Complete Text (Part 13)

the base of the prism at both surfaces, since the law governing refraction is operative to produce this result as the ray Fig. 25. Refraction of Light, as shown by a Prism : S I, incident ray ; 1 1', course through lens ; I' R, emergent ray. enters and as it leaves the prism. The relative relation of rays of light as they enter and leave the prism is not changed (Fig. 25). What is a lens ? A lens is a transparent medium having two regularly ground opposing surfaces, one or both of which are curved, the curve either describing the segment of a sphere or of a cylinder. Fig. 26. Forms of Lenses. What are the different forms of lenses ? They may be biconvex (1); biconcave (4); plano-convex (2); piano- OPTICAL PRINCIPLES. 99 concave (5) ; converging meniscus (3); diverging meniscus (6); ground as portions of a sphere or as portions of a cylinder (Fig. 26). Periscopic lenses are forms of the meniscus. What is the principal axis of a lens ? A line which cuts the lens at its centre and passes through its centre of curvature. What are secondary axes? Lines which pass through the optical centre of the lens (impinging on any part of its surface other than that corresponding to the principal axis), which are practically not changed in direction. What are biconvex and biconcave lenses? They are virtually two plano-convex or plano-concave lenses joined in such a manner that their principal axes coincide. What is the centre of curvature of a lens ? It is the centre of a sphere of which the curved surface of the lens forms a part. What is the radius of curvature of a lens ? A line extending from the centre of curvature of the sphere of which the curved surface of the lens forms a part. What is the optical centre of a lens ? It is the point through which all secondary axes pass after entering the lens. In the case of a medium with a single curved surface it corre- sponds with the centre of curvature of the surface. In the case of a medium with double curved surfaces, as in the bi- convex lens, the secondary axial rays are first directed to a point anterior to the centre of the lens, and then to a point posterior to the centre of the lens. These are termed "nodal points." The optical centre in these lenses lies between the nodal points. What is the principal focus of a lens ? It is a point at which parallel rays of light entering the lens are brought to a focus. In the biconvex or concave lenses their principal foci correspond with their centres of curvature if the index of refraction of the substance of which the lens is composed is 1.5. What is the principal focal distance of a lens ? It is the distance between the optical centre of a lens and the princi- pal focal point. What effect do the different forms of lenses produce on rays of light entering them? Convex lenses collect, concave lenses disperse, rays of light that pass through them. The former are called plus lenses, the latter minus lenses. 100 THE EYE. What are conjugate foci? Points in relation to a lens at which, if the object is placed at one point, an image will appear at the other point. What is an image of an object ? It is a picture or reproduction of the form of an object in a plane where some of the rajs of light from every point of the surface of an object are brought to a focus. The plane at which this occurs is known as the focal plane. What kind of images are known in physiological optics ? The real or inverted and the virtual or upright. The real image (Fig. 27) is always on the side of the lens opposite to the object, and may be Fig. 27. ^a A C, object, upright ; c a, real image, inverted. projected on a screen. The virtual image is on the same side with the object, and is formed only in the mind of the observer. All images A B, object, upright; a b, virtual image, upright. from concave lenses are virtual (Fig. 28), as are also those from convex lenses where the object is placed between the principal focus of the lens and the lens itself (Fig. 29). What is meant when rays of light are said to be projected ? The faculty that the eye possesses of referring all rays of light which OPTICAL PRINCIPLES. 101 enter it from an object to a point in space where the rays from all points of this object would, if prolonged, cross each other. The plane so formed may correspond with the object itself where the object is the direct source Fig. 29. Projection of Eays. of the rays, or to an image if the rays from the object are refracted before they enter the eye. How are lenses classified or numbered ? A lens is said to be a strong lens if it is of a high refractive power. There are a number of systems of notation for classifying lenses, the principal of which are the inch and dioptric systems. What is the inch system of notation ? It is a system based on the inch as the unit of focal distance. It differs in various countries according as the unit is of greater or of less length. The French, German, and English inches are all of different lengths, which causes some confusion in the use of the inch system. The system consists in the employment of the unit as a numerator and the focal distance of the lens as the denominator ; thus, a lens having a focal distance of 1 inch would be known as T ; having a focal distance of 4 inches, as . This system of notation is being replaced by the dioptric