Patent GB-358384-A (GB)
Title: Improvements in ophthalmic instruments
IPC: A61B3/103
Filed: 1931-01-12
Abstract: 358,384. Sight-testing instruments. ZEISS, C., [Firm of], Carl-Zeiss-Strasse, Jena, Germany. Jan. 12, 1931, No. 961. Convention date, Jan. 15, 1930. [Class 97 (i).] Relates to instruments for objectively determining the refraction of the principal point of the eye and the power of the lens required to correct ametropia, by finding by means of an observation device the locus of the images of surface elements of the fundus produced by a lens system and the optical system of the eye, the fundus being illuminated by a number of separate non-axial ray pencils in the path of which is interposed a test object that can be adjusted along, and rotated around, the optical axis. According to the invention the test object consists of two marks lying in planes at right-angles to the optical axis, these planes being capable of being brought into approximate coincidence. The instrument comprises a source of illumination 2 and a condenser 4 imaging the filament in the square aperture 6 of a diaphragm 5. Above the diaphragm is a plano-convex lens 7 which has four hexagonal transparent portions 9 .. 9<111>, Fig. 5, the remainder of this lens being opaque. Prisms are cemented to the transparent parts to deviate the transmitted pencils and produce four non-axial pencils by which the aperture 6 is imaged on the plane surface of a plano-convex lens 21, this lens being opaque except for four square transparent portions equally spaced around its periphery. The transmitted pencils are received by a mirror 18 and directed through a Dove prism 27 along the axis of a casing 34 in which are mounted rings 44, 45 carrying the plates 48, 49 bearing the two test marks. The ring 44 is mounted on a slide 35 which can be displaced by means of a screw 37 extending through the casing 34 and having at its outer end a pinion 38 engaging with teeth on a ring 12 rotatably mounted on the tube 3. A scale of dioptres 13 on the ring can be used against a fixed index 16. The ring 45 can be adjusted relatively to the ring 44 by a rack 42 and pinion 41, these relative movements being indicated on a dioptre scale on the outside of the casing 34. The rings 44, 45 are also provided with forks 50, 51 by which they can be rotated together about the optical axis, a pin 53 passing through the forks being secured to a rotatable head 32. A pointer 54 indicates on a scale 55 the angular setting of the rings. The mounting 26 for the prism 27 is rotatable within a sleeve 25 and is geared to the head 32 so that the prism rotates through half the angle of rotation of the rings. The test mark on the plate 48 comprises crosses of which the lines 56, 56<1>, Fig. 9, are portions of a diametral line and are marked in transparent rectangles 58, 58<1> and the mark on the plate 49 comprises similar crosses, the parts 57, 57<1>, Fig. 10, being on a diameter at right-angles to the line 56, 56<1>. The plates also have transparent portions 62 .. 63<1> and transparent central apertures 60, 61 covered by a green screen 64. The remaining parts of the plates are opaque. The light passing through the plates is received by a lens 72 which, in conjunction with the lens of the eye 86, produces images of the test marks on the fundus of the eye. The light returned from the fundus passes back through the plates and the prism 27, then through a central aperture 20 in the mirror 18, and is received by an observing device comprising an objective 78 and eyepiece 85. The mounting 75 for the objective 78 is slidable within the tube 74 and is provided with an external thread 79 engaged by an internal thread in a pinion 80 connected by gearing 82, 83 to the ring 12, the gearing being such that the image of the marks on the plate 48 produced by the objective 78 remains in a fixed plane. The eyepiece 85 is adjustable by a screw-thread 84. In the case of symmetrical ametropia the plates 48, 49 are displaced together axially by turning the head 12 until the marks are imaged on the fundus, in which position the said images are themselves imaged by the eye and the lens 72 so as to produce images of the marks covering the marks themselves. In the case of astigmatism the plates are adjusted axially and are rotated by turning the head 32 until the images of the marks 56, 56<1> completely cover the marks themselves and the plate 49 is then adjusted axially relatively to the plate 48 until the marks 57, 57<1> are completely covered by their images. The rotation of the prism 27 ensures the marks being always illuminated by the pencils passing through the lens 21 and also prevents rotation of the images seen by the observer.