VI 



DIOPTEIC MECHANISM OF THE EYE 



281 



struction of the figure that the size of object 00' is to the size of 

 image //' as the distance of the object from the nodal point C is 

 to the distance of the image from the same point. 



(e) Refraction of Light in a Centred Optic System. A more 

 complex dioptric system, as that of the eye, results when several 

 spherical surfaces are separated by media with different refractive 

 indices. When the centre of curvature of the respective surfaces 

 of separation are all on the same straight line, given by the optic 



o 



Fie;. 120. Formation of the image of a point. 



axis of the system, the system is said to jbe centred. In order to 

 determine the refraction of such a complex system, i.e. to construct 

 the image of a given object, it is necessary to pass from medium 

 to medium according to the above rules. Gauss (1841), by 

 mathematical calculation, demonstrated that the determination of 

 refractive power in any compound, centred system can be greatly 

 simplified if the radii of curvature of the respective surfaces of 



AW C 







O VV 2> J O 



Fi<;. 121. The six cardinal points of a centred dioptric system. 



separation, the refractive indices of the different media, and their 

 relative position upon the optical axis of the system are known. 

 He showed that each centred system may be replaced by a system 

 of six cardinal points (optic constants of Gauss). Given, for 

 instance, a system of four refractive media (M, M', M", M'" Fig. 

 121) separated by the three spherical surfaces AB, CD, El, which 

 all have their centre of curvature on the straight line XX formed 

 by the optic axis, the fundamental law laid down for a simple 

 system still holds good viz. that a definite point on the image 

 corresponds to each point of a luminous object. 



