Audi electric ophthalmoscopes were designed by Schweigger, Wolff and others. To determine the refractive state of the eyes, independent of influences on the part of the person to be examined.
In addition to ophthalmoscopy, examiners in Germany have also increasingly used retinoscopy (shadow test) over the past ten years. If you illuminate an eye with the ophthalmoscope, the eye hole (pupil) appears red. If you now make slight turns with the mirror, a part of the red glowing eyehole is darkened by a shadow from the edge. The appearance of the shadow occurs either in the same direction with the mirror rotation or in the opposite direction, and not by chance, but according to quite specific laws. The type of mirror used for the examination (whether plane or concave mirror) influences the movement of the shadow, as does the distance of the observer from the person being examined, and finally the refractive state of the eye being examined. If one keeps the mirror and the distance unchanged, the shadow migration depends only nosh on the refractive state. In an investigation, e.g. B. with a concave mirror from a distance of 1 m, the shadow goes in the red eye hole of the examinee only then in the same direction with the mirror rotation, if the examinee has a short-sightedness that is stronger than one diopter, the examined eye is weakly near-sighted or normal , or far-sighted, then the shadow moves at the same distance and same type of mirror in a direction opposite to the mirror rotation. In order to determine the degree of short-sightedness (or long-sightedness) more precisely, one holds ever stronger lenses (concave or convex lenses) in front of the examined eye and observes the movement of the shadow. For easier handling, the lenses are mounted on rotatable panes or on ladder-like frames, so that the lenses can be changed quickly by simply moving the carrier (Fig. 6).

One always finds a glass through which the shadow movement is so influenced that it turns into the exact opposite of the previous one. These concave (or convex) glass that has achieved the envelope indicates the degree of myopia (or hyperopia). If a plane mirror is used for the investigation, the behavior of the shadow is exactly the opposite of that of the concave mirror. This method was first described by Cuignet (1873) as keratoscopy, later by Landolt as pupilloscopy and by Parent (1880) as betinoscopy and properly justified. In addition to the above, there are a number of other names, such as dioptroscopy, fantoscopy, scotoscopy, retinoskiascopy, illumination test, photoptoscopy, but the most common are skiascopy(introduced by Chibret in 1886) and shadow test (after Pfliiger). Cf. Fick, determination of the refractive state of an eye by means of a shadow test (Wiesbaden 1891); Neusteitter , outline of the theory and practice of the shadow test (Munich 1900). Burch the retinoscope has been more or less supplanted: the optometers, apparatuses, which are used especially in mass examinations (schools, military enlistments, etc.) for the rapid determination of retraction and visual acuity. They are built on different principles. The eye to be examined usually looks for a test object through one or two combined lenses. Either this or the lenses are shifted, whereby the rays falling into the eye are changed in their direction, can be made either parallel, convergent or divergent in adaptation to the existing refraction. This can then be easily read off a scale. Despite the speed of determining the refraction, all of these optometric methods have not been able to hold up, since the eye looking into the instrument involuntarily accommodates, which affects (increases) the refraction. The radii of curvature of the breaking flinches are determined by means of ophthalmometry. With this one first determines the size of the mirror images reflected from the curved surface of items of known magnitude. The radius of the surface of curvature can be easily calculated from the imagery found, because the size of the object (a) is related to the size of the image (C) as the distance of the object from the mirror surface is to the distance of the image from the same ben. Since the image is created with convex mirrors in the focal point of the mirror, but the focal length of these mirrors is equal to half the radius of curvature (½r), one can use r instead of ‘distance of the image’; The ophthalmometer first constructed by Helmholtz (Fig. 7)

consists of two plane-parallel glass plates gg, which can be rotated about a common axis. If the plates are perpendicular to the axis, they are in the zero position (g1 g1). The plates can be moved by the same angle in opposite directions by means of a gear mechanism. The size of the angles can be read on a vernier. The observer (B) sees through the telescope (It) on the cornea of the person being examined (A) who is looking towards F, the two reflex images of two flames 1 and 2. If the glass plates are in the neutral position, they are without them Influence on the reflex images. But if they cross at some angle, the millennium of rays is shifted to one side, the other half to the other side: each of the flames then appears doubled. Now move the plates so that of the four images the middle ones coincide, one can calculate the distances between the reflex images of 1 and 2 from the read off angle of rotation Also calculate the radius of curvature of the cornea. Helmholtz have Coccias, Snellen n.a. ophthalmometer, however, because of its ease of use, the ophthalmometer by Jacob and Schiotz (Fig. 8)

