Patent GB-437764-A (GB)
Title: Improvements in sight-testing apparatus
IPC: A61B3/103
Filed: 1934-06-23
Abstract: 437,764. Sight-testing apparatus. REASON, R. E., and KAPELLA, Ltd., 104, Stoughton Street, Leicester. June 23, 1934, Nos. 18596/34 and 780/35. [Class 97 (i)] Relates to apparatus of the kind described in Specifications 413,029, 430,761, 430,836, and 432,727 comprising a projecting system for forming an image of a target on the retina, and an observing system for viewing the retina and the image formed thereon. According to this invention, in order to determine the refraction of the eye in different meridians, or to determine the axis of astigmatism, there is provided a rotatable reversing member which is common to the projecting and observing systems and has three or five reflecting surfaces. The apparatus shown in Fig. 1 comprises a lens 3 condensing light from a source 1 on to a target 2 in the form of a slit, the light diverging from the target passing through one of two slots 4, 5, Fig. 2, at right-angles to each other, in a plate 6 which can be moved diagonally by a screw 7. Beneath the plate 6 is a plate 8 having an aperture 9 and displaceable by a handle 10 so as to uncover either of the slots 4, 5, the transmitted light being received by a reflector 11 comprising two arms 11A, 11B, at rightangles to each other, so that by adjustment of the plate 8, light can be reflected at one or other of the arms and thence through one of two eccentric areas 90‹ apart on the subject's pupil. The reflected light passes through a lens 12 forming an image 13 of the target, a further image being formed by a lens 14 and the eye lens 15 on the retina. The lenses 12, 14 have their foci at the reflector 11 and pupil 15 respectively so that an image of the pupil is formed at the reflector. Between these lenses is mounted the rotatable reversing member comprising a prism 28 having three reflecting surfaces and with its entrance and exit faces approximately normal to the incident and emergent light. An image of the retina is formed at 13 by the lenses 14, 15 and a further image is formed at 18 by the lens 12, the rays forming the image 18 being divided, partially reversed, and observed as described in the above-mentioned Specifications. In order to focus the target image on the retina the whole of the system to the left of the lens 14 can be displaced along an axis 27, and in order to enable measurements to be made in different meridians, the prism 28 can be rotated about an axis 29 coinciding with the axis of the pupil and parallel to, but offset from, the axis 27. In order to observe the pupil and to align the observing system with respect thereto, lenses 30, 31 can be inserted in the observing system as described in Specification 430,836. To avoid ghost reflections from the face 34, Fig. 9, of the prism 28, an opaque mask 11<1> may be placed in the position of the image of the mirror 11 formed by the lens 12 and surface 34, or the lens may be placed so close to the prism that the reflected rays are thrown outside the field of view. Ghost images from the face 35 of the prism are avoided by inclining this face sufficiently to throw reflected light to one side. This face is also arranged at such a distance from the image 13 that light scattered at the face is prevented from entering the observing system and that errors due to aberration at the surface are avoided. To facilitate determining the proper adjustment to align the observing system with the pupil, a rotatable plate 39 bearing an index mark may be arranged in the plane of the pupil image formed when the lenses 30, 31 are in position, this plate being coupled to the prism 28 so as to rotate in the same direction as the prism but at twice the speed.