492 SIGHT. [BOOK in. 



medium. The direction of the rays of light in the aqueous 

 humour 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 humour; 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 humour, 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 humour is almost exactly the same as that of the aqueous 

 humour. 



Thus the apparently complicated natural eye may be simplified 

 into a 'diagrammatic eye/ in which the refracting surfaces are re- 

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

 anterior surface of the lens separating the lens from the aqueous 

 humour, and (3) the posterior surface of the lens separating the 

 lens from the vitreous humour. The media will similarly be 

 reduced to two ; the substance of the lens, and the aqueous or 

 vitreous humour. This 'diagrammatic eye' is of great use in the 

 various calculations which become necessary in studying physio- 

 logical optics ; for the magnitudes which are derived by calculation 

 from it represent the corresponding 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 intro- 

 duction of it, are as follow : 



Radius of curvature of cornea 8 mm. 



of anterior surface of lens 10 



of posterior 6 



Refractive index of aqueous or vitreous humour iff 



Mean refractive index of lens -}-f 



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 understood 

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

 undergoing any strain of accommodation. 



