, formation, the subject can see to some extent, but as the cataract advances to its maturation, vision

limit already, he cannot see the closer objects also clearly. Correction is made by convex lens which causes convergence of the incident rays (fig. IOB2. 1. 9). Fig. IOB2 .1. 9 3. Presbyopia After the age of 40, presbyopia is commonly seen In this condition, the crystalline lens becomes rigid, so that, during accomodation, even though the ciliary muscles are contracting, the lens fails to become more globular, le, it fails to increase its diopteric power. Such persons cannot see printed words or small objects, when held at 'usual' distance for reading. But when he holds the book rather at a greater than usual distance, he may be able to see. Such eyes are corrected by the use of 'reading glasses', that is, convex glass (which he is to use during reading only). N B Although, the term 'infinite distance' normally means an immeasurably long distance, in optics, it means only 20 feet (about 6 meters) or more, because if the object Js at 6 meters away, the rays from it are parallel to the eye. Near Point The 'hardening' of lens begins early in life although the signs of presbyopia begins to become noticeable only after the age of 40 years (or in some people after about 50 years). Consequently, bulging of the anterior surface of the lens due to contraction of ciliary body, that is, accomodation, is maximum in childhood The nearest point that can be seen clearly is called the 'near point' At the age of 10 years, the near point usually is 9 cm away. Throughout adult life the near point recedes farther and farther At the age of 70 years the near point may be 100 cm away. In the myopic, some compensation of near point often occurs, that is, the near point is nearer than the emmetropics of his age. It is quite common to find a myope over 50 years, reading or threading the eye of the needle without the help of reading glass (the author of the current book, who is 54* years being one such example). 4. Astigmatism In some corneas, the curvature varies from meridian to meridian. Thus, in a given cornea, the curvature in a transverse direction (i. e. the direction which runs from nasal to the temporal side) may be less than that in the vertical meridian Such an eye is called an 'astigmatic' eye. As the power of a body acting as a fens depends upon its curvature, the refractive power of an astigmatic lens will be different in different meridians. As a result, images from all the portions of the object can not be simultaneously focussed on the retina and so the object can not be clearly seen If a person, suffering from astigmatism, is given a diagram consisting of radiating lines, he will fail to see all the lines simultaneously Correction of astigmatism is made by using a cylindrical lens. The errors of refractions described above are all examples of pathological conditions But even in normal persons, some errors may sometimes be seen Indeed no optical system is perfect and the optical system of our eyes is surprisingly free of imperfections, yet, it has a few, relatively minor, imperfections. These are: (i) spherical aberration, and (n) chromatic aberration. 5. Spherical aberrations Even in a normal person, the power of the lens at the extremes of its periphery, is not identical with the power in its central part Therefore, when the pupil is widely dilated (e.g. in darkness or after administration of atropine) some blurring of vision may occur and is called spherical aberration. * Written while the book was in its 1st edition. 6 Chromatic aberration Particularly at the periphery of the lens of the eye, rays of the different wave lengths are refracted differently, causing what is known as chromatic aberration. Red light is refracted least whereas the violet light the most and consequently on looking at a source of 'white light' (containing in full, the 'Vibgyor'), the red light falls posterior to and the blue or violet anterior to the retina. Cataract In advanced age, the lens may become opaque and fail tc transmit light, this is due to denaturation ultimately leading to coagulation, of lens protein Finally, in the coagulated protein, there may be calcium deposition, making, it still harder and more opaque. The whole process (from the onset of denaturation and coagulation to its complete opaci. ty) requires some time, (may be a few years). A matured cataract fie where the coagulation and opacity is complete) has to be removed surgically and to compensate for the loss of refraction, a convex lens (+ lens) of say around 1 D power has to be given In the initial phases of catarac, formation, the subject can see to some extent, but as the cataract advances to its maturation, vision becomes increasingly poor Exposure to strong sunlight or ultraviolet light probably favors cataract formation (by helping the process of denat uration and coagulation of the lens protein). It is advisa. ble, therefore, to use sunglass in the outdoors of a tropica country like India TESTING OF VISUAL ACUITY (fig. 10B2. 1. 11) To test the visual acuity for distant vision, visual char (Snellen's chart), is used Such a chart contains letters (o the alphabet) of different sizes For example, the letter I on the topmost row (fig. IOB2. 1. 11) has the biggest size and a normal man can see it early from a maximum distance o 60 meters Letters of the 2nd row can be seen learly with emmetropic eyes, from a maximal distances of 36 m and so on. The subject stands (or sits), 6 m away from such a charl If he can see, a letter of the top row, (which an emmetropic man can see from 60 m), but no more, then his vision is 6/60 This means he can see an object, only when he stands at 6 n distance which one should normally see from a distance o 60 m. Obviously a normal vision is 6/6. Physiological basis of visual acuity In fig 10B2 1 10, an object, mn is placed