From Big Medical Encyclopedia

FURTHER POINT OF CLEAR SIGHT (punctum remotum — R) — the point, most remote from an eye, in space, to-ruyu an eye clearly sees at absolute rest of accommodation.

For an emmetropichesky eye this point lies at infinitely far distance from it since an eye can connect on a retina parallel rays of light (see. Emmetropia ).

Fig. 1. The scheme of a path of rays in an emmetropichesky eye: further point of clear sight (D. of t.) is in infinity (1 — an eye; 2 — a crystalline lens; 3 — a retina).

In relation to an eye the concept «infinity» is applied in a little conditional values. Almost infinitely far it is possible to consider the distance equal of 5 m and more. The beam of light getting in an eye through a pupil is so narrow that the beams making it even if they proceed from the objects which are from an eye at distance of 5 m have so insignificant discrepancy that are practically considered as parallel (fig. 1).

Fig. 2. The scheme of a path of rays in a miopichesky eye: and — parallel beams meet before a retina; — short-sightedness of 2,0 D, D. t. is in 50 pieces from an eye; in — short-sightedness of 5,0 D, D. t. is in 20 cm from the 6th eye.

Refraction of an eye — this static condition of an eye, at Krom do not change the refracting force it, position of a retina. At a myopia (see Short-sightedness) an eye can connect on a retina beams only to a certain rate of divergence. A point, from a cut there are these beams, and its D. t will be. I. h. It is always ahead of an eye at the known final distance (fig. 2, a). This distance for each miopichesky eye will be a miscellaneous depending on degree of short-sightedness (fig. 2, and c). Than closer D. t is located to an eye. I. h., that short-sightedness since at stronger discrepancy of beams their stronger refraction or increase in the perednezadny size of an eye that rays of light could connect in focus on a retina is necessary is more severe.

At a hypermetropia (see. Far-sightedness ) an eye can connect on a retina only such beams which to an entrance to an eye would have the meeting direction (fig. 3, a). However in the nature of such beams does not exist. Therefore for a gipermetropichesky eye there cannot be D. of t. I. h., edges really would be somewhere ahead of an eye.

Fig. 3. The scheme of a path of rays in a gipermetropichesky eye: and — the dotted line designated the course of the imagined meeting beams (which are not existing in the nature) which refract on a retina; — parallel beams meet behind a retina (are designated by a dotted line) eyes, D. of t. — imaginary.

Under D. of t. I. h. at a hypermetropia understand not the valid, but imaginary point, edges is behind an eye (fig. 3, b). This point designates itself that degree of a convergence of beams what they shall have to an entrance to an eye in order that after refraction to connect on a retina. A Gipermetropichesky eye (at rest of accommodation) cannot connect clearly on a retina neither the parallel, nor dispersing beams.

Therefore, each eye has strictly certain provision D. of t. I. h. Provision D. of t. I. h. in space defines a type of a refraction, and D.'s distance of t. I. h. from an eye — degree of a refraction, edge is expressed in dioptries and is determined by a formula: D = 1/R, where R — and D.'s distance of t. I. h. from an eye, expressed in meters. At D.'s myopia of t. I. h. is ahead of an eye. The distance to it is counted from an eye in the opposite direction of the beams which are rather getting into an eye and is negative (-R). Degree of a myopia is equal (— 1/R) [e.g., distance to D. of t. I. h. equally to 0,5 m, degree of a myopia is equal in dioptries 1 / (— 1/2) = — 2,0].

At a hypermetropia distance to D. of t. I. h. (imaginary) it is counted on the course of the direction of light rays, i.e. is positive (e.g. if to present that D. of t. I. h. is at distance of 0,5 m behind an eye, degree of a hypermetropia will be equal to +2,0).

See also Accommodation of an eye , the Closest point of clear sight , Refraction of an eye .

V. I. Morozov, V. I. Pozdnyakov.