380 PHYSIOLOGICAL PHYSICS, [Chap. xxix. 



The position of the optical constants can be deter- 

 mined by a mathematical formula, provided the 

 indices of refraction, the radii of curvature, and the 

 thicknesses of the different media be given. 



In the case of the eye, these values, according to 

 Listing, are as follows : 



Index of refraction for air ... ... 1 



Index of ref rac ;ion for aqueous humour. . . i ^ 1 '3379* 



Index of refraction for crystalline lens ... \\ = 1'4545* 



Index of refraction for vitreous body ... V^ = 1-3379* 



Radius of curvature of cornea ... ... 8 mm. 



Radius of curvature of anterior surface of 



crystalline lens ... ... ... 10 mm. 



Radius of curvature of posterior surface 



of crystalline lens ... ... ... 6mm. 



Distance of the anterior face of the cornea 



from the anterior surface of the 



crystalline lens ... ... ... 4 mm. 



Thickness of the crystalline lens ... 4 mm. 



* Helrnholtz gives 1'3365 for aqueous humour ; 

 1 '3382 for vitreous body ; 

 1/4415 for crystalline lens. 



The centres of curvature of the different media, 

 are in the same straight line, THE OPTICAL AXIS OF 

 THE EYE, which passes through the centre of the 

 globe and the summit of the cornea. 



Using the above values, the positions of the car- 

 dinal points of the human eye on the optical axis, 

 calculated from the summit of the cornea, are as 

 follows : 



Anterior principal focus ... 12-8326 mm. 



Posterior principal focus ... 22-6470 mm. 



Anterior principal point ... 2-1746 mm. ] Difference, 



Posterior principal point ... 2*5724 mm. / 0-3978. 



First nodal point ... ... 7 '2420 mm. 1 Difference, 



Second nodal point 7'6398 mm. j 0-3978. 



Of these, the anterior principal focus is in front of 

 the cornea, the others are behind. 



