REFRACTION IN THE EYE 463 



KELATIONS OF THE VISUAL Axis 



It has already been stated that the refracting 

 surfaces of the eye are centred, often imperfectly, 

 upon a right line, the optical axis. This line 

 normally meets the retina between the yellow 

 spot and the optic papilla or exit of the optic 

 nerve. To see a luminous point clearly, the 

 image of the point must fall on the centre of the 

 yellow spot. The line passing from the centre of 

 the yellow spot through the nodal point to the 

 luminous point is termed the visual axis. Unless 

 the luminous point already lie in the visual axis, 

 it must for distinct vision be brought there by 

 the rotation of the eyeball. The object is then 

 said to be " fixed " by the eye. The point about 

 which the eye rotates is the centre of rotation. 

 The line between the luminous point and the 

 centre of rotation is the line of fixation. 



The line of fixation and the visual axis should 

 nearly coincide. Generally, the visual axis 

 and the optical axis do not coincide. In other 

 words, the visual axis is generally a secondary 

 axis, and the planes of the refracting surfaces 

 are oblique to it. The optical axis passes to 

 the inner side of the yellow spot. It inter- 

 sects the visual axis at the nodal point. Hence 



