THE FAE AS AN omCAL INSTRUMENT 270 



At each of these three surfaces the light passes from a medium 

 of one density to auotlier and, therefore, as the surfaces are 

 convex, the incident rav is bent towards the central axis. 



We have seen that refraction is always accompanied by a 

 certain amount of reflection, and this is the cause of the pheno- 

 menon known as Sansori's Images. If a candle is held at a short 

 distance from one side of the eye, the observer can distinguish 

 the images formed by each of the refracting surfaces. The image 

 produced by the cornea where the change in refractive index is 

 from 1 to 1-37 is much brighter than the images from the lens, 

 where the change is from 1-33 to 1-42 and from 1-42 to 1-33. 

 In the case of the two latter also there 

 is a certain amount of absorption of 

 light by the media. The images from 

 the cornea and the anterior surface 

 of lens are upright, that from the 

 posterior surface of lens is inverted. 



Curvature of the Surfaces. The 

 images of the candle are not all of 

 the same size, because the radii of 

 curvature of the three reflecting and 

 refracting surfaces are different. The 

 central image is the largest because 

 the anterior surface of the lens is 

 the least curved, while the inverted 

 image is the smallest because it is 

 reflected from the posterior concave 

 surface of the lens which has the greatest curvature, i.e. smallest 

 radius of curvature (Table XXXVIII.). 



no. 



-ansDu's Images. 



The images, from left to riglit, are 

 from the anterior surface of tlie cornea, 

 anterior surface of lens, and posterior 

 surface of the leas. (From Goulden'.s 

 " Refraction.") 



TABLE XXXVIII 



Radii of Curvatures of Cornea and Lens 



Cornea (anterior surface) ..... 



(posterior surface) ..... 



Lens (anterior surface) .... 



(posterior surface) .... 



mm. 

 7-98 

 6-22 

 10-20 

 6-17 



Positions on Optic Axis. If we add to our knowledge of the 

 refractive indices and radii of curvature of the components of the 

 optical system of the eye, measurements of their distances from the 

 retina, we shall be in a position to calculate the dioptric values of 

 these media. 



The distances are given in Table XXXIX. 



