496 THE SPECIAL SENSES. 



lens in converging all the rays of light, emanating from a given 

 point, to an accurate focus, at the surface of the retina. To accomplish 

 this, the density of the lens, the curvature of its surfaces, and its 

 distance from the retina, must all be accurately adapted to each 

 other. For if the lens be too convex, and its refractive power con- 

 sequently too great, the rays will be converged to a focus too soon, 

 and will not reach the retina until after they have crossed each 

 other and become partially dispersed, as in Fig. 158. The visual 

 impression, therefore, coming from any particular point in the 

 object is not concentrated and distinct, but diffused and dim, from 

 being dispersed more or less over the retina, and interfering with 

 the impressions coming from other parts. This is the condition 

 which is present in myopia, or near-sightedness. On the other hand, 



Fig. 158. Fig. 159. 



MYOPIA. PRESBYOPIA. 



if the lens be too flat, and its convergent power too feeble, as in 

 Fig. 159, the rays will fail to come together at all, and will strike 

 the retina separately, producing a confused image, as before. This 

 is the defect which exists in presbyopia, or long-sightedness. In 

 both cases, the immediate cause of the confusion of sight is the 

 same, viz., the rays coming from the same point of the object 

 striking the retina at different points ; but in the first instance, this 

 is because the rays have actually converged, and then crossed ; in 

 the second, it is because they have only approached each other, but 

 have never converged to a focus. 



Another important particular in regard to the action of the lens 

 is the accommodation of the eye to distinct vision at different distances. 

 It is evident that the same arrangement of the refractive parts, in 

 the eye, will not produce distinct vision when the distance of the 

 object from the eye is changed. If this arrangement be such that 

 the object is seen distinctly at a certain distance, as in Fig. 160, 

 and the object be then removed to a remoter point, as in Fig. 161, 

 the image will become confused ; for the rays will then be con- 



