THE OPTICAL SYSTEM OF THE EYE 



521 



this is the arrangement found in the lens of the eye, the power of 

 refraction is very much greater than an ordinary lens would possess of the 

 same curvature as the lens and the same refractive index as the average 

 density of its substance. The optical propertied of the lens are therefore 

 unique, and it is interesting to find the same chemical substance in the lens 

 of different morphological groups of animals. This would point to the 

 substance with the required optical properties being somewhat rare. To 

 show the effect which the increasing density of the lens produces, the refrac- 

 tive indices of its parts may be compared with its equivalent R.I. (that is 

 the refractive index of a glass lens of the same size, shape and focal length). 

 The refractive index of the periphery of the lens is 1'37, and that of the central 

 nucleus 1*41, the mean being about 1-39. But the equivalent density of the 

 lens is found to be 1'42, that is, greater by -03 than the mean refractive index 

 of its substance. The lens lies in contact with two transparent media 

 both of which have an approximate refractive index of 1'34. The power 

 of the lens if its composition was uniform would therefore be proportional to 

 the difference between its own mean R.I. (1-39) and that of its surroundings 

 (1'34), that is to 0-05. Owing to its peculiar structure its equivalent R.I. is 

 1-42, and therefore its power is proportional to the difference between 

 that and 1-34, that is to 0-08. Owing to its structure the lens has 

 therefore increased in power in the ratio of 0-08 to 0'05. Now since the range 

 of accommodation depends, -other things being equal, on the power of the 

 lens, we see that the peculiar structure of the lens has nearly doubled its 

 range. The graduation in the densities of the different layers of the lens 

 has a further advantage which will be described later, in that it reduces 

 the spherical aberration of the eye as a whole (see page 531 ) and also reduces 

 the amount of scattered light within the .eyeball. 



THE OPTICAL CONSTANTS OF THE EYE. In the case of the 

 rystalline lens, two methods are available for the determination of the radii 



OBSERVERS EYC 



1 



FIG. 262. Diagram to show a method of determining the curvature of the anterior 

 surface of the cornea. The images of the lamps A and B are caused to coin- 

 cide by shifting the position of the double image prism. The greater the 

 curvature of the cornea the closer must the double image prism be to the eye. 



curvature of the anterior and posterior surfaces, namely measurements 

 on the excised lens, either in the air or preferably suspended in a fluid of 



