VISION. 



single medium. Instead, the eye must be regarded as a compound 

 refracting system, composed of a spherical surface and a biconvex 

 lens. 



The cardinal points are 

 Two. principal points, 

 Two nodal points, 

 Two principal foci. 



In the diagram, Fig. 43, the cardinal points are shown all 

 upon the optic axis, f-a. At 6, two principal points, situated so 



FIG. 43. Described in text. 



close together in the anterior chamber that they may be regarded 

 as one. At / is the first principal focus, and at a the second. 

 The nodal points correspond nearly to the optical centre of the 

 refractive system. Rays passing through these points are not 

 refracted. They are situated about 7 mm. behind the cornea. 



THE FORMATION OF RETINAL IMAGES. A luminous point 

 placed above the principal axis has its image formed upon the 

 retina below this axis, and vice versa. Replace these points by an 

 object and the same thing occurs. The retinal image is, as it 

 were, a mosaic, composed of innumerable foci of the object. 



Construct a simple diagram of the human eye, showing the 

 formation of an image, say an arrow or a candle, upon the retina. 

 Is the image erect or inverted ? If inverted, why do we see it erect ? 



The human eye has aptly been compared to a camera, the 

 refracting media representing the camera lenses, and the retina 

 its sensitive plate. 



[183] 



