| 摘要 |
<p>An optical instrument for visual acuity testing has a lens system (14,16,18) with an occular or objective lens (15) all on the same optical axis. One of the lenses (14) is of spherical type of fixed focal length while the others (16,18) are of variable cylindrical strength and cylindrical axis. A test mark (50) is provided in the spherical lens (14) and this (50) and the lens system (14,16,18) are so arranged that relative motion along the optical axis between two points is possible. The two points are somewhat shorter and longer respectively than the fixed focal length of the lens (14).</p> |
