IMAGE FORMATION 



727 



tures into one mean curved surface of known curvature, and the different 

 refracting media into one mean medium the refractive power of which is 

 known. 



The simplified or so-called schematic eye, formed upon this principle, 

 suggested by Listing as the reduced eye, has the following dimensions: 



From the anterior surface of the cornea to the principal point = 2 . 3448 mm. 



From the nodal point to the posterior surface of lens = o .4764 mm. 



Posterior principal focus lies behind cornea =22. 8237 mm. 



Anterior principal focus in front of cornea = 12 . 8326 mm. 



Radius of curvature of ideal surface = 5. 1248 mm. 



In this reduced or simplified eye the principal posterior focus, about 

 22.8 mm. behind the spherical surface, would correspond to the position of 

 the retina behind the anterior surface of the cornea. The ideal refracting 

 surface would be situated about midway between the posterior surface of 

 the cornea and the anterior surface of the lens. 



The optical axis of the eye is a line drawn through the centers of curva- 

 ture of the cornea and lens, and when prolonged backward it cuts the retina 

 between the optic disc and fovea centralis. This differs somewhat from 

 the visual axis which passes through the nodal point of the reduced eye to 



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

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



the fovea centralis, and forms an angle of five degrees with the optical axis. 

 The visual or optical angle is the angle included between the lines drawn 

 from the opposite borders of any object through the nodal point. It has 

 been shown by Helmholtz that the smallest angular distance between two 

 points which can be appreciated is fifty seconds, the size of the retinal image 

 being 3 . 65^. This practically corresponds to the diameter of the cones at 

 the fovea centralis which is 3^, the distance between the centers of two 

 adjacent cones being 4/4. 



The image of an object formed upon the retina may be considered as a 

 series of points, from each of which a pencil of light diverges to the eye, and 

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

 ive surface perpendicularly to the surface is not refracted^ but passes 



