DIOPTRIC MECHANISMS, 745 



surfaces are all centred on a line called the optic axis, which meets the retina 

 at a point somewhat above and to the inner (nasal) side of the fovea cen- 

 tral is. In passing from the outer surface of the cornea to the retina the 

 rays of light traverse in succession the cornea, the aqueous humor, the lens, 

 and the vitreous humor. Refraction takes place at all the surfaces bound- 

 ing these several media, but particularly at the anterior surface of the 

 cornea, and at both the anterior and posterior surfaces of the lens. Since 

 the anterior and posterior surfaces of the cornea are parallel, or very nearly 

 so, the rays of light would suffer little or no change of direction in passing 

 through the cornea, if it were bounded on both sides by the same medium. 

 The direction of the rays of light in the aqueous humor would, therefore, 

 remain the same if the' cornea were made exceedingly thin ; if, in fact, its 

 two surfaces were made into one, forming a single anterior surface to the 

 aqueous humor; or, which comes to the same thing in the end, since the 

 refractive power of the substance of the cornea is almost exactly the same 

 as that of the aqueous humor, the refraction at the posterior surface of the 

 cornea may be neglected altogether. Thus the two surfaces of the cornea 

 are practically reduced to one. The lens varies in density in different parts, 

 the refractive power of the central portions being greater than that of the 

 external layers ; but the refractive power of the whole may, without any 

 serious error, be assumed to be uniform. The refractive power of the vitreous 

 humor is almost exactly the same as that of the aqueous humor. 



618. Thus the apparently complicated natural eye may be simplified 

 into a " diagrammatic eye," in which the refracting surfaces are reduced to 

 three, viz. : (1) the anterior surface of the cornea, (2) the anterior surface of 

 the lens separating the lens from the aqueous humor, and (3) the posterior 

 surface of the lens separating the lens from the vitreous humor. The media 

 will similarly be reduced to two : the substance of the lens and the aqueous 

 or vitreous humor. This " diagrammatic eye " is of great use in the various 

 calculations which become necessary in studying physiological optics ; for 

 the magnitudes which are derived by calculation from it represent the cor- 

 responding magnitudes in an average natural eye with sufficient accuracy to 

 serve for all practical purposes. The values adopted by Listing for 'the 

 constants of this " diagrammatic eye," and to him we are indebted for the 

 introduction of it, are as follows : 



Radius of curvature of cornea 8 mm. 



of anterior surface of lens ]0 ' 



of posterior 6 " 



Refractive index of aqueous or vitreous humor \fif 



Mean refractive index of lens | 



Distance from anterior surface of cornea to anterior surface of lens 4 mm. 



Thickness of lens 4 " 



The calculated position of the principal posterior focus, i. e., the point at 

 which all rays falling on the cornea parallel to the optic axis are brought to 

 a focus, is in the diagrammatic eye 14.6470 mm. behind the posterior surface 

 of the lens, or 22.6470 mm. behind the anterior surface of the cornea. 

 That is to say, the fovea centralis must occupy this position in order that a 

 distinct image of a distant object may be formed upon it. It must be under- 

 stood that these values refer to the eye when at rest, i. e., when it is not 

 undergoing any strain of accommodation. 



Accommodation. 



619. When an object, a lens, and a screen to receive the image are so 

 arranged in reference to each other that the image falls upon the screen in 



