16. Crossed Lines showing the Effect of Astigmatism 113 17. Another Design showing the same 114 18. Star-like Images of Luminous Points 116 19. Sutures of the Crystalline Lens 117 20. Multiple Images of a Luminous Point 120 21. The same, showing an increased number of Images 122 22. The same when a Slit is held before the Eye 123 23. Multiple Images of an Electric Lamp Filament 125 24. The same seen through a Slit 126-128 25. Illusion of Length 132 26. Another form 135 27. Another form 136 28. Another form 137 29. Another form 138 30. Illusion of Inclination 143 31. Zöllner's Lines 144 32. Slide for showing Illusions of Motions 147 33. Illusion of Motion 149 34. Illusion of Luminosity 152 35. Illusion of Colour 155 36. Recurrent Vision demonstrated with a Vacuum Tube 176 37. The same with a Rotating Disk 178 38. Apparatus for showing Recurrent Vision with Spectral Colours 181 39. Charpentier's "Dark Band" 187 40. Charpentier's Effect shown with the Hand 189 41. Multiple Dark Bands 192 42. Temporary Insensitiveness of the Eye after Illumination 194 43. Visual Sensations attending a Period of Illumination 199 44. Benham's Artificial Spectrum Top 200 45. Demonstration of Red Colour-borders 205 46. Black and White Screens for the same 209 47. Rotating Disk for the same 210 48. Demonstration of Blue Colour-borders 215 49. Disk for Experiments on the Origin of the Colour-borders 217 50. Disk for the Subjective Transformation of Colours 224 CHAPTER I. LIGHT AND THE EYE. In the present scientific age every one knows that light is transmitted across space through the medium of the luminiferous ether. This ether fills the whole of the known universe, as far at least as the remotest star visible in the most powerful telescopes, and is often said to be possessed of properties of so paradoxical a character that their unreserved acceptance has always been a matter of considerable difficulty.