242 LAB OR A TOR Y GUIDE IN PH YSIOLOG V. 



dicular to the middle of the base line may be called the 

 median plane. Any point in this plane which is fixed by 

 the two eyes in binocular vision may be called \he point 

 of binocular fixation. The line joining this point to the 

 middle of the base line would lie in the median plane, 

 and would be called the median line. 



If the point of fixation be at a great distance (infinity) 

 the lines of fixation of the two eyes would be parallel to 

 the median line. In this case there would be no con- 

 vergence. If, however, the point of fixation be near there 

 will be a convergence of the two lines of fixation toward 

 that point. The amount of convergence is greater the 

 nearer the point, and is called the angle of convergence. 

 The angle of convergence is then the angle between the 

 line of fixation at infinity and the line of fixation at the 

 given distance less than infinity, the given distance be- 

 ing measured on the median line, beginning at the base 

 line. 



The geometric situation is indicated in the accom- 

 panying figure (Fig. 34). Let C represent the center 

 of rotation of the left eye, M the middle of the base line 

 and the origin of the median line; CP the line of fixa- 

 tion of an object at infinity; MM' the median line; the 

 line CM is one-half the base line; represent the distance 

 CM by b. The angle D'CP is the angle of convergence 

 when D' is the point of binocular fixation. As 

 to the exact measure of the angle, it is evident from 

 the figure that the line MD', which we may represent by 

 d, is the cotangent of the angle of convergence (ang. c). 



The unit of measurement for the angle of convergence 

 is the meter angle (Ma) of Nagel. The meter angle is 

 the angle of convergence when the point of binocular 

 fixation is 1 m. distant (d = l,000 mm). Ma equals the 

 angle whose cotangent is g (cot Ma=!j). The aver- 



