ramsden, or common astronomical, called positive because the image is formed outside the field-glass; (c) the erecting or terrestrial for ordinary telescopes, which presents the object in an erect position.
1790. Roy, in Phil. Trans., LXXX. 155. The common eye-piece with two convex glasses.
1831. Brewster, Optics, xliii. 360. Achromatic eyepieces … may be composed of two or three lenses.
1867. J. Hogg, Microsc., I. ii. 40. The two [eye-glass and field-glass], when combined, are termed the eye-piece.
1878. Lockyer, Stargazing, 111. The Achromaticity of the Huyghenian Eyepiece.
b. attrib. as eyepiece micrometer. (See quot.)
1874. Knight, Dict. Mech., Eye-piece Micrometer, a graduated slip of glass introduced through slits in the eyepiece tube, so as to occupy the center of the field.
2. Australian.
1880. Leeds Mercury, 16 Nov., 7. The power of a lazy free-selector to pick out the eye-piece of a squatter’s run.
