390 THE BODY AT WORK 



And what is true of the retina as a whole is true, apparently, of 

 each individual cone. In what way does light act upon a 

 cone ? It is one of the most fascinating problems in physi- 

 ology. Round it our thoughts revolve whenever we are trying 

 to form conceptions of the nature of stimulation, sensation, 

 and perception. Each of the two theories which we have ex- 

 pounded above helps to group together certain of the more 

 striking phenomena of colour-vision, but neither gives a satis- 

 fying explanation of their causation. 



The sensitiveness of the retina is in a remarkable degree 

 adjusted to the intensity of the light. When a dark room is 

 entered, the pupil dilates ; but one's power of distinguishing 

 objects continues to increase after the pupil has reached its 

 maximum size. At the end of ten minutes the eye may be 

 twenty-five times as sensitive as it was when the room was 

 entered. This adaptation to darkness is due in large degree to 

 the substitution of rods for cones as the organs on which vision 

 chiefly depends. But it cannot be wholly due to this, since 

 it occurs when one is working with a red light. Probably the 

 red used in a " dark room " is not sufficiently near the end of 

 the spectrum to be completely without influence upon visual 

 purple, but it is a colour to which rods are comparatively 

 insensitive. Other evidence also points to an adaptation of 

 cones as well as of rods. 



Accommodation of the eye for distance is brought about by a 

 mechanism which allows the lens to change in shape. It 

 becomes more convex when a near object is looked at than it 

 was when adjusted for an unlimited distance, which is its con- 

 dition when the eye is at rest. Adjustment for near objects 

 involves muscular action, and is accompanied by a sense of 

 effort, however slight. Whilst the eye is at rest the lens is 

 mechanically compressed against the anterior layer of its sus- 

 pensory ligament. Accommodation for near vision is effected 

 by the ciliary muscle, which is placed in the shelf of tissue which 

 projects into the interior of the eyeball. This muscle is made 

 up of a ring of circular fibres, and to the outer side of this, 

 of fibres which radiate backwards and outwards. The longi- 

 tudinal, or radiating, fibres obtain their purchase by attach- 

 ment to the firm wall of the globe just beyond the cornea. 

 They spread into the front of the loose chorioid membrane 



