ix Tin-: 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 &quot;the 

 &quot;devil c.iii cite Scripture for his purpose.&quot; Those who take the 

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

 abstain from doing the very thing for whirh 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 : &quot; If I 

 &quot;am asked why I have spent so much time in explaining the imper- 

 &quot; feet ion of the eye, I answer, as I said at first, that 1 have not done 

 &quot;so in order to depreciate the performances of this wonderful 

 &quot;organ, or to diminish our admiration of its construction. It was 

 &quot;my object to make my readers understand, at the outset of our 

 &quot; inquiry, that it is not any mechanical perfection of the organs 

 &quot;of our senses which secures for us such wonderfully true and 

 &quot;exact impressions of the outer world. The extraordinary value 

 &quot;of the eye depends on the way in which we use it : its peifeetion 

 &quot;is practical, not absolute, consisting not in the avoidance of every 

 &quot;error, but in the fact that all its defects do not prevent its render- 

 &quot; ing us the most important and varied services.&quot; This &quot; practical 

 &quot;perfection &quot; he afterwards defines as &quot;adaptation to the wants of 

 &quot;the organism ;&quot; the defects of the eye as an optical instrument 

 being &quot;all so counteracted, that the inexactness of the image which 

 &quot;results from their presence very little exceeds, under ordinary 

 &quot; conditions of illumination, the limits which are set to the delicacy 

 &quot;of sensation by the dimensions of the retinal cones.&quot; 



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 centra! spot known as the ///&amp;lt;/.///&amp;lt;/ ////V&amp;lt;/, 

 is shown by Pioiessor Helmholt/. to be luiiy 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, 

 v\hi&amp;lt;li we wish to examine minutely, upon this sensitive spot; 

 * &quot; 1 ojajl.ir Lcctuns, p . 26. 



