. A text-book of human physiology . rays oblique. The first case is the simplest (for the second see page 525). Suppose wehave a lens, which in the horizontal meridian has a refractive power of 10diopters; in the vertical meridian a refractive power of 13 diopters. It isevident that the beam after refraction will no longer have a common focus,for the incident rays in the horizontal meridian are brought to a focusyV m- behind the lens and those falling in a vertical meridian J^ m. behindthe lens. Further study of the ])rol)lem has shown that if no account be taken ofthe spherical aberration, th
. A text-book of human physiology . rays oblique. The first case is the simplest (for the second see page 525). Suppose wehave a lens, which in the horizontal meridian has a refractive power of 10diopters; in the vertical meridian a refractive power of 13 diopters. It isevident that the beam after refraction will no longer have a common focus,for the incident rays in the horizontal meridian are brought to a focusyV m- behind the lens and those falling in a vertical meridian J^ m. behindthe lens. Further study of the ])rol)lem has shown that if no account be taken ofthe spherical aberration, the light rays instead of being converged to fociat the focal points of the two meridians are converged into a focal line per-pendicular to the principal ray at each of those points. The first focal linecorresponds to the focal point of the meridian with the strongest refractivepower and is perpendicular to that meridian—i. e., in the plane of the weakestmeridian. The second focal line corresponds to the focal point of the weakest. Fig. 21S.—Illustrating .spherical aberration. The rajs parallel to the axis of the system areconverged to foci nearer and nearer the convex surface the farther they are removed fromthe axis. meridian and is perpendicular to that meridian—i. e., in the same plane as thestrongest meridian. In front of the first focal line the beam of rays forms in a cross sectionan ellii)se with the longer in the direction of the first focal line, beyondthe second focal line the beam forms an ellipse with the longer axis in thedirection of the second focal line. A transition from the one elongationto the other takes place between the two focal lines, the upright ellipsebecoming first a circle and then a procumbent ellipse (Fig. 219). In an astigmatic eye, therefore, a homocentric ^ l)undle of rays cannot bebrought to a single focus. When the eye is adjusted for the most refractivemeridian, the images on the retina are all drawn out in tb(> direction of the
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