Patent GB-413029-A (GB)
Title: Improvements in apparatus for the objective determination of the refraction of the eye
IPC: A61B3/103
Filed: 1933-01-24
Abstract: 413,029. Sight-testing apparatus. CLARKE & CO., Ltd., C., 16, Wigmore Street, and FINCHAM, E. F., 70, Hillfield Avenue, Hornsey, both in London. Jan. 24, 1933, No. 2307. [Class 97 (i).] Apparatus for the objective determination of the refraction of the eye comprises an illuminated target and a lens for projecting light from the target to the eye, and an observing system having its axis displaced from the axis of the target and lens, the observing system comprising means whereby the observed retinal image may be divided into two or more relatively shifted or reversed parts. Fig. 1 shows the optical arrangement, the target 13 and projection lens 19 having a common axis b and receiving light from a source 14 by way of a condenser 15, colour filter 16, prism 17, and a slotted diaphragm 18 which is displaced slightly from the axis b. The light emerging from the lens 19 is twice reflected by a rhomboidal prism 20 so as to enter the eye c at a point slightly displaced from the axis and to form an image on the retina, which image lies on the axis a when the eye is emmetropic. The observing apparatus consists of a telescope comprising lenses 11, 12 and an eyepiece 10, and of an image-splitting system 21, behind the eyepiece, comprising a pair of parallel-sided plates 22, 22a one above the other and equally and oppositely inclined to the axis a, and two prisms 23, 24, joined together. The prism 24 has twice the height of the prism 23 as shown in Fig. 10. Light incident on the plates is divided into two beams as shown in Fig. 8, the upper beam passing wholly through the prism 24, within which it is totally reflected, and the other beam being refracted by the compound prism 23, 24 so that both beams emerge from the same face of the prism 24 and two images are formed. These images coincide when the incident rays enter along the axis of the system, i.e., when the image formed on the retina lies on the axis a. The images can be made to coincide by displacing the target 13 along its axis b, and the position of the target can be indicated on a scale which can be graduated to indicate the refractive errors of the eye. Two thin prisms 25, 25a may be provided in front of the prism 24 to eliminate haziness of the image in the region of the line of division. The apparatus is carried by a base 26 on which it can be adjusted laterally, longitudinally, and vertically by screws 31, 32, 33, and also about the vertical axis under the control of a screw 34. The telescope 35, to which is secured the illuminating and target tube 40, is mounted in a bracket 36 so as to be rotatable about its own axis to enable the eye to be examined in any meridian, the telescope being secured in any desired position by a clamp 37 to an angularly graduated plate 38. The target 13 is carried by a tube 43 which can be adjusted within the tube 40 by means of a rack 44 and pinion 46, and carries a pointer 48 co-operating with a scale 49. An auxiliary slidable target 51 is provided to fix the accommodation of the eye. A modification is described wherein both the position of the target and the angle of rotation about the axis a can be read from the eyepiece end of the telescope.