524 PHYSIOLOGY 



of a small, nearly spherical lens of very great density. In the fish therefore 

 the lens takes the principal part in the refraction of the light so as 

 to form an image. 



REDUCED EYE. It is an interesting fact that, owing to the closeness 

 of the principal and nodal points to one another, it is possible to imagine the 

 media of the eye replaced by 'a single optical surface without introducing 

 any appreciable error. To this system is given the title ' reduced eye.' 

 Its constants are given in the following Table: 



Radius of surface .... .6 mm. 



Position of principal point ...... . 2'.'} mm. 



Position of nodal point . . 7*1 mm. 



Position of retina . . . . . . . .22-6 mm. 



Refractive index ..'.... . 1'33 mm. 



Focal length . . . lf>'5 mm. 



THE ACCOMMODATION OF THE EYE 



The above description has been made on the supposition that the rays 

 entering the eye consist of parallel bundles, or in other words that the objects 

 seen are at an infinite distance from the eye. But during near vision such 

 is by no means the case. If there were no means of varying the focus of 

 the eye it would not be possible for divergent rays (those coming from near 

 objects) to be brought to a focus on the retina. The mechanism for varying 

 the focus of the eye is called the accommodation. 



THE THEORIES OF ACCOMMODATION. Of these three are of historic interest 

 only. (1) That during accommodation the cornea increases in curvature (similar 

 to the bird's eye). This was disproved by Thomas Young who placed his eye under 

 water, replaced the corneal refraction by a convex lens, and then found the amplitude 

 of accommodation unaffected. (2) That the eye elongates in near vision, thus causing 

 the rays from near objects to focus on the retina (similar to the arrangement in the mollusc 



Fit:. -<)4. Methods by which accommodation of the refraction of the eye tor ol.jerts 



at different distances could be effected. 



(A) By lengthening the eyeball. (B) By increasing curvature of cornea. 

 ((') .By moving lens forward. (D) By increasing curvature of lens. 



Pecten). This also Thomas Young disproved by placing two iron rings which could be 

 clamped together, one in front of and one behind the eyeball. Having very prom inen t eyes 

 he could do this if the eye being tested were rotated strongly inwards. The phosphene 

 caused by the pressure of the posterior ring, which extended to the fovea, did not change 

 in appearance during accommodation. He thus found no evidence for an elongation 

 of the eye during accommodation. (3) That the lens during near vision advances 

 towards the cornea (similar to the mechanism in the fish's eye). This \ie\\ \\as 

 disproved by Tscherning, who calculated that the lens would have to advance 

 nearfylOmm. in order to give the full amplitude found tor the eye, whereas the anterior 



