THE EYE AS AN OPTICAL INSTRUMENT. 189 



the field of vision to an angular magnitude which can be covered 

 by the nail of one's forefinger when the hand is stretched out as 

 far as possible. In this small part of the field our power of vision 

 is so accurate that it can distinguish the distance between two 

 points, of only one minute angular magnitude, i.e. a distance 

 equal to the sixtieth part of the diameter of the finger-nail. 

 This distance corresponds to the width of one of the cones of 

 the retina. All the other parts of the retinal image are seen 

 imperfectly, and the more so the nearer to the limit of the 

 retina they fall. So that the image which we receive by the 

 eye is like a picture, minutely and elaborately finished in the 

 centre, but only roughly sketched in at the borders. But 

 although at each instant we only see a very small part of the 

 field of vision accurately, we see this in combination with what 

 surrounds it, and enough of this outer and larger part of the 

 field, to notice any striking object, and particularly any change 

 that takes place in it. All of this is unattainable in a telescope. 

 .But if the objects are too small, we cannot discern them at 

 all with the greater part of the retina. 



When, lost in boundless blue on high, 

 The lark pours forth his thrilling song, 1 



the ' ethereal minstrel ' is lost until we can bring her image to a 

 focus upon the central pit of our retina. Then only are we able 

 to see her. 



To look at anything means to place the eye in such a position 

 that the image of the object falls on the small region of per- 

 fectly clear vision. This we may call direct vision, applying 

 the term indirect to that exercised with the lateral parts of the 

 retina indeed with all except the yellow spot. 



The defects which result from the inexactness of vision and 



the smaller number of cones in the greater part of the retina 



are compensated by the rapidity with which we can turn the 



eye to one point after another of the field of vision, and it is 



1 The lines in the well-known passage of Faust : 



Wenn ttber uns im blauen Raum verloren 



Ihr schmetternd Lied die Lerche singt. 



