VISION >tf 



maintain the proper intra-ocular pressure on which the geometrical 

 figure of the eyeball, and therefore its efficiency as an optical instrument, 

 depend. The balance between secretion and absorption is accurately 

 adjusted in health, but in disease it may be upset, as in glaucoma, where 

 the intra-ocular tension is so much increased as to interfere with the 

 circulation, and injuriously affect the nutrition and function of the retina. 

 Experimentally, occlusion of all the arteries supplying the head causes 

 a rapid fall of tension, and the cornea becomes wrinkled and slack to 

 the touch. On restoring the circulation after not too long an interval, 

 the tension gradually returns to normal, and then becomes markedly 

 hypernormal, even when the general arterial pressure is still low. This 

 is probably due to the crippling of the elements which secrete and 

 absorb the intra-ocular fluids, or of the capillary walls, so that a proper 

 adjustment can no longer be attained, as happens in a tissue rendered 

 cedematous by temporary anaemia. Where asphyxia of the eyeball is 

 avoided or is brief the intra-ocular pressure varies directly as the blood- 

 pressure in the ocular vessels within a wide range (Henderson and Starling) . 



Refraction in the Eye Formation of the Retinal Image. The 

 amount of refraction which a ray of light undergoes at a curved 

 surface depends upon two factors the radius of curvature of the 

 surface, and the difference between the refractive indices of the 

 media from which the ray comes and into which it passes. The 

 smaller the radius of curvature, and the greater the difference ol 

 refractive index, the more is the ray bent from its original direction. 

 A ray of light passing into the eye meets first the approximately 

 spherical anterior surface of the cornea, covered with a thin layer 

 of tears. Since the refractive index of the tears is much greater 

 than that of air, the ray is strongly refracted here. The anterior 

 and posterior surfaces of the cornea being practically parallel, and 

 the refractive indices of the tears and aqueous humour being nearly 

 equal, but little refraction takes place in the cornea itself. At the 

 anterior and posterior surfaces of the lens the ray is again refracted, 

 since the refractive index of the aqueous and vitreous humours is 

 less than that of the lens. The following tables show the radii of 

 curvature of the refracting surfaces and the refractive indices of 

 the dioptric media, as well as some other data which |are of use in 

 studying the problems of refraction in the eye : 



In accommodation for 



Far Vision. Near Vision. 



fCornea - - - - 7-8 mm. 7-8 mm. 



Radius of curvature of-! Anterior surface of lens - 10-0 ., 6-0 ,, 

 \ Posterior surface of lens - 6-0 ,, 5-5 ,, 

 'Anterior surface of cornea and an- 

 terior surface of lens - 3-6 ,, 3-2 ,, 



Distance 

 between 



Anterior surface of cornea and pos- 



terior surface of lens - 7-6 ,, 7-6 



Anterior and posterior surface of lens - 4-0 ,, 4-4 



.Posterior surface of lens and retina - 14-6 ,, 14*6 

 Antero-posterior diameter of eye along the axis -22-2 ,, 22-2 



62 



