130 ELEMENTARY PHYSIOLOGY. XXIII. 



Many variations are possible, portions of the designs 

 alone may disappear, portions may reappear together, 

 but rarely if ever do both images disappear at the same 

 time. 



Cover one design up with a sheet of white paper, the 

 other design is seen without interruption. Now suddenly 

 remove the sheet of paper and it will be observed that 

 the design now uncovered will immediately take the 

 place of the other and will keep it unchallenged for 

 several seconds. 



28. Visual illusions, a. Draw a horizontal line 

 10 cm. long. Then, keeping the line horizontal in front of 

 you, draw a vertical line through it of what seems to you 

 of equal length. Measure it. 



b. Draw a horizontal line about 2cm. long; under 

 this at a distance of about 1 cm. draw another line 

 parallel to it and of equal length. At each end of one 

 line draw a V, making the limbs of the two Vs point 

 towards one another (thus giving the line an arrow-head 

 at each end); at each end of the other line draw a V, 

 making the. limbs of the two Vs point away from one 

 another. The former line will look shorter than the 

 latter. 



c. Measure on a card equal squares; without putting in the out- 

 line of the squares, fill in one with fine vertical lines one to two 

 millimetres apart and the other with similar horizontal lines the 

 same distance apart. Place them a short distance off, they will ap- 

 pear not square but oblong, the side at right angles to the direction 

 of the lines looking longer than the side parallel to the direction of 

 the lines. 



