THE EYE AS AN OPTICAL INSTRUMENT. 213 



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 parti- 

 cularly 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,* 



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 po- 

 sition that the image of the object falls on the small region 

 of perfectly 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 this rapidity of movement which 



' The lines in the well-known passage of Faust : — 



"Wenn iiber uns im blauen Eaum rerloren 

 Ihr schmetternd Lied die Lerche singt. 



