THE EYE AS AN OPTICAL INSTRUMENT. 179 



riches which it spreads before our sense. It is by the eye alone 

 that we know the countless shining worlds that fill immeasur- 

 able space, the distant landscapes of our own earth, with all the 

 varieties of sunlight that reveal them, the wealth of form and 

 colour among flowers, the strong and happy life that moves in 

 animals. Next to the loss of life itself that of eyesight is the 

 heaviest. 



But even more important than the delight in beauty and ad- 

 miration of majesty in the creation which we owe to the eye, is the 

 security and exactness with which we can judge by sight of the 

 position, distance, and size of the objects which surround us. For 

 this knowledge is the necessary foundation for all our actions, from 

 threading a needle through a tangled skein of silk to leaping from 

 cliff to cliff when life itself depends on the right measurement 

 of the distance. In fact, the success of the movements and ac- 

 tions dependent on the accuracy of the pictures that the eye gives 

 us forms a continual test and confirmation of that accuracy. If 

 sight were to deceive us as to the position and distance of external 

 objects, we should at once become aware of the delusion on 

 attempting to grasp or to approach them. This daily verification 

 by our other senses of the impressions \ve receive by sight 

 produces so firm a conviction of its absolute and complete truth 

 that the exceptions taken by philosophy or physiology, however 

 well grounded they may seem, have no power to shake it. 



No wonder then that, according to a wide-spread conviction, 

 the eye is looked on as an optical instrument so perfect that 

 none formed by human hands can ever be compared with it, 

 and that its exact and complicated construction should be 

 regarded as the full explanation of the accuracy and variety 

 of its functions. 



Actual examination of the performances of the eye as an 

 optical instrument carried on chiefly during the last ten years 

 has brought about a remarkable change in these views, just as in 

 so many other cases the test of facts has disabused our minds 

 of similar fancies. But as again in similar cases reasonable 

 admiration rather increases than diminishes when really impor- 

 tant functions are more clearly understood and their object 

 N 2 



