394 THE BODY AT WORK 



see it either red with a blue fringe or blue with a red fringe, 

 according as the eye is focussed for red or for blue. The 

 purple glass having absorbed all intermediate rays, we become 

 a^are that we cannot focus the two extreme ends of the 

 spectrum at the same place. Since a greater effort of accom- 

 modation is needed to focus red, we judge that the bright 

 object is nearer to us when it appears red than when it appears 

 blue. 



Spherical aberration is another fault of the lens. The rays 

 which enter its margin are brought to a focus sooner than those 

 which pass through its centre. This is due to the fact that 

 its surfaces are regularly curved, whereas a glass lens is cor- 

 rected by grinding it flatter towards the margin. This defect 

 is partly corrected by the cornea, which has an ellipsoidal 

 surface, and partly by the greater density of the centre of the 

 lens. Yet it is still necessary for the eye to be " stopped 

 down " by the iris when a near object is looked at, although 

 less light is entering the eye than when it is directed to the 

 horizon a condition which would lead a photographer to open 

 his iris-diaphragm. 



Of all the imperfections of the eye which the mind ignores, 

 the most remarkable is the gap in the field of vision, due to the 

 gap in the sensitive layers of the retina, which occurs where 

 the optic nerve enters it the blind spot. Hold this page of 

 the book 10 inches from the face, keeping the lines of print 

 horizontal. Close the left eye and look at X with the right 

 eye. The black disc disappears, because its image is focussed 



on the blind spot. Since the picture on the retina is reversed, 

 it is clear that the optic nerve enters the globe to its inner side, 

 and slightly above its horizontal meridian. But, unless we 

 employ an unusual test, we are quite unconscious of the fact 

 that a definite hole is punched in the picture. The mind fills 

 it in, and the way in which it does so is extremely suggestive. 



