16. An illusion of contrast 55 17. Equal circles which appear unequal due to contrast (Ebbinghaus' figure) 56 18. Equal circles appearing unequal owing to contrasting concentric circles 56 19. Circles influenced by position within an angle 57 20. Contrasting angles 57 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 48. Distortions of contour due to contact with other contours 87 49. An illusion of direction 88