122 



ESSENTIALS OF PHYSIOLOGY. 



tion in the direction of rays of light takes place at the anterior surface 

 of the cornea, anterior surface of the lens, and posterior surface of the 

 lens. It has been calculated, however, that the net result of the 

 refraction in the eye is the same as that which would occur in a uniform ' 

 medium the anterior surface of which is 2 '3 mm. behind the anterior 

 surface of the cornea, and the nodal point O47 mm. in front of the 

 posterior surface of the lens. Such a theoretical arrangement is known 

 as the reduced or schematic eye (fig. 35). 



By means of the schematic eye the size of an image on the retina 

 can be ascertained. In the normal or emmetropic eye at rest parallel 

 rays are brought to a focus on the retina, and for practical purposes 

 rays proceeding from a point 6 metres or more distant from the eye 

 may be regarded as parallel. If, therefore, a diagram of the reduced 

 eye and of an object at 6 metres distance be drawn to scale, and if 



lines be drawn from the 

 periphery of the object 

 through the nodal point 

 to the retina, the size 

 of the retinal image can 

 be measured. The size 

 of the image of an 

 object nearer than 6 

 metres can also be cal- 

 culated, it being as- 

 sumed that the eye is 

 accommodated for the object. The angle subtended by the object, and 

 therefore by its image in the retina, is spoken of as the visual angle. 

 As the distance of the nodal point from the retina in an emmetropic 

 eye is known (15-5 mm.), the size of the retinal image of an object 

 is easily ascertained if the visual angle is measured. 



The limit of retinal discrimination corresponds with a visual angle 

 of sixty seconds, that is, in order that two points of light may be 

 distinguished as separate points they must subtend an angle at the 

 nodal point of not less than sixty seconds. A visual angle of this size 

 corresponds with the diameter of a single cone in the fovea centralis ; 

 it subtends a base of 4'38^ on the retina, and the cones in the fovea 

 vary in diameter between 2 and 5/u,. 



SIGHT-TESTING. 



The acuteness of vision is tested either by means of groups of dots 

 of varying sizes or by means of special test types. The test card is 

 usually placed at a distance of 6 metres from the eye to be tested, and 



. FIG. 35. Diagram showing formation of an image 

 in reduced eye. 



