166 THE SENSES. 



minuter details. This is known as the accommodation of the eye. 

 In optical instruments this process of accommodating the instru- 

 ment to the focal distance is called focusing. So is accommodation 

 an automatic focusing of the eye. 



How is accommodation effected? 



By the ciliary muscle. The crystalline lens is suspended in its 

 capsule by the suspensory ligament, and this is controlled by the 

 ciliary muscle. At rest the eye is focused for seeing at a distance, 

 and the lens is held somewhat flattened by the tension of the sus- 

 pensory ligament. When focusing upon a near object the ciliary 

 muscle contracts, and the lens is permitted to project more as the 

 tension of the ligament relaxes. 



What is the near-point? 



It is the nearest point to the eye at which vision is distinct, the 

 shortest focus of the crystalline lens. It is usually about ,5 or 6 

 inches. 



What is an emmetropic eye ? 



It is the normal eye ; that is. an eye in which parallel rays or 

 rays from objects at a distance are focused upon the retina without 

 an effort at accommodation. Such a distance, for practical pur- 

 poses, is considered to be any point beyond 20 feet. Absolutely 

 emmetropic eyes are not common. 



What is myopia ? 



Near-sight. In this case the rays from a distance are focused in 

 front of the retina, and the image is blurred. Such an eye is per- 

 manently focused for near objects. 



How is myopia produced? 



In two ways, by the antero-posterior diameter of the eye being 

 too great, or by the convexity of the lens being exaggerated. In 

 either case the focus of the lens will fall in front of the retina. 



How is myopia corrected ? 



By the use of a concave lens which diverges the rays, and in this 

 way prevents their coming to a focus too soon. Such glasses are 

 seldom needed except for distant vision. 



What is hypermetropia ? 



Far-sight. In this case the lens focuses behind the retina in 



