CH. LV.] ACCOMMODATION. 759 



is formed on the retina. The retina is a curved screen, but 

 the images fall only on a small area of the retina under normal 

 circumstances ; hence for practical purposes this small area may 

 be regarded as flat. 



The question then arises, Why is it that objects do not appear 

 to us to be upside down 1 This is easily understood when we 

 remember that the sensation of sight occurs not in the eye, but 

 in the brain. By education the brain learns that the tops of 

 objects excite certain portions of the retina, and the lower parts 

 of objects other portions of the retina. That these portions of 

 the retina are reversed in position to 

 the parts of the object does not matter 

 at all, any more than it matters when 

 one's photograph arrives home from 

 the photographer's that it was wrong 

 way up in the photographer's camera 

 one puts it right way up in the 

 photograph album. 



ACCOMMODATION. Fig. 580. Diagram showing three 



reflections of a candle, i, From 



m , . , . the anterior surface of cornea ; 



1 he power Of accommodation, Or 2 from the anterior surface of 



the n,l,,i,t<Um of the eye to vision at l^VieZ. XrTuS 



iff/emit distances, is primarily due tO explanation, see text The 



- experiment is best performed 



an ability tO vary the shape Of the lens; by employing an instrument 



... . i , invented by Helmholtz, termed 



its front surface becomes more or less a phakoscope (see &g. 582). 



convex, according as the distance of the 



object looked at is near or far. The nearer the object, the more 

 convex, up to a certain limit, the front surface of the lens becomes, 

 and vice versa ; the back surface takes no share in the production of 

 the effect required. The posterior surface, which during rest is 

 more convex than the anterior, is thus rendered the less convex of 

 the two during accommodation. The following simple experiment 

 illustrates this point: If a lighted candle be held a little to one 

 side of a person's eye an observer looking at the eye from the 

 other side sees three images of the flame (fig. 580). The first 

 and brightest is ( i ) a small erect image formed by the anterior 

 convex surface of the cornea; the second (2) is also erect, but larger 

 and less distinct than the preceding, and is formed at the anterior 

 convex surface of the lens ; the third (3) is smaller, inverted, and 

 indistinct ; it is formed at the posterior surface of the lens, which 

 is concave forwards, and therefore, like all concave mirrors, 

 gives an inverted image. If now the eye under observation is 

 made to look at a near object, the second image becomes smaller, 



