746 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



posterior surface of the lens, from 7.1-7.5 mm. The thickness of the lens 

 therefore varies from 4 to 4.6 mm. The distance from the posterior surface 

 of the lens (Kuapp) to the retina is about 13 mm. The coefficient of re- 

 fraction is for the lens 1.455, and for the vitreous and aqueous humors 1.3366. 

 In order that the course of a ray of light may be followed in any system of 

 refractive media, it is necessary that the position of the five cardinal points 

 should be known namely, that of the two principal points, which are the 

 points where the principal refracting surfaces are cut by the axis ; that of 

 the two nodal points, or those points in the refractive media to which a ray 

 of light is directed before, and appears to proceed from, after its refraction ; 

 and lastly, of the focal, or rather of the two focal points, one behind the lens 

 and the other in front of the cornea. The relative positions of these cardi- 

 nal points are as follows : 



1. Principal point, 



2. Principal point, 



1. Nodal point, 



2. Nodal point, 

 2. Focal point, 

 1. Focal point, 



2.1746 mm. 



25724 " 



0.7850 " 



0.3602 " 



14.6470 " 



12 8326 " 



behind the anterior surface of the cornea. 



u 11 u i< 



in front of the posterior surface of the lens. 



t! U U U 



behind " " " 



in front of the anterior corneal surface. 



The two principal points thus lie at 0.4 mm. distance from one another, 

 nearly in the middle of the anterior chamber of the eye. The two nodal 

 points, also about 0.4 mm. from each other in the posterior part of lens ; 

 the second focal point for parallel rays close to or in the retina. The two 

 nodal points lie so close to one another that they may be regarded as single, 

 and visual rays passing through them as rectilinear. In like manner, the 

 two principal surfaces may be regarded as marked in the spherical surface, 

 h h (Fig. 260), which represents the refractive surface of the eye. Every 

 point of a retinal image corresponds to a point of the object; and if it be 

 desired to determine which are the corresponding points of the object and of 

 the image, it is only requisite to join them by a line passing through the 

 united nodal point. The retinal images are consequently inverted. 



612. The Eye maybe regarded as an optical instrument of great per- 

 fection, adapted to produce on the surface of the Retina, a complete image 

 or picture of luminous objects brought before it; in which the forms, colors, 

 lights, and shades, etc., of the object are all accurately represented. By the 

 different refractive powers of the transparent media through which the rays 

 of light pass, and by the curvatures given to their respective surfaces, both 

 the Spherical and Chromatic aberrations are corrected in a degree sufficient 

 for all practical purposes; so that, in a well-formed eye, the picture is quite 



free from haziness and from false col- 

 ors. The power by which it adapts 

 itself to variations in the distance of 

 the object, so as to form a distinct 

 image of it, whether it be six inches, 

 six yards, or six miles off, is ex- 

 tremely remarkable, and cannot be 

 regarded as hitherto completely ex- 

 plained. It is obvious that, if we 

 fix upon any distance as that for 

 which the eye is naturally adjusted 

 (say 12 or 14 inches, the distance at 

 which we ordinarily read), the rays 

 proceeding from an object placed 

 nearer to the eye than this would 

 not be brought to focus upon the 



FIG. 200. 



O 



F F', Axis of the Eye; it H', Principal points; 

 K K, Nodal points; h h, Principal refracting sur- 



face; o B, Visual ray. 



retina, but would converge towards a point behind it 



upon 

 whilst on the con- 



