464 THE OUTGO OF ENERGY 



the nodal point becomes the vertex of an 

 angle, the angle gamma, 7, the legs of which 

 are the anterior portion of the optical and visual 

 axes. The angle 7 usually reaches 5, but may 

 reach 10. 



In emmetropia and hypermetropia, the visual 

 axis passes through the cornea on the inner side 

 of the optical axis ; angle 7 is then positive. The 

 eyeball must rotate outwards in order to fix an 

 object. Thus the visual axes seem to diverge. 

 Hence the angle 7 must be considered in esti- 

 mating the degree of a divergent squint. 



In myopia, the visual axis may coincide with 

 the optical axis or pass through the cornea on 

 the outer side of the optical axis. In the latter 

 case, angle 7 is negative. In this condition the 

 eyeball must rotate inwards in order to fix the 

 object. The deviation inwards may be confused 

 with convergent squint. 



Draw a diagram showing angle 7. 



Visual Angle. Draw an arrow in front of a 

 diagram of the reduced eye. Draw lines from 

 the nodal point through and beyond the two ex- 

 tremities of the object. 



The angle included between the lines drawn 

 from the nodal point to the extremities of the 

 object is termed the visual angle. 



Apparent Size. Within the lines marking the 



