DIOPTRIC MECHANISMS OF THE EYEBALL 593 



It will be seen that both the principal points lie in the anterior 

 chamber, while the nodal points fall in the back part of the lens. The 

 posterior focus of the eye falls upon the retina. For many purposes 

 we may simplify our calculations by running the principal points 

 and nodal points together. In such a reduced eye the single principal 

 point is situated 2-3 mm. behind the anterior surface of the cornea, and 

 the single nodal point 0-47 mm. in front of the hinder surface of the 

 lens. If a circle be drawn from the single nodal point as a centre 

 through the single principal point as a circumference we get a surface 

 II in the figure, which represents the anterior refracting surface 

 of such a reduced eye. 



A reference to the Table of Constants of the human eye shows 

 that, whereas the anterior focal distance of the cornea is 23 mm., that 

 of the whole eye is 15 mm. and that of the lens 44 mm. It is evident 

 from this that the anterior surface of the cornea is the most important 

 refractive surface of the eye, and that, in the convergence of rays 

 necessary for the formation of an image on the retina, it is the refrac- 

 tion at this surface which plays the greatest part. Under water this 

 refraction is of course abolished, since the refractive indices of the 

 aqueous humour and cornea are practically identical with that of 

 water. The eye therefore becomes long-sighted ; it is impossible 

 to get any clear image of near objects, and distant objects can only be 

 seen with a strong effort of accommodation. A smaller effect is pro- 

 duced by removal of the lens, an operation often undertaken when this 

 body has become opaque as a result of cataract. Such an operation 

 not only abolishes the power of accommodation of the eye, but diminishes 

 the refractive power of the eye at rest by ten dioptres, so that a lens 

 of this power has to be placed at the front of the eye in order to render 

 possible clear vision of distant objects. 



* 



PATH OF THE RAYS IN THE FORMATION OF THE 



RETINAL IMAGE 



In the reduced eye the construction of the path of the rays is 

 very simple. When the eye is focused for the object which is being 

 looked at, the rays from any point of the object come to a point on the 

 retina. All that is necessary therefore is to draw lines from points 

 of the obj ect through the single nodal point to the retina, as is shown 

 in Fig. 261. The image thus produced, like that produced by a 

 bi- con vex lens, is real, inverted, and diminished in size. The further 

 the distance of the object from the retina the smaller will be its image 

 on the retina. The angle formed at the junction of two lines drawn 

 from the extreme points of an external object to the nodal point is the 

 ' visual angle.' 



In the schematic eye a visual angle of sixty seconds corresponds 



38 



