DIOPTEIC MECHANISM OF THE EYE 



285 



of curvature of the limiting surfaces; (c) the distance of these 

 from each other and from the surface of the retina. 



(a) The facts collected by various authors in relation to the 

 refractive indices of the different transparent media of the eye, 

 extracted from the living body or fresh cadaver, differ very slightly. 

 The following table shows the figures obtained with the most 

 reliable methods, particularly with Abbe's refractometer : 



Comparison of these figures brings out two important facts: 

 the refractive indices of the aqueous humour and the vitreous 

 humour are approximately the same ; the crystalline lens is not 

 an optically homogeneous body, but consists of concentric layers 

 of different consistency, so that the refractive index increases 

 progressively from the periphery to the more central layers. To 

 facilitate the study of dioptrics in the eye we may picture the 

 crystalline lens as replaced by an optically homogeneous lens of 

 the same form and the same total power of refraction as the lens. 

 This can be determined directly on lenses extracted from the 

 dead body and suspended in the air, to ascertain the position of 

 the focal points ; or it may be calculated from known data of the 

 refractive indices of the different strata of the lens. 



By both methods Matthiessen arrived at the result that the 

 total index of the crystalline lens is 1*4371, which is considerably 

 above the refractive index of the nucleus of the lens. According 

 to Tscherning the total index given by Matthiessen is too high- 

 he thinks 142 nearer the true figure. More recently (1902) 

 Treutler has deduced the total refractive index of the lens from 

 the diminution of refracting power in the eye that has lost its lens, 

 and estimates it at 1-4215, which almost exactly coincides with 

 that of Tscherning. 



Neglecting these slight differences, it is remarkable that the 

 total index of the lens is higher not only than the mean index of 

 its different layers, but also than the maximal index of its nucleus. 

 This fact, which at first seems paradoxical, is easily understood if 

 we reflect that the nucleus of the lens is limited by far more highly 

 convex surfaces than the lens as a whole, and the latter accord- 

 ingly has a higher refractive index than a homogeneous lens con- 



