642 



THE SENSES 



this pencil has for its center or axis a ray which impinging upon the refrac- 

 tive surface perpendicularly to the surface is not refracted, but passes through 

 the nodal point and is prolonged backward to the retina, whereas the diverging 

 rays are also made to converge to a principal posterior focus behind the lens, 



FIG. 455. Diagram of the Method of the Formation of an Inverted Image Exactly Focussed 

 upon the Retina. The dotted line is the ideal surface of curvature. 



or the chief axis of the pencil of light proceeding from the point in question, 

 and 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 cornea 

 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 



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 Focussed 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. 



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. 