system because of its uniformity. What is the dioptric system? It is based on the French dioptre as a unit. The dioptre is a lens whose focal distance is 1 metre, or 39.4 English inches, in length. A lens having a focal distance of 39.4 inches is known as a lens of 1 dioptre. A lens with 20 inches focal distance is known as a lens of 2 dioptres. A lens of a focal distance of 79 inches is known as a lens of i dioptre focal power. What are trial-glasses ? They are sets of lenses, usually arranged in pairs, composed of spheri- cals and cylindrical ranging from i dioptre to 20 dioptres. They are marked in dioptres and in equivalents in inches, and are used for the 102 THE EYE. purpose of determining what glass is appropriate for the correction of any error of refraction. What is a dioptric surface? It is the surface between two transparent media of different densities. THE EYE AS AN OPTICAL INSTRUMENT. What is the dioptric system of the eye ? The cornea, aqueous humor, crystalline lens, and vitreous humor. What is the index of refraction of each ? Of the cornea, aqueous and vitreous humors it is, approximated, 1.3365 (Helmholtz) ; lens, 1.4480 (Aubert). What is the principal focal distance? The eve acts as a collecting or plus lens, and has a principal focal dis- tance of 22.2 mm. (Helmholtz), 22.8 mm. (Landolt). What is necessary for distinct vision ? The dioptric media must be transparent, the curvature of the surfaces must be regular, the location of the retina must correspond with the principal focal plane, and the optic-nerve tracts must be in perfect func- tional condition. What is the point of most acute vision in the eye ? The fovea centralis. It is a small area of the retina, about J^ inch in diameter, situated TV inch to the temporal side of the optic-nerve en- trance. It is usually a very little to the outer side of the posterior pole of the globe. What is the optical axis? It is a line cutting the centre of the cornea and the posterior pole of the globe. What is the visual line? It is a line extending from the fovea centralis through the optical cen- tre of the eye. How do you determine the acuteness of vision ? Usually by the employment of test-cards on which are arranged letters of different sizes suitable to be seen at different distances by the normal eye. These cards are known as test-cards, and are made for near and distant vision. Those intended for distant vision are usually read at 20 feet, which is termed infinity ; that is, a distance at which rays of light emanating from a point are virtually parallel when they enter the eye. To distinguish detail in an object it is necessary that rays of light from each of the different points on the object make a visual angle of 1 min- THE EYE AS AN OPTICAL INSTRUMENT. 103 ute. In the construction of a letter it is found that five points must he seen — the bars, which are three in number, and the spaces between, two in number. Hence the visual angle made by the rays of light from such an object must be 5 minutes, and this is the basis on which the letters on the test-cards are constructed. The test-cards usually employed are those devised by Snellen, and contain letters that should be read at from 15 to 200 feet. The distance at which the letters should be seen is used as the numerator, the distance at which the letter is seen as the denominator ; thus, if a patient at a distance of 20 feet sees the letters that should be read at that distance, the vision is said to be §§. If he can read only the letter that should be read at 100 feet, the vision is $fc. Test-cards con- structed on the same principle are also used for the ordinary reading dis- tance. Can the refraction of the eye he changed at the will of the patient ? Yes. A change can be made in the curve of the dioptric surface of the lens by which the refraction is increased. This is known as the power of accommodation. By what means is the eye enabled to accommodate itself to see objects distinctly at different distances? By the contraction of the ciliary muscle, also known as the muscle of accommodation, the suspensory ligament which is attached to it becomes relaxed. The elastic properties of the lens produce a bulging forward of its anterior surface, symmetrically increasing the acuity of curvature of its surface. This shortens the principal focal distance of the dioptric system of the eye, and makes the focusing of rays of light coming from different distances within infinity possible. What is meant by the term " amplitude of accommodation " ? It is the distance between the nearest point at which objects may be seen distinctly and the farthest point at which objects maybe seen. The far-point is known as the punctum remotissimum ; the near-point, as the punctum proximum. How is the near-point determined? By ascertaining, by actual measurement, how close to the eye an object may be brought and still be seen distinctly. The far-point may be deter- mined by noting the distance, unimproved by a plus glass, at which an object may be seen distinctly. How much of the power of accommodation is available for con- stant use? About two-thirds ; in other words, it is necessary that the individual should hold about one-third of his amplitude of accommodation in reserve in order that he may do continuous work without distress. 