THE SENSE OF SIGHT. 247 



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 

 anteroposterior 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 variations 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 intensity, 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 sensibility reaches its maximum. It is 

 at this point that the image is found when vision is most distinct. 

 The macula and fovea are always in the line of direct vision. From 

 the macula toward the periphery of the retina there is a gradual 

 diminution in sensibility, and a corresponding decline in the dis- 

 tinctness of vision. In those portions of the retina lying outside the 

 macula, the indistinctness of vision depends not only on diminished 

 sensibility, but also upon inaccurate focusing of the rays. 



Blind Spot. Although the optic nerve transmits the impulses 

 excited in the retina by the ethereal vibration, the nerve-fibers them- 

 selves are insensitive to light. At the point of entrance of the optic 

 nerve, owing to the absence of the rods and cones, the rays of light 



