The anomalies of eye refraction are extremely widespread. Especially important problem of ophthalmology is nearsightedness (myopia). In different areas of our country the frequency of this disease among the schoolboys composes 3.5-33%. A high nearsightedness — the most often reason of physical inability owing to eye diseases. All this puts the struggle with myopia on a level of the state problem and indicates necessity of realization of active and broad measures of prevention of this disease and its complications, that should be realized not only by oculists, but also by doctors of a general profile.
The refraction is a refractive ability of any optic system (refractivity). A refraction of the eye is its refractivity in the condition of accommodation, i.e. refractive force of the eye owing to its anatomic constitution.
OPTIC SYSTEM OF THE EYE
The refractive apparatus of the eye consists of the cornea, aqueous humor (liquid of the anterior chamber), lens and vitreous. The refractive apparatus of the eye consists of a few refractive surfaces and environments and represents a difficult compound optic system.
The refraction force of any optic system is measured in diopters. The unit of the refractive force is the force of the lens with a focal length of 1 m. The less a focal length, the more the force of lens.
For example, the force of lens with a focal length of 50 cm amounts to two diopters; the force of lens with a focal length of 25 cm amounts to four diopters etc.
So for the aim to calculate refractive force of the eye optic system, it is necessary to know a focal length of this system. The focal length is calculated on the basis of optic values.
The optic values consist of: 1) curvature radiuses of the cornea and lens surfaces; 2) factors of environments refraction; 3) distances between refractive surfaces, i.e. between the cornea and anterior surface of the lens between anterior and posterior surfaces of the lens. Knowing these values, it is possible to calculate a position of cardinal points, i.e. two main points, two nodular and two main focuses, and then focal length (these concepts are known for you from a course of physics).
The optic values are determined with the help of very delicate and sometimes very difficult methods of investigation. As these values have personal oscillations in this connection the mean values are obtained as a result of measurements of series of eyes. Such an eye with mean optic indices is named a schematic eye.
The schematic eye of Gullstrand is most widespread, it consists of six refractive surfaces: the anterior and posterior surfaces of the cornea, anterior surface of the lens, anterior and posterior surfaces of the lens nucleus, posterior surface of the lens. They divide seven environments: air, the cornea, fluid, anterior and posterior cortical layers of the lens and its nucleus and vitreous. The refractive force of a schematic Gullstrand's eye is 58.64 D. 43.05 D belong to the cornea and 19.11 D — to lens in accommodation's rest. The schematic eye is used for the solution of many problems of physiologic optics but in the whole number of cases the calculation of the date which is necessary for the clinical purposes is possible with the help of the most simplified scheme of the optic eye model. It is named as the reduced eye in which there is one average index of refraction, one refractive surface and one main plane. The reduced eye of Verbitski is the perfect model. The constants of this eye include the index of refraction — 1.4; the radius of refractive surface curvature — 6.8 mm; radius of retina surface — 10.2 mm; length of eye axis — 23.4 mm.
Last years the simplified schemes of optic indices calculation gain large practical value for definition of focal point of the optic eye system at opticoreconstractive operations.