426 NATURE AND MAN. 



comparison the slight want of optical perfection wliich — as I have 

 already shown you — is inseparable from it. 



Let us now turn our attention to the fact that it is only in the 

 sensitive spot of the retina, the macula lufea, that we have the 

 most perfect provision, in the elaborateness of its structure, for the 

 reception and transmission of the visual picture. The " rods," 

 and " cones," as they are called, of that spot are much smaller 

 tlian they are in any other part of the retinal surface ; and our 

 vision of objects whose picture falls upon it is proportionately 

 distinct and minute. Now to me it seems that the inferior visual 

 perfection of the rest of the retina, far from being disadvantageous, 

 is a positive advantage. How completely the disadvantage is 

 compensated by the facility with which we move our eyes, I have 

 already shown in Professor Helmholtz's own words. The direction 

 of their axes which is required to bring upon the macula lutea the 

 image of any object at which we wish to look, is given without 

 any conscious exertion of our own ; we have only to " will " to 

 look at the object, and the muscles of our eyes automatically bring 

 their axes into convergence upon it. If you look at the eyes of a 

 person who is reading or writing, you will see them move from left 

 to right as he follows each line across the page, and then turn 

 suddenly to the left again as he begins the next line ; and yet he 

 is not conscious of giving them any such direction. So, again, if 

 we fix our gaze on any object, and move our head upwards or 

 downwards, or from side to side, another person looking at our 

 eyes will see them move in the opposite direction, so that their 

 axes continue to point to the object at which we are looking.* 

 Now while the disadvantage of the limitation of distinct vision to 

 the macula lutea is thus fully compensated, I hold that this limita- 

 tion is postively advantageous in this way, — that we see the object, 

 or the part of the object, at which we will to look, with much g?-eater 

 distinctness than we should do if the whole of the visual picture 

 which we receive at one time were as complete and vivid as that 

 portion of it which is formed on the central spot of the retina. 

 For our W6V//«/ receptivity of this picture depends upon tho. attention 

 we give it ; so that the more completely our attention is con- 



* Any one may make this experiment for himself, by looking at his own 

 eyes in a looking-glass, and moving his head either horizontally or vertically. 