104 THE EYE. How is accommodation influenced by age? The amplitude of accommodation is shortened at the expense of the punctum proximum, which recedes, and after it has passed the point of 8 inches from the eye the condition is known as presbyopia, or the vision of the old, and glasses are required to make near vision easy. Amplitude of Accommodation at Different Ages {Landolt). Years. Range of Accommodation. Far-point. 10 . . 14 D . . . . ... 2| 15 . . 12 " . . . . ... 3 20 . . 10 "/... . . . 4 25 . . 8.5 "... . , . . 4* 30 . . 7 " . . . . ... 5* 35 . . 5.5 "... . . . . 7 40 . . 4.5 "... . ... 8f 45 . . 3.5 " . . . . . . .11* 50 . . 2.5 "... . . . . 16 55 . . 1.75" .... . . .23 60 . . 1 " . . : . . . . 39.5 65 . . . . . . .0.75" 70 . . 0.25" 75 . . 0 " What may be the refractive conditions of the eye when at rest regarding parallel rays of light ? The rays of light may be focused on the retina ; they may come to a focus in front of the retina or behind the retina. What terms are used to indicate these conditions? Where the rays fall at a focus on the retina the condition is termed emmetropia (Fig. 30) ; where the focus falls in front of the retina, myopia ; where they fall behind the retina, hypermetropic/,. Fig. 30. Emmetropic Eye : parallel rays focused on the retina. What is meant by the term " ametropia " ? Any condition of refraction other than emmetropia. THE EYE AS AN OPTICAL INSTRUMENT. 105 What is emmetropia? A condition of the eyeball in which the antero-posterior diameter is of the right length, the principal focus of the dioptric system at 22.2 mm. What is myopia? It is a condition of the eyeball in which the anteror posterior diameter is elongated (Fig. 31). The condition may be congenital, acquired, spasmodic, or real. Fig. 31. Myopic Eye. What is congenital myopia ? It is a condition in which the eye, when formed, is too long in its antero-posterior diameter, or in which the dioptric system is more strongly refractive than in the normal e3Te. What is acquired myopia ? A condition in which there has been an antero-posterior elongation of the globe due to weakness of its coats. This form of myopia is marked by sclero-choroiditis of greater or less degree. It may supervene on a previous hypermetropia, emmetropia, or myopia. It is a diseased condition, and should always be regarded as an element of danger to vision. • ... What is spasmodic myopia? It is a form of myopia produced by spasmodic contraction of the muscle of accommodation, and may convert a hypermetropic or emme- tropic eye into a myopic eye, or a myopic eye into an eye more highly myopic. The latter is not common. How may the actual condition be determined? By the use of a mydriatic, which acts to paralyze the muscle of accom- modation and to place the accommodative apparatus in a condition of rest. What is the course of the rays of light coming from the retina in the myopic eye ? Since the retina is placed at a distance greater than the principal focal point of the dioptric system from the optical centre, the rays of light coming from the retina converge as they pass from the myopic eye. 106 THE EYE. What is the relation of punctum remotissimum (P. R.) and punc- tum proximum (P.P.) in myopia? The amplitude of accommodation is not lessened, but P. R. is brought within infinity, and is found at the point where rays of light emanating from the retina come to a focus. P. P. is brought correspondingly nearer to the eye. What is the danger in high degrees of progressive myopia ? If the posterior elongation of the eye continues, the choroid and retina in the vicinity of the fovea centralis are much altered in structure ; the extreme stretching and thinning that the retina is subject to induce detachment of this membrane in some cases. Vision becomes imper- fect, and in high grades of myopia may be entirely lost. Hemorrhages Fig. 32. Fig. 33. Section of a Highly-myopic Eyeball: the Myopic Crescent of a Small Posterior retina has been removed. Staphyloma (Wecker and Jaeger). sometimes take place into the retina and vitreous, and interfere largely with vision (Figs. 32, 33). Fig. 34. Correction of Myopia. How is myopia corrected ? By placing before the eye a concave or dispersing lens equivalent to the degree of myopia (Fig. 34). THE EYE AS AN OPTICAL INSTRUMENT. 107 What is hypermetropia ? It is a condition of the eye in which the antero-posterior diameter is too short (Fig. 35). It is not the result of a disease-process, and need give no apprehension of loss of vision, as is the case in acquired myopia. Fig. 35. Hypermetropic Eye. What is the conduct of rays of light coming from the retina in hypermetropia ? Since the retina is placed at a shorter distance from the optical centre than the principal focal distance, the rays of light coming from the retina diverge as they pass out of the eye, the degree of divergence correspond- ing to the degree of the hypermetropia. What effect does this have on P. R. and P. P. ? In emmetropia rays of light emanating from the retina pass out of the eye parallel to each other, and P. R is said to be at infinity. In hyper- metropia, since the rays of light pass out in a divergent direction, P. E, is said to be beyond infinity to a degree represented by the lens which serves to bring the rays of light coming from the hypermetropic eye par- allel to each other. P. P. is removed from the eye to