202 RECENT PROGRESS OF THE THEORY OF VISION. 



measurement of the distance. In fact, the success of 

 the movements and actions dependent on the accuracy 

 of the pictures that the eye gives us forms a con- 

 tinual test and con6rmation 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 im- 

 pressions we 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 con- 

 viction, 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 important functions are 

 more clearly understood and their object better esti- 

 mated, so it may well be with our more exact knowledge 

 of the eye. For the great performances of this little 

 organ can never be denied ; and while we might con- 

 sider ourselves compelled to withdraw our admiration 

 from one point of view, we must again experience it 

 from another. 



Regarded as an optical instrument, the eye is a camera 

 obscura. This apparatus is well known in the form used 

 by photographers (Fig. 27). A box constructed of twc 



