192 SPECIAL PHYSIOLOGY 



(e) Is the power of convergence apparently normal in all members 

 of the class? If not, describe minutely any variations. 



VII. MISCELLANEOUS EXPERIMENTS. 



(a) Schemer's Experiment. (1) Prick two smooth holes in a card 

 at a distance from each other less than the diameter of the pupil. 

 Fix two long, fine needles or straws in two pieces of wood or cork. 

 Fix the cardboard in a piece of wood with a groove made in it with 

 a fine saw, and see that the holes are horizontal. Place the needles 

 in line with the holes, the one about eight inches, the other about 

 eighteen inches from the card. 



(2) Close one eye, and with the other look through the holes at the 

 near needle, which will be distinctly seen, while the far needle will be 

 double, both images being somewhat dim. 



(3) With another card, while accommodating. for the near needle 

 close the right-hand hole, the right-hand image disappears, and if the 

 left-hand hole be closed the left-hand image disappears. 



(4) Accommodate for the far needle, the near needle appears 

 double. Now close the right-hand hole, and the left-hand image 

 disappears; and on closing the left-hand hole the right-hand image 

 disappears. (Practical Physiology, Stirling.) 



(5) Explain the phenomena, drawing figures which show just what 

 must take place in the eye. 



(6) The Blind Spot. (6) Marriotte's Experiment. On a white 

 card make a black cross and a circle about three inches apart. 

 Closing the left eye hold the card vertically about ten inches from 

 the right eye, so as to bring the cross to the left side of the circle. 

 Look steadily at the cross with the right eye, when both the cross and 

 circle will be seen. Gradually bring the card toward the eye, keeping 

 the axis of vision fixed upon the cross. At a certain distance the 

 circle will disappear i. e. } when its image falls on the entrance of 

 the optic nerve. On bringing the card nearer the circle reappears, 

 the cross, of course, being visible all the time. 



(7) Map Out the Blind Spot. Make a cross on the center of a sheet 

 of white paper and place it on a table about ten or twelve inches from 

 you. Close the left eye and look steadily at the cross with the right 

 eye. Wrap a penholder in white paper, leaving only the tip of the 

 penpoint projecting, dip the latter in ink, or dip the point of a white 

 feather in ink, and keeping the head steady and the axis of vision 

 fixed, place the penpoint near the cross and gradually move it to 

 the right until the black becomes invisible. Mark this spot. Carry 

 the blackened point still farther outward until it becomes visible again. 

 Mark this outer limit. These two points give the outer and inner 

 limits of the blind spot. Begin again, moving the pencil first in an 



