194 



SPEC I A L PH YSIOL OGY 



After drawing the circles, draw meridians, which divide each 

 quadrant into three to nine subdivisions. The complete blackboard 

 chart will have the appearance and proportions shown in Fig. 81. 

 The circles and meridians should be traced permanently in white 

 enamel upon the surface of the blackboard. Any marks upon the 

 board with chalk may then be erased without disturbing the perim- 

 eter circles. 



The most satisfactory test objects are pieces of fresh crayon not 

 over 1 cm. in length. They may be held in wire holders of convenient 

 length. 



Each blackboard must be provided with a rest or contrivance to 

 ensure that the subject's eye is 20 cm. from the surface of the board. 

 Whether this takes the form of a rod of wood extending out from the 

 board and so adjusted that when the subject rests the most prominent 

 infraorbital region upon its end the cornea will be 20 cm. from the 

 center of the chart, or whether it takes some other form that ensures the 

 same results, is of little consequence. 



3. Experiments and Observations. In all the observations which 

 are subsequently indicated it is taken for granted that the visual axis 

 is perpendicular to the surface of the chart, that the center of the 

 chart is the point of fixation, and that the accommodation is kept 

 uniform i. e. } the eye is either uniformly focused on the center of the 

 blackboard perimeter or uniformly relaxed ; further, that the eye not 

 under observation be closed or closely shaded. 



