534 PHYSIOLOGY 



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 produced 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. 



PART 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 object through the single nodal 

 point to the retina, as is shown in Fig. 263. The image thus produced, like 



FIG. 263. Path of the rays in the formation of an image on the retina. 



that produced by a bi-convex 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 to a 

 distance on the retina between the two ends of the image of -00438 mm. 

 Few people can distinguish two points of light the line joining which sub- 

 tends a smaller visual angle than sixty seconds. If we measure the histo- 

 logical elements in the centre of the retina which are responsible for distinct 

 vision, viz. the cones, we find that in the yellow spot each cone is about -002 

 to -005 mm. thick. The limits of our power of distinguishing two luminous 

 points is approximately in agreement with the diameter of each end- organ 

 of vision. In order that the images of the two points may give rise to dis- 

 tinct sensations their images must fall upon different cones. This fineness 

 of vision exists only at the yellow spot, the accuracy of vision in the peripheral 

 parts of the retina being very much lower. Not only is the image less 



