522 PHYSIOLOGY 



known optical properties, or by estimating the apparent size of the images 

 of an object which 'are formed by reflection at its .surfaces. In order that 

 the latter method shall succeed, a device must be employed for eliminating 

 the effect of chance movements of the eye while under observation. This 

 was first done by Thomas Young, by employing a method used in astronomy, 

 namely that of doubling the image to be measured and then adjusting the 

 lower edge of one image to be in coincidence with the upper edge of the 

 other. If the eye moved during the determinations, both images moved 

 together and therefore difficulties in adjustment were avoided. In the case 

 of the cornea this method is alone available because only in the living state 

 is the true curvature preserved. In the case of the lens the determinations 

 are complicated by the fact that the refraction of the cornea has to be allowed 

 for. Further, the images that are seen are neither bright nor sharply defined ; 

 but in spite of this considerable accuracy is attainable. The following 

 are the approximate values given by these "methods. 



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



Radius of lens, anterior surface . . . . . . .10 mm. 



Radius of lens, posterior surface . . . . . 6 mni. 



THE REFRACTIVE INDICES (optical densities) of the eye media 

 are determined on the excised eye by means of the Abbe refractometer. It 

 is found that the cornea and aqueous are so nearly alike that for all practical 

 purposes they may be regarded as one, particularly as the posterior corneal 

 surface has nearly the same centre as the anterior. The refractive indices 

 may therefore be given as follows : 



Refractive index of cornea and vitreous . . . . . 1-34 



Refractive index of lens (equivalent) ..... . I '42 



Refractive index of aqueous humour ..... .1 :>.", 



THE APPLICATION OF GAUSS' THEOREM. In addition to tin- 

 above data we require to know the distance between the principal 

 surfaces ; these are found to be : 



Distance from cornea to anterior lens surface .... :Mi mm. 

 Distance from cornea to posterior lens siul.u . . . . ?<> nun. 



from cornea to the retina ..... 22-0 mm. 



These values being known it is possible by calculation to determine the 

 path of any ray through the eye. The problem is however made very much 

 simpler by the application of Gauss' theorem, which may be briefly stated 

 as follows. Any system of spherical optical surfaces, the centres of which 

 lie along a straight line, possesses six cardinal points, namely two principal 

 points h and h', two nodal points K'andK', and two focal points, the anterior 

 $ and the posterior <'. It is found that these have certain properties which 

 may be summarised as follows : 



An object placed at the first principal point is found after refraction 

 to be at the second. Further, the image and its object are found to be of 1 he 

 same size. A ray passing through the first nodal point on its way into 

 the system appears to corm from the other on its way out. bui its direction 



