44 DR KNOX on the Comparative Anatomy of the Eye. 



I. General Idea of Vision. 



It has been said by a distinguished physiologist, that we can- 

 not obtain an immediate knowledge of real distance by sight 

 alone. Our countryman, Dr PORTERFIELD, however, demon- 

 strated that, within certain limits, perhaps within the range of 

 distinct vision, our knowledge of distance is perfect, and that it 

 depends on the organ being double and symmetrical. Beyond 

 this point, the mode by which we judge of distance becomes 

 complex, for we have to avail ourselves of other senses, and more 

 particularly of that of feeling. We reason also respecting the 

 distance of objects, from their degrees of illumination, from their 

 obscuring each other, from their magnitude, &c. With all these 

 aids, many individuals judge very erroneously of distance, whilst 

 others decide upon it with the utmost precision. This faculty 

 depends, therefore, on a peculiarly organised brain, conjoined 

 with much experience. The superiority of one military genius 

 over another is in no small degree attributable to this talent. 

 History is filled with the errors of those, who, miscalculating dis- 

 tance, have either crowded their troops into a space unable to 

 contain them, or, by too great an extension of their lines, have 

 presented a feeble front, incapable of resisting the enemy. Such 

 errors must necessarily lead to serious disasters. 



A particular eye sees distinctly objects placed at only a cer- 

 tain distance, hence men are either short or long sighted ; but 

 we know that experience exerts great influence over this. Even 

 the vulture, whose eye is beyond all doubt the most keen and 

 piercing, is conducted by experience to a knowledge of the prey 



