Patent GB-432727-A (GB)
Title: Improvements in sight-testing apparatus
IPC: A61B3/103
Filed: 1934-02-03
Abstract: 432,727. Sight - testing apparatus. FINCHAM, E. F., 70, Hillfield Avenue, Hornsey, and CLARKE & CO., Ltd., C., 16, Wigmore Street, both in London, REASON, R. E., and KAPELLA, Ltd., 104, Stoughton Street, Leicester. Feb. 3, 1934, No. 3568. [Class 97 (i)] Relates to apparatus of the kind described in Specifications 413,029 and 430,761 comprising means for projecting an image of the target on to the retina, and a viewing system, including image dividing and reversing means, for observing the image. According to this invention, in order to facilitate determination of the refractive errors and the axis of astigmatism of the eye, the projecting system is adapted to form, through a small eccentric area of the pupil, an image of a target line either in the plane including the meridian passing through the said area, or in the plane of the meridian at rightangles thereto, and the viewing system is adapted to observe the image through another area of the pupil, the image dividing and reversing means being arranged to divide the image line into two or more parts and to reverse at least one part with respect to the remainder about an axis parallel to the line. Fig. 1 illustrates the principle of the invention, 1 being the pupil and 2 the retina of the observed eye. Light forming an image 3 of the target line can be projected through an area 4 of the pupil lying in a meridian a at right-angles to the line 3. The image is observed by a system of the kind described in the above Specifications so that, if it is not in focus on the retina, two images are seen which are relatively displaced in a direction at right-angles to the line, the refraction of the eye in the meridian a being determined by adjusting the target until the two images are in alignment. To find the axis of astigmatism, the refraction in different meridians must be measured and the maximum or minimum determined. To facilitate the determination of this axis the light may alternatively be projected through an area 7 in a meridian b containing the line 3. An observable displacement of the image is then produced only by refraction in a direction at rightangles to the meridian b, so that if this meridian contains, or is at right-angles to, the axis of astigmatism, the image is displaced only along its length and is therefore seen as an unbroken line, whether or not it is in focus. In any other direction the line is displaced laterally as well as longitudinally and a double image is seen. The axis of astigmatism is thus found by determining the meridians in which the image is unbroken. The refraction of the eye is measured as before by passing the light through the area 4. Alternatively only a single area of the pupil may be utilized, the target then consisting of two lines at right-angles which can be imaged alternatively or simultaneously on the retina, and the reversing device in the observing system being rotatable so that its imagedividing line is at right-angles to either one of the target lines. Fig. 2 shows an apparatus of this form. Light from a source 8 is projected through an aperture 11, an image of which is formed on the pupil 1 by a lens 14 to define the eccentric area 4. The light passes through the target 13 of the form shown in Fig. 3 and through the lens 14 and is directed through the area 4 by a prism 15, an image being formed on the retina 2 by the lenses 1, 14. The observing system comprises a lens 17, a dividing, reversing, and recombining system 16, 20, 24, as described in Specification 430,761, and an eyepiece 21. The target can be moved along the axis of the lens 14 for focusing, and the observed image is focused by moving the parts 16 to 21 along the axis of the lens 17. These two focusing movements may be arranged to be made by a single operation, for example by a pin 35 directly coupling the displaceable parts if the lenses 14, 17 have the same focal length. The observing system can be rotated about the axis 6 to a position appropriate to the target line being employed, and the whole apparatus can be turned about this axis to enable measurements to be made in any meridian. Another form is described in which focusing is effected by movement of a single lens which is common to both the projection and the observing systems. To fix the accommodation of the eye being tested, an image of a fixation object may be formed on the retina on or near the axis of the observing system by means including a transparent reflector disposed in the observing system.