228 MICRO-PROJECTION FOR LARGE CLASSES [Cn. IX 



FIG. 125. DIAGRAM SHOWING VISUAL ANGLE. 



The nodal point or optic center of the eye is placed at the center of the circle, 

 and the rays from the extremities of the object which cross at this nodal point 

 show the visual angle. 



It is clearly seen from the diagram that the object must increase in length 

 in direct proportion to its distance from the eye if the visual angle remains 

 constant. 



Visual Angle The angle between the lines extending from the extremities 

 of the visible object and crossing at the nodal point (n) of the eye. 



Axis The straight line extending along the principal optic axis of the eye 

 to the visible object on one side and to the retina on the other side of the nodal 

 point (n). 



n Nodal point or optic center of the eye. 



ri Retinal image. The size of the retinal image of a given object depends 

 upon the visual angle and the visual angle depends upon the distance of the 

 object from the nodal point. 



For I meter (.3 mm. -=- .0075 = 400 diameters magnification). 

 For YJ, meter (.075 -: .0075 = 100 diameters magnification). 

 For 10 meters (3 -: .0075 = 4,000 diameters magnification). 

 For anything like a satisfactory view of the corpuscles, it would be desirable 

 to double these magnifications. 



