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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 12. The angle X. 



the entrance-pupil. The pupillary axis is usually 

 about 5 degrees temporalward from the primary line 

 of sight, as appears in figure 12. 



Size of Retinal Image 



The chief ray concept is also useful in dealing with 

 the size of the retinal image of an external object. The 

 angle 7 subtended by two object points at the center 

 of the entrance-pupil is called the visual angle; it 

 is the angle between the two lines of sight (see fig. 9). 



The term "size of the retinal image' refers to the 

 linear distance between the retinal images M" and 

 Q," of the two object points M and Q,. When the eye 

 is out of focus, the retinal images are blur circles and 

 the size M"Q_" represents the distance between the 

 centers of the two blur circles formed on the retina. 

 If the angle y' subtended by the centers of the two blur 

 circles at the center of the exit-pupil is expressed in 

 radians, 



linear separation of the 



centers of two blur circles 

 y' = . 



distance from the exit pupil to the retina 



The ratio of 7' to 7 is one of the more important 

 constants of the eye. In the case of the Helmholtz 

 schematic eye 



7'/7 = 0.81. 



With this ratio and the distance from the exit-pupil 

 to the retina specified, one can compute the linear 

 separation of two blur circles on the retina for a given 

 value of 7. 



The expression 'size of the retinal image' is often 

 misinterpreted to mean the size of the blur circle 

 formed by a single object point, but this is something 

 which is quite different from the linear separation 

 between the centers of two blur circles. 



REFRACTION AND ACCOMMODATION OF THE EYE 



The eye has an adjustable focusing mechanism. 

 The first section of this chapter has explained how the 

 eye forms an image of an object when the focusing 

 mechanism is fixed. This section describes how the 

 changes in focus may be described and specified. 



Refraction of the Eye 



If a given point on the primary line of sight pro- 

 duces a bundle of rays which comes to a point focus 

 on the retina, the eye is said to be focused for this 

 point. Another way of stating this is to say that the 

 object point producing the bundle is conjugate to 

 the retina. 



The spectacle point 5 (14 mm in front of the cornea) 

 is used as the reference point for specifying the loca- 

 tion of the point R which is conjugate to the retina, 

 as represented in figure 13.4. Stating the distance 

 from R to S adequately describes the refractive state 

 of the eye, but it is customary to use the reciprocal of 

 this distance and call it the refraction of the eye. It 

 is measured in diopters when the distance RS is given 

 in meters. 



When the eye is astigmatic, it is necessary to 

 specify separately the refraction in the two principal 

 meridians. To visualize this problem it is better to 

 start with a point on the retina penetrated by the 

 foveal chief ray and trace a bundle of rays back out 

 of the eye through the entrance-pupil. The line of 

 sight represents the chief ray of this bundle, and the 

 principal meridians are the planes which intersect at 

 right angles at the line of sight. 



The o to 180 degree meridian which is the reference 

 meridian for the location of the principal meridians 

 lies in the plane of regard which is defined by the 

 centers of the two entrance-pupils and the point of 

 convergence of the two primary lines of sight. In figure 

 1 3^ the line of sight is perpendicular to the paper and 

 penetrates the front of the eye at A. The angles 0i and 



-I4fnm 

 A. SPECTACLE POINT 



FIG. 13. Reference points and planes for specifying the 

 refraction of an eye. 



