Fig. 33.
A, The normal eyeball, in which, when the ciliary muscle is relaxed, parallel rays are brought to a focus on the retina. B, A hypermetropic eyeball. Its depth being less than normal, parallel rays are not brought to a focus on the retina when the eye is adjusted for distant vision without the aid of a convex glass. C, A myopic eyeball. Its depth being more than normal, a concave lens is needed to diminish the convergence of parallel rays.
A star or a distant gas-lamp is seen as a point of light with rays. Usually this figure, which has given origin to the expression “star-shaped,” shows three greater rays alternating with three lesser rays. Such an image is not produced by a point of light near to the eye, since it is due to the puckering of the lens when flattened against its ligament. It brings into evidence the three axes on the front of the lens and the three axes which alternate with them on the back, with regard to which the lens-fibres are disposed.
As an adaptation of living tissues to optical purposes the eye is above admiration, yet it presents many defects, which an optician corrects in the instruments which he manufactures. A remarkable fact in the physiology of vision is our unconsciousness of the imperfections of its organ. An unusual experiment is needed to bring them to our notice. If we look through a common glass lens uncorrected for unequal refraction of rays of different wave-lengths, we recognize that a bright object is shown with a colour-fringe, yet we take no cognizance of the colour-fringes which surround the images of all bright objects focussed upon our retinæ. If we think about the matter, we recognize a feeling that blue in a window of stained glass appears farther away than red; but this might well be due to association. Blue glass is chiefly used for the sky. If we look at a bright object through purple glass, we her red with a blue fringe or blue with a red fringe, according as the eye is focussed for red or for blue. The purple glass having absorbed all intermediate rays, we become aware that we cannot focus the two extreme ends of the spectrum at the same place. Since a greater effort of accommodation is needed to focus red, we judge that the bright object is nearer to us when it appears red than when it appears blue.
Spherical aberration is another fault of the lens. The rays which enter its margin are brought to a focus sooner than those which pass through its centre. This is due to the fact that its surfaces are regularly curved, whereas a glass lens is corrected by grinding it flatter towards the margin. This defect is partly corrected by the cornea, which has an ellipsoidal surface, and partly by the greater density of the centre of the lens. Yet it is still necessary for the eye to be “stopped down” by the iris when a near object is looked at, although less light is entering the eye than when it is directed to the horizon—a condition which would lead a photographer to open his iris-diaphragm.
Of all the imperfections of the eye which the mind ignores, the most remarkable is the gap in the field of vision, due to the gap in the sensitive layers of the retina, which occurs where the optic nerve enters it—the blind spot. Hold this page of the book 10 inches from the face, keeping the lines of print horizontal. Close the left eye and look at X with the right eye. The black disc disappears, because its image is focussed on the blind spot. Since the picture on the retina is reversed, it is clear that the optic nerve enters the globe to its inner side, and slightly above its horizontal meridian. But, unless we employ an unusual test, we are quite unconscious of the fact that a definite hole is punched in the picture. The mind fills it in, and the way in which it does so is extremely suggestive. It lies about it—in a downright ingenuous fashion if it is confident of credence, in a more subtle way if a simple falsehood is likely to be challenged. In place of the black disc make nine conspicuous crosses:
