DESIGN IN THE ORGANIC WORLD. 423 



unfairness of quotation ; a single passage, detached from its con- 

 text, often conveying a meaning altogether different from that 

 which it bears when taken with its context, so that even "the 

 "devil can cite Scrii)ture for his purpose." Those who take the 

 anti-theological side are specially bound, as it seems to me, to 

 abstain from doing the very thing for which they would severely 

 blame their opponents ; and yet I have seldom met with a case 

 so unfair, as the citation of this statement without any of the 

 qualifications which it subsequently receives. Thus, after show- 

 ing that these defects scarcely reveal themselves in our ordinary 

 vision — some of them requiring most refined methods of observa- 

 tion for their detection — Professor Helmholtz continues : " If I 

 "am asked why I have spent so much time in explaining the imper- 

 " fection of the eye, I answer, as I said at first, that I have not done 

 "so in order to depreciate the performances of this wonderful 

 "organ, or to diminish our admiration of its construction. It was 

 " my object to make my readers understand, at the outset of our 

 " inquiry, that it 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 extraordinary value 

 " of the eye depends on tlie way in which we use it : its perfection 

 "is practical, not absolute, consisting not in the avoidance of every 

 "error, but in the f;ict that all its defects do not prevent its render- 

 " ing us the most important and varied services." This " practical 

 "perfection" he afterwards defines as "adaptation to the wants of 

 "the organism ;" the defects of the eye as an optical instrument 

 being " 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." * 



An optical defect which has long been known to ophthalmolo- 

 gists, — the inferiority in the sensitiveness of the retinal surface 

 generally, to that of the central spot known as the macula lutea, — 

 is shown by Professor Helmholtz to be fully compensated by the 

 facility and rapidity with which we move the eye, in such a manner 

 as to bring the image of the object, or of any part of the object, 

 which we wish to examine minutely, upon this sensitive spot ; 



* " Popular Lectures," p. 226. 



