THE SENSE OF SIGHT 701 



media and occupies by far the largest portion of the interior of the eyeball. 

 From its position it gives support to the retina. Anteriorly it presents a con- 

 cavity, in which the crystalline lens is lodged. The vitreous humor consists 

 of water (97 per cent.), organic matter and salts, enclosed in a transparent 

 membrane, the tunica hyaloidea. The mass of the vitreous humor is pene- 

 trated by a species of connective tissue. 



These bodies, the cornea and aqueous humor, the lens and the vitreous 

 humor, together form a refracting system by which parallel rays of light, 

 which enter the eye when it is in repose, are converged and brought to a focus 

 on the retina. 



The relations of the parts entering into the structure of the eye are shown 

 in Fig. 283. 



THE PHYSIOLOGY OF VISION 



The Retinal Image. The general function of the eye is the formation 

 of images of external objects on the free ends of the percipient elements of 

 the retina, the rods and cones. The existence of an image on the retina 

 can be readily seen in the excised eye of an albino rabbit, when placed 

 between a lighted candle and the eye of an observer. Its presence in the 

 human eye can be demonstrated with the ophthalmoscope. It is this image, 

 composed of focal points of luminous rays, that stimulate the rods and 

 cones, which is the basis of our sight perceptions, and out of which the mind 

 constructs space relations of external objects. In only two essential re- 

 spects as far as space relations go, does the image on the retina differ from 

 the appearance of the object, aside from the fact that the object has usually 

 three, the image only two, dimensions viz., in size and position. What- 

 ever the distance, the image is generally smaller than the object; it is also 

 reversed, the upper part of the object becoming the lower part of the image, 

 and the right side of the object the left side of the image. 



The Dioptric or Refracting Apparatus. The formation of an image is 

 made possible by the introduction of a complex refracting apparatus consist- 

 ing of the cornea, aqueous humor, lens, and vitreous humor. Without these 

 agencies the ether vibrations would give rise only to a sensation of diffused 

 luminosity. Rays of light emanating from any one point that is, homo- 

 centric rays arriving at the eye must traverse successively the different re- 

 fracting media. In their passage from one to the other, they undergo at the 

 surfaces changes in direction before they are finally converged to a focal point. 

 In order to follow mathematically the rays in all their deviations through the 

 media, to determine their focal points and to construct an image, a knowl- 

 edge of the form of the refracting surfaces, the refractive indices of the dif- 

 ferent media, and the distance of the surfaces from one another must 

 be known. 



The following constants are now accepted: The radius of curvature 

 of that portion of each refracting surface used for distinct vision is for the 

 cornea 7.829 mm., for the anterior and posterior surfaces of the lens 10 and 

 6 mm., respectively. The indices of refraction of the different media are 

 as follows: cornea and aqueous humor, 1.3365; lens, 1.4371; vitreous body, 

 1.3365. The distance from the vertex of the cornea to the lens is 3.6 mm.; 

 the thickness of the lens, 3.6 mm.; the distance from the posterior surface 

 of the lens to the retina, 15 mm. As the two surfaces of the cornea are prac- 



