7 2 & THE SENSES 



through the nodal point and is prolonged backward to the retina. The 

 diverging rays are refracted to converge to a posterior conjugate focus 

 behind the lens on the chief axis of the pencil of light proceeding from the 

 point in question. This focus, if the image is to be clear, should fall on the 

 retina. 



Thus from each point of an object a corresponding image is formed on 

 the retina, so that an image of the distant object is produced. It is an inverted 

 image. Whether the image is blurred or not depends upon the refractive 

 power of the media and upon the distance of the anterior surface of the cor- 



FIG. 456. Diagram of the Course of a Ray of Light, to Show how a Blurred or 

 Indistinct Image is Formed if the Object be not Exactly Focused upon Retina. The 

 surface CC should be supposed to represent the ideal curvature. The nodal point should 

 be nearer the posterior surface of lens as in figure 455. 



nea from the retina. If the refractive media are too powerful or the eye too 

 long, the image is formed in front of the retina, figure 456; if the reverse, the 

 image is formed behind the retina, and in both cases an indistinct and blurred 

 image is the result. 



Accommodation. The distinctness of the image formed upon the 

 retina is mainly dependent on the perfection with which the rays emitted 

 by each luminous point of the object are brought to a focus upon the retina. 

 If this focus occurs at a point either in front of or behind the retina, indis- 

 tinctness of vision ensues, in the way we have just described, with the pro- 

 duction of a halo. The focal distance, i.e., the distance of the point at which 

 the luminous rays are collected from a lens, besides being regulated 

 by the degree of convexity and density of the lens, varies with the distance 

 of the object from the lens, being greater as the distance is shorter, and vice 

 versa. In other words, the luminous points on the object and the focal points 

 on the retina are conjugate foci. Hence, since objects placed at various 

 distances from the eye can within certain limits be seen with almost equal 

 distinctness, there must be some provision by which the eye is enabled to 

 adapt itself, so that, at whatever distance the luminous object may be, the 

 focal point may always fall exactly upon the retina. 



Accommodation is the act of adapting the eye to vision at different distances. 

 It is obvious that the effect might be produced in either of two ways, viz., 



