VISION. 



227 



it evident that if the line A B represent the plane sur- 

 face of the blackboard and if the eye be placed at O the 

 equal increments of 10 on the quadrant become a series 

 of increasing increments upon the surface of the 

 board. The numbers at the right (Fig. 31) show just 

 how many centimeters the radius of each successive 

 circle should be provided the distance of the eye from 

 the board be taken at 20 centimeters. 



13 J 



7.3 - 



11.1 



It,.!- 



FIG. 31. 



FIG. 31. For de- 

 scription see 



FIG. 32. Showing method of ruling a black- 

 board for use in perimetry. The radii of the cir- 

 cles are given at the line A B in Fig. 31. 



After drawing the circles, draw meridians which divide 

 each quadrant into three to nine subdivisions. The 

 completed blackboard chart will have the appearance 

 and proportions shown in Fig. 32. The circles and 



