THE EYE AS AN OPTICAL INSTRUMENT. 201 



And history teaches us, what we shall have opportunity 

 of seeing in the present inquiry, that the most important 

 practical results have sprung unexpectedly out of investi- 

 gations which might seem to the ignorant mere busy 

 trifling, and which even those better able to judge could 

 only regard with the intellectual interest which pure 

 theoretical inquiry excites. 



Of all our members the eye has always been held the 

 choicest gift of Nature — the most marvellous product of 

 her plastic force. Poets and orators have celebrated its 

 praises; philosophers have extolled it as a crowning 

 instance of perfection in an organism ; and opticians have 

 tried to imitate it as an unsurpassed model. And indeed 

 the most enthusiastic admiration of this wonderful organ 

 is only natural, when we consider what functions it per- 

 forms ; when we dwell on its penetrating power, on the 

 swiftness of succession of its brilliant pictures, and on 

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

 the eye alone that we know the countless shining worlds 

 that fill immeasurable 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 admiration of majesty in the creation which we owe 

 to the eye, is the security and exactness with Avhich we 

 can judge by sight of the position, distance, and size of 

 tlie objects which surround us. For this knowledge is 

 the necessary foundation for all our actions, from thread- 

 ing a needle through a tangled skein of silk to leaping 

 from cliff to cliff when life itself depends on the right 



