SENSE OF SIGHT. 221 



an object may be seen distinctly. The distance between these two points is 

 the range of accommodation. 



Optic Defects. Astigmatism is a condition of the eye which prevents 

 vertical and horizontal lines from being focused at the same time, and is 

 due to a greater curvature of the cornea in one meridian than in another. 



Spheric aberration is a condition in which there is an indistinctness of an 

 image from the unequal refraction of the rays of light passing through the 

 circumference and the center of the lens; it is corrected mainly by the iris, 

 which cuts off the marginal rays, and transmits only those passing through 

 the center. 



Chromatic aberration is a condition in which the image is surrounded by a 

 colored margin, from the decomposition of the rays of light into their elemen- 

 tary parts. 



Myopia, or shortsightness, is caused by an abnormal increase in the antero- 

 posterior diameter of the eyeball, or by a hypernormal refracting power of 

 the lens. It is generally due to the first cause; the lens, being too far removed 

 from the retina, forms the image in front of it, and the perception becomes 

 dim and blurred. Concave glasses correct this defect by preventing the rays 

 from converging too soon. 



Hypermetropia, or longsightness, is caused by a shortening of the antero- 

 posterior diameter or by a subnormal refractive power of the lens; the focus 

 of the rays of light would, therefore, be behind the retina. Convex glasses 

 correct this defect by converging the rays of light more anteriorly. 



Presbyopia is a loss of the power of accommodation of the eye to near 

 objects, and usually occurs between the ages of forty and sixty; it is remedied 

 by the use of convex glasses. 



The Iris. The iris plays the part of a diaphragm, and by means of its 

 central aperture the pupil regulates the quantity of light entering the interior 

 of the eye; by preventing rays from passing through the margin of the lens 

 it diminishes spheric aberration. The size of the pupil depends upon the 

 relative degree of contraction of the circular and radiating fibers; the varia- 

 tions in size of the pupil from variations in the degree of contraction depend 

 upon different intensities of light. If the light be intense, the circular fibers 

 contract, and diminish the size of the pupil; if the light diminishes in inten- 

 sity, the circular fibers relax and the pupil enlarges. 



Point of Most Distinct Vision. While all portions of the retina are 

 sensitive to light, their sensibility varies within wide limits. At the macula 

 lutea, and more especially in its most central depression, the fovea, where the 

 retinal elements are reduced practically to the layer of rods and cones, the 



