VISION. 



MISCELLANEOUS EXPERIMENTS. 



Blind Spot. On a white card make a small black cross, and 

 about 8 cm. away, in a horizontal direction, draw a black dot. 

 Looking intently at the dot, one eye having been screened, hold the 

 card about 25 cm. from the eye and then move it slowly toward the 

 eye. At what distance does the cross disappear? Why? 



Imperfect Visual Judgments. Draw one line (horizontal), 6 cm. 

 long, and one perpendicular to the same length but not joining. 

 Which line appears the longer and why ? 



Make three black dots on the same imaginary horizontal line 

 and equidistant from each other. Mark them x, y, and z. Con- 

 nect x and y with a series of equidistant dots of the same size. 

 Which appear farther apart, x and y or y and z ? 



Draw two horizontal lines 5 cm. long. On the upper one make 

 arrow-heads pointing toward each other. On the lower make 

 arrow-heads pointing away from each other. Which line appears 

 the longer? 



Draw three parallel lines. On the upper one draw a series of 

 short, parallel intersecting lines, cutting the longer line at an angle. 

 Do the same with the middle line, except to make the angle of 

 intersection equal and opposite to that of the first series. Prepare 

 the lower line the same as the upper line. Do the lines still appear 

 parallel ? 



Sanson-Purkinje Images. Darken the room. Hold a lighted 

 candle a little to one side and in front of the subject's eye. The 

 observer, looking at the eye from the other side, sees three images 

 of the flame. The first and brightest is a small, erect image 

 formed by the anterior convex surface of the cornea. The sec- 

 ond, larger and less distinct, is formed by the anterior convex sur- 

 face of the lens. The third, smaller, inverted, and indistinct, is 

 formed by the posterior surface of the lens. Let the subject ac- 

 commodate for a near object. Describe the change in relation 

 that takes place in the size and clearness of the second image and 

 its proximity to the first. 



