Patent GB-357931-A (GB)
Title: Improvements in methods and instruments for measuring the curvature of the cornea
IPC: A61B3/107
Filed: 1931-03-09
Abstract: 357,931. Sight testing apparatus. ZEISS, C., [Firm of], Carl-Zeiss-Strasse, Jena, Germany. March 9, 1931, No. 7208. Convention date, March 13, 1930. [Class 97 (i).] In an instrument for determining corneal curvature by measuring the distance between the reflected images produced by the cornea of two test objects arranged in a plane containing the axis of the eye, these images being imaged by an optical observing device, the view of the two test objects is presented to the eye under a constant angle and optical means are provided to separate from the pencil of imaging rays of the observing device two partial ray pencils. The curvature of the cornea is determined from the relative positions of the images of the test objects. The marks are rotatable around the axis of the observation device so that their plane can be made to coincide with a principal section plane of an astigmatic eye. Preferably a second pair of marks is provided on the same observing device, the plane of these marks being at right-angles to that of the first-mentioned marks so that the curvatures of the two principal section planes can be determined at one observation. The instrument shown comprises four collimators 3 mounted on a microscope 14 so that their axes lie in two planes at right-angles to each other and each containing the axis of the microscope. The axes of the collimators intersect that of the microscope at the same point 10. Each collimator contains a lamp 4 and a condenser consisting of two plano-convex lenses 6, 7, the test objects being marked on the plane lens surfaces 8 .. 8<111> furthest from the lamp, which surfaces are in the focal planes of lenses 9. The test marks in the collimators lying in a horizontal plane consist of a cross 29, Fig. 6, having its horizontal line elongated and two spaced horizontal lines 30, 30<1>, Fig. 5, marked with scales 31, 31<1> of radii of curvature and refractive powers respectively. The marks in the other two collimators are similar, Figs. 7 and 8, but arranged vertically. The observing microscope 14 comprises an objective 15 in the focal plane of which is a plate 35 bearing a luminous spot 36 to be observed by the eye 12 under examination. The ocular 16 of the microscope is mounted in a sleeve 17 carrying a plate 18 on which are four radial marks 19, Fig. 11, spaced at 90‹ from each other. The microscope contains a reversing system comprising two plano-convex lenses 20, 21, the plane surface of the lens 20 being formed as a diaphragm having four rectangular apertures 23 .. 23<111>, Fig. 3, behind which are disposed refracting prisms 24 .. 24<111>. The whole assembly of microscope and collimators can be rotated in a bearing 1 about the axis of the microscope, a graduated dial 25 being provided for measuring the angle of rotation. In using the instrument the sleeve 17 is adjusted until the images of the test objects are seen in focus with the lines 19. Then, supposing the eye to be astigmatic and its principal section planes not horizontal and vertical, the whole instrument is turned in the bearing 1 until, as shown in Fig. 11, the longer lines of the crosses 29, 32 bisect the spaces between the lines 30, 301 and 33, 331 respectively. The meridians of the principal planes are then obtained from the graduations 27 on the dial 25, and the radii of curvature and refractive powers in these planes from the positions of the shorter lines of the crosses in relation to the scales 31, 31<1> and 34, 34<1>.