THE SENSE OF VISION 



611 



e.g., those of the aqueous humour and the cornea the refracting 

 surfaces may be regarded as three in number: anterior surface of 

 the cornea, the anterior and posterior surfaces of the lens. There 

 are three refractive media the aqueous humour, the lens, and the 

 vitreous humour. These surfaces and media are arranged in such 

 a manner that rays of light travelling from a distance i.e., parallel 

 rays are brought to a focus upon the retina at the point known as 

 the yellow spot, the principal focus of the eye. The line drawn 

 through the centres of curvature of the cornea and lens to this spot 

 is known as the optical axis of the eyeball (dr, Fig. 341). 



Many careful measurements have been made to determine the 

 cardinal points of the normal eye. The following measurements have 

 been deduced: 



First (anterior) principal focus of eye 

 Second (posterior) principal focus 

 First principal point 

 Second principal point 

 First nodal point 

 Second nodal point 



12-8326 mm. in front of cornea. 

 22-647 inn:, behind cornea. 



1-7532 



2-1101 



6-9685 



7-3254 . 



The principal points and nodal points are so close together that 

 they may be combined, giving what is known as the schematic or 

 reduced eye. In such a schematic eye, the path of the rays leading 

 to the formation of the image upon the retina can be mapped out. 

 The rays of light travelling from A and B, the extreme point of the 

 image, and falling parallel upon the surface, are not refracted, but 

 pass straight through the nodal point (K) to the retina. The angle 

 formed by these lines is known as the visual angle. The rays parallel 

 to the axis from A and B are refracted through the principal focus, 

 and cut, on the retina, the first rays passing through the nodal point 

 at a' and &' respectively. Hence an inverted object (a 1 b') is produced 

 upon the retina. 



The eye, however, is not a mathematically correct instrument. 

 The various refractive surfaces are not usually so centred that the 

 optic axis coincides with the line drawn from the point viewed to the 

 fovea centralis of the retina the visual axis. The angle of one axis 

 to the other, where they meet at the nodal point, is usually about 

 5 degrees, but may be as great as 12 degrees. Moreover, since the 

 centre about which the eye rotates is in the optical and not in the 

 visual axis, the line of regard (the line joining the point- view to the 

 aentre of rotation of the eye) does not coincide with the line of vision. 



There is a certain amount of spherical aberration in the normal 

 eye. This is not of much consequence, since it is corrected by the 

 action of the iris. There is also some chromatic aberration, which 

 is not, however, generally appreciated psychologically. It can be 

 demonstrated by looking at the sky through the upper part of a 

 window, and holding the edge of a card parallel to the upper side of 

 the window-frame, passing it from below upwards and from above 

 downwards. When the card covers half the pupil, the window-frame. 



