ACCOMMODATION 295 



a smaller circle, while the meridional fibres cause the 

 cihary processes to be drawn towards the pupil and sUghtly 

 forward. The effect of contraction of this muscle is there- 

 fore to release the tension of the suspensory Ugament 

 and to allow the lens to become more spherical. The 

 refractive power of the lens is in this way increased and 

 divergent rays are focussed on the retina. The ciliary 

 muscles are supplied by the third nerve. 



Accommodation is always accompanied by contraction 

 of the pupil. This results in a clearer definition of the 

 image, owing to the cutting off of the rays which strike 

 the periphery of the lens, and to the increased depth of 

 focus. 



The clearness of the image formed upon the retina is 

 limited by diffraction — that is to say, the tendency of the 

 edges of the wave of light to spread and form patterns. 

 Diffraction is a physical process, and is therefore inevitable. 

 There remain to be considered two other optical errors 

 and the means taken by the eye to overcome them. 



Chromatic Aberration. — The waves of short length (those 

 at the blue end of the spectrum) are deflected by the 

 refracting media more than the long red waves. The 

 normal eye is so shaped that yellow — that is, the most 

 intense^rays focus on the retina, red rays behind and 

 green and blue rays in front. Around a central spot of 

 yellow there are therefore formed a small halo of red, a 

 small halo of green and a large halo of blue. The red 

 and green halos combine to form yellow, while the blue is 

 too diffuse to stimulate the retina. 



Spherical Aberration. — If the refracting media were of 

 miiform density and their surfaces of uniform curvature, 

 rays striking the cornea peripherally would come to a focus 

 in front of those passing centrally. This is obviated in 

 two ways : (1) the centre of the lens is more highly refractive 

 than the periphery ; (2) the curvature of the anterior 

 surface of the cornea is less peripherally than it is centrally. 



