THE EYE AS AN OPTICAL INSTRUMENT. 201 



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 adaptation 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 perfection of the races of animals and 

 plants has been carried on. Wherever we scrutinise the con- 

 struction of physiological organs, we find the same character of 

 practical adaptation to the wants of the organism ; although, 

 perhaps, 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 inexactness 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 some- 

 what changed conditions, we become aware of the chromatic 

 aberration, the astigmatism, the blind spots, the venous shadows, 

 the imperfect transparency of the media, and all the other de- 

 fects of which I have spoken. 



The adaptation of the eye to its function is, therefore, most 

 complete, and is seen in the very limits which are set to its 

 defects. Here the result which may be reached by innumerable 

 generations working under the Darwinian law of inheritance, 

 coincides with what the wisest Wisdom may have devised 

 beforehand. A sensible man will not cut firewood with a razor, 

 and so we may assume that each step in the elaboration of the 

 eye must have made the organ more vulnerable and more slow 

 in its development. We must also bear in mind that soft, 

 watery animal textures must always be unfavourable and diffi- 

 cult material for an instrument of the mind. 



One result of this mode of construction of the eye, of which 



