VISUAL SENSATIONS, FROM EXCITATION OF THE RETINA 739 



of myopia, and is due either to an abnormal shortening of the eyeball 

 antero-posteriorly or to a decrease in the convexity of the refracting 

 surfaces, or both. Parallel rays entering the eye at rest are focused be- 

 hind the retina. An effort of accommodation is therefore required to 

 focus parallel rays on the retina. When the rays are sharply divergent, 

 as in viewing a very near object, the accommodation is insufficient to 

 focus them. Thus, not only do distant objects, normally seen without 

 effort, require an act of accommodation, but near objects are focused 

 only by the maximal muscular contraction of the accommodative mech- 

 anism. The eye is under a constant strain which produces in the end 

 various nervous, as well as ocular, disorders. This defect is obviated 

 by the use of convex glasses, which render the pencils of light more con- 

 vergent. Such glasses are especially needed for near objects, as in read- 

 ing, etc. They are also required for distant vision to rest the eye by 

 relieving the ciliary muscle from constant work. 



Presbyopia. Presbyopia is a condition of diminished range of ac- 

 commodation. It takes place with considerable uniformity from youth to 

 old age. It is not a disease, but a physiological process which every eye 

 undergoes as its owner grows older. It is due to a gradual diminution of 

 elasticity of the lens by a sort of sclerosis from the center toward the 

 periphery. It begins even in childhood, but advances so slowly that it is 

 not until the age of twenty-five or so that a distinct, though small, nucleus 

 is present. With advancing years the process goes on until finally the 

 lens becomes inelastic and is unable to assume a shape convex enough to 

 focus rays from a near object upon the retina, as in reading. The defect 

 is remedied by the use of convex lenses equivalent to the loss in accom- 

 modation. 



Visual Sensations, from Excitation of the Retina. Light is the 

 normal agent in the excitation of the retina. The only portion of the retina 

 capable of reacting to the stimulus is the rod and cone layer. The proofs of 

 this statement may be summed up thus: i. The point of entrance of the optic 

 nerve into the retina, where the rods and cones are absent, is insensitive to 

 light and is called the blind spot. The phenomenon itself is very readily 

 demonstrated. If we close one eye, and direct the other upon a point at 

 such a distance to the side of any object that the image of the latter must 

 fall upon the retina at the point of entrance of the optic nerve, its image is 

 lost. If, for example, we close the left eye, and direct the axis of the right 

 eye steadily toward the circular spot in figure 467, while the page is held at 

 a distance of about six inches from the eye, both dot and cross are visible. 

 On gradually increasing the distance between the eye and the object, by 

 removing the book farther and farther from the face, keeping the right eye 

 steadily on the dot, it will be found that suddenly the cross disappears from 

 view, while on removing the book still farther it suddenly comes into view 



