THE SENSE OF SIGHT. 607 



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 per- 

 cipient 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, which stimulates the rods and cones, which is 

 the basis of our sight-perceptions, and out of which the mind con- 

 structs space-relations of external objectsf^ln only two essential 3 ' 

 respects does the image on the retina differ from the object, aside ['/ ' l ' 

 from the fact that the object has usually three, the image only two, J t-jV 

 dimensions viz., in size and relative arrangement of its parts. 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 of the 

 image. -^ 



The Dioptric Apparatus. The formation of an image is made 

 possible by the introduction of a complex refracting apparatus con- 

 sisting 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, homocentric rays arriving at the eye must 

 traverse successively the different refracting media. In their passage 

 from one to the other, they undergo at the surfaces changes in direc- 

 tion before they are finally converged to a focal point. In order to 

 mathematically follow the rays in all their deviations through the 

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

 knowledge of the form of the refracting surfaces, the refractive indices 

 of the different media, and the distances 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 practically parallel, and as 

 thejndex oj^ refraction of the aqueous humor is_jfie same as thai of 

 thTcorneaTthey jnayBe ^regards d as burTOgm^mrn. The refracting 

 Tulrfaces^rnay therefore" Fe reduced to the anterior surface of the 



