EYE-PIECE

in a telescope, microscope, or other optical instrument, the lens, or combination of lenses to which the eye is applied. As the use of & single lens limits the field ef view, eyepieces are generally formed from two tenses or lens systems, called the eyejens and the field-lens, placed at a distance apart. The Ramsden or ositive eye-piece consists of two enses of equa! focal length placed at a distance apart equal to two-thirds of the focal length of either lens. The object locked at requires to be a short distance in front of the field-lens. In the Huyghens or negative eye-piece the focal lengths of the field and eye lenses are in the ratio 3 to 1, and the distance between the lenses is twice the focal length of the eye-lens. As a result the image formed by, say, the objective of a telescope is in focus when between the two lenses.