THE EYE AS AN OPTICAL INSTRUMENT. 227 



is not any mechanical perfection of the organs of our 

 senses which secures for us such wonderfully true and exact 

 impressions of the outer world. The next section of this 

 inquiry will introduce much bolder and more para- 

 doxical conclusions than any I have yet stated. We have 

 now seen that the eye in itself is not by any means so 

 complete an optical instrument as it first appears : its 

 extraordinary value depends upon the way in which we 

 use it : its perfection is practical, not absolute, consisting 

 not in the avoidance of every error, but in the fact that 

 all its defects do not prevent its rendering us the most 

 important and varied services. 



From this point of view, the study of the eye gives us 

 a deep insight into the true character of organic adapta- 

 tion generally. And this consideration becomes still more 

 interesting when brought into relation with the great and 

 daring conceptions which Darwin has introduced into 

 science, as to the means by which the progressive perfec- 

 tion of the races of animals and plants has been carried 

 on. Wherever we scrutinise the construction of physio- 

 logical organs, we find the same character of practical 

 adaptation to the wants of the organism ; although, per- 

 haps, there is no instance which we can follow out so 

 minutely as that of the eye. 



For the eye has every possible defect that can be found 

 in an optical instrument, and even some which are peculiar 

 to itself ; but they are all so counteracted, that the inexact- 

 ness of the image which results from their presence very 

 little exceeds, under ordinary conditions of illumination, 

 the limits which are set to the delicacy of sensation by 

 the dimensions of the retinal cones. But as soon as we 

 make our observations under somewhat changed condi- 

 tions, we become aware of the chromatic aberration, t]\e 

 astigmatism, the blind spots^ the yenous shadows, the 

 11 



