228 LABORATORY GUIDE IN PHYSIOLOGY. 



meridians should be traced permanently in slate-colored 

 enamel upon the surface of the blackboard. Any marks 

 made upon the board with chalk may then be erased 

 without disturbing the perimeter circles. 



Make test objects in this manner. To a soft pine disc 

 3 or 4 cm. in diameter and 1 cm. thick fix a 20 cm. 

 handle of hard wood. The whole should be given a dead 

 black surface, India ink is good for this purpose. Upon 

 the disc one may fix with a pin the test object : a circle 

 or a square or other form in white, yellow, green, blue 

 or red. 



Each blackboard chart must be provided with a rest 

 or contrivance to insure 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 promi- 

 nent infra-orbital region upon its end, the cornea will be 

 20 cm. from the center of the chart; or whether it takes 

 some other form that insures the same result is of little 

 consequence. 

 3. Experiments and Observations. 



In all the observations which are subsequently indi- 

 cated, it is taken for granted that the visual axis is per- 

 pendicular to the surface of the chart, that the center of 

 the chart is the point of fixation, and that the accommo- 

 dation is kept uniform, i. e., the eye is either uniformly 

 focused on the pole of the blackboard perimeter or uni- 

 formly relaxed; further that the eye not under observation 

 be closed or closely shaded. 



(1) Examine the upper median quadrant by sweeping a 

 white circle or square around arc. 60, keeping the test 

 object as near the surface of the chart as possible. If 

 the subject does not see it at all, try latitude 50. Hav- 

 ing located the circle which seems to be near the boun- 



