286 PHYSIOLOGY CHAP. 



sisting of a medium of the same refractive index as the nucleus of 

 the lens. According to Hermann we may conceive the lens to be 

 formed of a highly bi-convex lens c and two concave-convex 

 lenses a, and I (Fig. 126). " The latter neutralise part of the effect 

 of c, and the less so in proportion as their refractive index is 

 lower. Since a, b have a lower refractive index than c the total 

 action of the lens is greater than if it had the same index as c, i.e. 

 if the lens were homogeneous and had the high refractive power 

 of the nucleus throughout." 



It will be seen later on that the stratified constitution of the 

 lens is not without importance in the dioptrics of the eye. 



We stated that the refractive index of the cornea is rather 

 higher than that of the lachrymal secretion and the aqueous 

 humour, which bathe its two surfaces. But from the point of 

 view of optics the study of the eye is considerably simplified by 

 the fact that the cornea is a lamella with parallel surfaces, and is 

 therefore, like a watch glass, unable to alter 

 the direction of a ray of light passing through 

 it, and is capable only of slightly displacing it 

 parallel to itself. The cornea may therefore 

 be neglected and the eye considered diagram- 

 matically as though it consisted of three 



FIO. i28.-Dtagn,m to show media Onl 7> the aqueous humour, lens and 

 the physical structure of vitreous body separated by surfaces that are 



the crystalline lens, which i 



consists of a strongly bi- approximately centred. 



SnSSSrtASS ^ Petit ( 1723 ) ^tempted to estimate 



con j ex i enses a and b - the radius of curvature of the cornea and the 



(After Hermann.) , 



two surfaces of the lens, on eyes extracted 

 from the dead body. These measurements, however, were very 

 incomplete and unreliable, owing to the rapid shrivelling of the 

 tissues, and Kohlrausch's method was afterwards adopted, in which 

 the height of the images reflected from the corneal surface and the 

 anterior and posterior surfaces of the lens are measured in the 

 living eye. These are known as the Sanson-Purkinje images, 

 from the authors who first described them. 



The radius of curvature of the cornea is estimated by measur- 

 ing the height of the corneal image of a luminous object, the size 

 and distance of which are known, starting from the law that the 

 size of the luminous object is to that of its reflected image as 

 the distance of the luminous object from the convex surface of 

 the mirror is to the half of the mirror's radius. Two points of 

 light are generally employed as the luminous object. The 

 distance of the images from the two points is measured by the 

 ophthalmometer (of Helrnholtz, Aubert, or Javal). 



To observe Purkinje's images the flame of a candle must be 

 placed about 50 cm. away at the height of the observer's eye, so 

 that the line which unites the flame with the observed eye forms 



