Determination of the Focal Length of a Lens by experiment. I. Celestial Sphere.-The imaginary spherical surface by which space is apparently bounded, the eye of the spectator being the centre. II. Secondary Circles.-Great circles perpendicular to the plane of a given circle. III. Zenith and Nadir.-The points in which a plumb-line at any place, produced indefinitely, may be supposed to meet the celestial sphere above and below that place respectively. |