Part 3

21. Owing to perspective the right angles appear oblique and vice versa 58

22. Two equal diagonals which appear unequal 58

23. Apparent variations in the distance between two parallel lines 59

24. A striking illusion of perspective 60

25. Distortion of a square due to superposed lines 61

26. Distortion of a circle due to superposed lines 62

27. Illustrating fluctuation of attention 65

28. The grouping of the circles fluctuates 66

29. Crossed lines which may be interpreted in two ways 67

30. Reversible cubes 68

31. The reversible "open book" (after Mach) 69

32. A reversible tetrahedron 69

33. Reversible perspective of a group of rings or of a tube 70

34. Schröder's reversible staircase 70

35. Thiéry's figure 71

36. Illustrating certain influences upon the apparent direction of vision.

By covering all but the eyes the latter appear to be drawn alike in both sketches 73

37. Zöllner's illusion of direction 77

38. Parallel lines which do not appear so 79

39. Wundt's illusion of direction 79

40. Hering's illusion of direction 80

41. Simple effect of angles 81

42. The effect of two angles in tilting the horizontal lines 83

43. The effect of crossed lines upon their respective apparent directions 83

44. Another step toward the Zöllner illusion 84

45. The two diagonals would meet on the left vertical line 85

46. Poggendorff's illusion. Which oblique line on the right is the prolongation of the oblique line on the left? 85

47. A straight line appears to sag 86