OPTICS 



279 



ability to turn around the visual line as an axis. In reality 

 such a turning does not take place, but for each position of 

 the visual line the whole eye has a definite position. This 

 is due to the peculiar coordination of the innervation of the 

 eye muscles. The position of the eye for any given position 

 of the visual axis is such as if it moved from the primary to 

 the new position by rotating around an axis perpendicular 

 to the visual axis in the primary and the new position. 



r.sup. 



FIG. 36. ACTION OF THE EYE MUSCLES. 



Lines described by the line of vision in the visual field when the eyeball is 

 turned from its primary position by some muscle of the eye. The movements of 

 the point of vision [Blickpunkt] corresponding to the angle of rotation are indi- 

 cated on the lines in degrees. The distance of the plane of the paper from the 

 point of rotation of the eye equals the line dd. The position which the horizontal 

 meridian in a primary position assumes is indicated at the end of each line. 



(fr) Action of the individual muscles (see Figs. 35 and 36). 

 -The change in position of the eye may be stated as fol- 

 lows : 



i. Change in the position of the anterior surface of the 



