Patent GB-242626-A (GB)
Title: Improvements in apparatus for the objective measurement of the refractive value of the principal point of the eye
IPC: A61B3/103
Filed: 1925-10-29
Abstract: 242,626. Zeiss, C. [Firm of]. Nov. 5, 1924, [Convention date]. Sight-testing apparatus.- Apparatus for the objective measurement of the refractive value of the principal point of the eye, of the character described in Specifications 148,754 and 191,015 and without restriction of the measuring range is provided with an observation device having a diaphragm relatively to which the light aperture is so disposed that the diaphragm and light aperture are imaged side by side in the pupil of the eye to be examined. A telescope h carried on an adjustable bar b mounted in the stand a is provided with an eye-lens i<1>, an objective h<1>, h<2>, a diaphragm m and two plates k, l in the focal plane of the objective. The plate k carries the graduation marks r, s, Fig. 3, and the plate l four lines t<1> - - t<4>. An illuminating tube n carried by the telescope h is provided with an optical system for the projection of a test mark o on one of the lenses through the diaphragm p by means of a prism o eccentrically to the optical axis of the telescope. The test mark comprises an opaque disc with apertures v<1> - - v<4> and apertures x and w. the latter denoting the optical axis. An additional lens h<3> is fitted in the telescopic tube and used in conjunction with the lens d mounted on the stand a. For reading the diopter scale b<2> a prism h<4> is also fitted to the tube h. The pencils by which the diaphragms m, p are imaged in the pupil of the eye are, parallel between the lenses h<4>, d. For the examination of astigmatism the tube h together with the illuminating apparatus and the plate l are rotatable about the optical axis of the telescope in the support e<1>. In use the eye y of the patient is placed with the pupil y<1> in the focal plane of the lens d, the luminous circle w serving to position it on the axis. In this position the diaphragm m, and the light aperture p are imaged side by side in the pupil y<1>, their relative position indicated on the scale r determining the measuring meridian. In a normal eye the image observed in the eyepiece i<1> will be, shown in Fig. 3, with the lines t<1> - - t<4> in the centres of the rectangles v<3> - - v<4>. In an axially symmetrical ametropic eye, the image will be indistinct and the lines laterally displaced in the direction of the measuring meridian. Adjustment of the telescope by the pinion g until the image is distinct and the marks central will give a reading on the scale b<2> of the refractive error of the eye. In an astigmatio eye the marks t<1> - - t<4> will appear displaced both horizontally and vertically and the illuminating device is then rotated until the marks t<1>, t<3> are in the middle of the corresponding rectangles v<1>, v<3>. The telescope) is then moved until the other lines are in their respective rectangles, and thus indicating on the scale r one of the principal meridians and on the scale b<2> the error in that meridian. Rotation through a right angle of the illuminating apparatus and re-adjustment of the telescope gives the error in the other meridian. Specification 198,667 also is referred to.