GB148754A - Improvements in apparatus for the objective measurement of the refractive value of the principal point of the eye - Google Patents

Abstract

148,754. Zeiss, C., [Firm of]. July 23, 1919, [Convention date]. Sight-testing apparatus.-The refractive error of the eye is measured by an instrument based on the principle that when a lens c is placed in front of the eye, with relaxed accommodation, so that the distance between the adjacent principal points of the eye and lens is equal to the focal length of the lens, the error of the eye in diopters is proportional to the distance of the conjugate point of the retina, with respect to both eye and lens from the second focal plane of the lens c. The image of the retina formed by the lens c is examined by means of a lens g, the emergent parallel rays being then observed by a telescope consisting of an objective m<1> and eye-piece p. A " Scheiner " disk h with two small holes h<1>, h<2> is rigidly fixed in the focal plane of the lens g. The retina is illuminated by a glow lamp r, an image of which is formed by lenses s<1>, s<2> and the prism t on the diaphragm h adjacent to the holes h<1>, h<2>. The lenses g, c form images of the lamps r and the holes h<1>, h<2> in the pupil of the eye, so that the retina is illuminated by diffused light and a virtual aperture consisting of two small holes is formed in the pupil of the eye. Two separate small pencils of light from the retina then pass through the instrument to prisms k<1>, k<2> and prisms l<1>, l<2> respectively, from which they pass through objectives m<1>, m<2> and prisms n<1>, n<2>, o<1>, o<2> to a common eye-piece p. The face o<3> of the prism o<2> is partially silvered so that the observer's eye sees two partial images. The lens g is moved until coincidence of the partial images is obtained, and the error of the eye in diopters is read off on a uniform scale d' by means of a pointer a<2>. By rotating the diaphragm h and the illuminating tube the error in any meridian may be determined, the meridian being indicated by a scale d<2>. Alternatively, these parts mav be fixed and a reflecting prism rotated through half the angle to the vertical of the meridian it is desired to examine. A mark s<3> on the lens o<1> is focussed on the retina to ensure relaxed accommodation.