492 PHYSIOLOGY 



to the distance at which an object is placed. Such a method would be found ineffective 

 except for relatively near objects however, because of the small change of focus which 

 is involved. A superior method would be to endow a particular part of the retina with 

 increased sensitiveness, next to provide two complete organs of vision instead of one, 

 both capable of rotation in all directions, and then to mount them as far apart as 

 possible, so that as they are turned relatively to one another, in order to view near or 

 distant objects, the amount of such relative deviation may be estimated and so a means 

 be'provided of appreciating distance. But the provision of increased sensitiveness in a 

 particular part of the retina also tends to diminish the disturbing effects of the rest and 

 therefore improves at the same time the power of concentrating the attention on a 

 particular object. This in its turn greatly simplifies the task of producing the lens system , 

 which forms the image of external objects, since only a part of the image is required 

 to have the maximum sharpness, namely that corresponding to the most sensitive 

 region of the retina, this part being always used whenever an object of particular 

 interest is being examined. In order to employ a simple lens system to the greatest 

 advantage, that part of the image should be used which lies immediately in front of the 

 axis of the lens ; for it is here that the best definition is found. The most sensitive | a it 

 of the retina should be placed therefore in this position. It remains to describe a further 

 improvement which may be effected in the perception of distance, when a pair of eyes 

 are used which move in co-ordination. Suppose, for example, that there are two objects- 

 one more distant than the other, which appear to the right eye to be in line. Then to 

 the 4eft eye the more distant one will appear to lie to the left of the other. There is 

 thus a relative displacement of the two images of the objects, which will be found 

 to increase as the distance between the objects increases. If the objects do not appear 

 in line to either of the eyes, it will still be found that there is a constant difference between 

 the positions of the two images formed on the retinae. If there be a suitable mechan- 

 ism for estimating the amount of this displacement, there is at once provided a 

 very accurate method of judging distance. One form which this mechanism could take 

 will be considered later. 



In the development of a hypothetical organ of vision, which has been traced above 

 from a simple and inefficient to an elaborate and improved type, we have seen that each 

 modification had to be introduced in order to make use of the application of some well- 

 known physical property ; not once has the impossibility of obtaining some obvious 

 beneficial feature to be faced. The eye as we find it in man is almost identical with 

 this organ which we have developed as it were from first principles. The eye therefore 

 provides an excellent example of the efficiency with which evolution has been controlled 

 by natural laws, and of the small extent to which the limitations of the materials avail- 

 able have prevented the introduction of desirable features. 