has become popular.
The instrument is based on the same principles as that of Helmholtz, but the doubling of the reflex images is not achieved by parallel glass plates, but by a birefringent one of the calcite crystal or a birefringent glass prism system. The ophthalmometer consists of a tube R fastened to an adjustable stand iv, with the eyepiece 0. In the tube there is the calcite crystal or the prism system between two objective lenses of the same focal length attached. The eye to be examined is at the focal point of the outer lens, in which there is a crosshair on the inside, on which the eyepiece is to be focused. On the tube is a metal arc (M) with a graduation and two white sliding marks (L and Li). The circular arc with the marks can be rotated around the axis of the tube at the same time as the prisms. The patient places his head in the wooden frame C and places his chin on the adjustable support K. The eye not to be examined is covered by the plate e, while the eye to be examined looks into the opening of the tube. The examiner now sees the marks (L and Li) duplicated on the patient’s cornea, moves the marks until the middle images touch and then reads the distance from the arc division. The value found immediately corresponds to the refractive power of the cornea in dioptres. In conclusion, one only needs to divide the distance of the object from the focal point of the cornea (= 337 mm) by the value found to find the radius of curvature of the cornea. Since the arc can be brought into any meridian by turning it, the values sought for each of them can easily be found. The instrument is therefore also used to determine astigmatism and is called the setting-monitor.
When examining the functions of the eye, the first thing to note is the sharpness. A thorough examination of the eye, because of the smallness and delicacy of its individual parts, and because of the multiplicity of the functions of the organ, is not possible without a large apparatus of instruments. Viewing with the naked eye in daylight is generally only sufficient. the examination of the lids, the conjunctiva, the muscles and gross changes in the eye itself. It is limited by the smallest distance that two points may have in order to still be perceived separately. If you connect both points with the nodal point of the eye, you get the visual angle; the normal human eye still perceives objects presented to it with an angle of 1 Alin. to appear. This fact is used by Snellen in making
his sebum samples (optotypes). The letters are arranged so that each part of the same enters the angle of 1 min. the whole letters enter with an angle of 5 min. (akb) be seen (Fig. 9).

Therefore the letters are of different sizes for different distances. Snellen has now written on his letter boards, next to each row, the number of meters at which the letters in question should normally be seen. sees e.g. E.g. an eye has the letter with the designation 6 in 6 m, then this eye 6/6 = 1, i.e. full visual harp; sees another eye in the same distance must be the letter with the designation 6/12=2 the normal visual acuity. Examination of the visual field is important for many diseases, particularly those affecting the Amgen background, but also the visual pathways in the brain. It is understood to be the area in which we can perceive other objects, even if they are indefinite, at the same time as a sharply fixed point. When you fix the fixation point with the posterior eye center (the macula lutea), but all other points with peripheral parts of the retina, one also distinguishes between central, direct and peripheral, indirect vision. The quickest way to determine the limits of the field of vision is as follows: Doctor and patient stand facing each other and look firmly into one of the opposite eyes, while the other is closing. The physician now moves the outstretched hand along the fixation line from the periphery until the patient notices it, and thereby satisfies himself whether the limits of the patient’s field of vision approximately correspond to his own. Forster’s perimeter (Fig. 10)

permits a more precise measurement of the field of view. The same consists in a graduated semicircle (H) which is rotatable in all meridians around its vertex. In the center of the resulting hollow sphere is an adjustable chin rest that the patient places on his head. The eye to be examined fixes the center point of the perimeter arc with the other covered, the doctor pushes a white mark (m) from the periphery into the visual field using a crank device and best on the scale of the arc the number at which the patient has perceived thesis. The normal facial fold is 90° on the outside, 55° on the inside, 55° on top and 60° on the bottom. The light sense is examined with Forster’s light sense meter (Fig. 11).

It is important to determine the lowest still perceptible lighting limit (stimulus threshold) or the smallest difference in brightness (difference threshold). Per Apparatus consists of a closed box into which the patient can see through two openings (a and ai); on the opposite wall are large black lines on white paper (P). The box receives the light from the outside through an opening (L) that can be adjusted with a screw, the size of which can be read on a scale (M). You determine the opening size and thus the degree of brightness that the eye needs to just recognize the lines.
The investigation of the sense of color is of great importance for many branches of industry, especially those which have to observe colored signals, such as railway officials, ducks. The three most commonly used examination methods are: 1) The Holmgren method, which consists of taking one sample from a large number of colored embroidery wool samples and asking the examiner to search for all samples of the same color. Put the latter to a green sample z. e.g. grave or pink-colored or the like, then it is colorless, 2) the table of Daae, consisting of ten color rows of seven colored feathers each, but of which only two rows contain fields of the same colour. If the person to be examined gives other rows as having the same color, then he is also colorless; 3) Stilling’s pseudoisochromatic tables show colored numbers or letters on a background of a different color. The numbers cannot be distinguished from the reason by the need for colors.

   This article has been translated from an 1888 Antique German Publication. It deals with the early inventions of opthamology tools and practices.

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