822 



THE EYE AND VISION 



[CH. LYIII. 



angle of incidence to that of the angle of refraction ; this is explained 

 in the small text beneath fig. 521. 



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

 about 23 mm. behind the spherical surface, would correspond to the 



FIG. 521. If P P' is a line which separates two media, the lower one being the denser, and A O is a ray 

 of light falling on it, it is bent at O towards the normal or perpendicular line N N'. A O is called 

 the incident ray, and O B the refracted ray ; A O N is called the angle of incidence (i), N' O B the 

 angle of refraction (r). If any distance O X is measured off along O A, and an equal distance O X, 



along O B, and perpendiculars drawn to N N'; then-^p index of refraction. 



position of the retina behind the anterior surface of the cornea. The 



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 centres of 



curvature of the cornea and lens, 

 prolonged backwards to touch the 

 retina between the porus opticus 

 and f ovea centralis, and this differs 

 from the visual axis which passes 

 through the nodal point of the 

 reduced eye to the fovea centralis ; 

 this forms an angle of 5 with 

 the optical axis. But for practical 

 purposes the optical axis and the visual axis may be considered to 

 be identical. 



The visual or optical angle (fig. 522) is included between the lines 



FIG. 522. Diagram of the optical angle. 



