LIST OF DIAGRAMS.
| FIG. | PAGE. | |
| [1.] | Image of Slit and Spectrum | [12] |
| [2.] | Diagram of the Eye | [24] |
| [3.] | Abney’s Colour-patch Apparatus | [45] |
| [4.] | Partially Intercepted Spectrum | [49] |
| [5.] | Stencil Cards | [52] |
| [6.] | Helmholtz’s Curves of Colour Sensations | [72] |
| [7.] | König’s Curves | [73] |
| [8.] | Stencil Card for Complementary Colours | [77] |
| [9.] | Another form | [79] |
| [10.] | Slide for Mixing any two Spectral Colours | [80] |
| [11.] | Refraction of Monochromatic Light by Lens | [87] |
| [12.] | Refraction of Dichromatic Light | [89] |
| [13.] | Narrow Spectrum as seen from a Distance | [97] |
| [14.] | Spectrum formed with V-shaped Slit | [103] |
| [15.] | Bezold’s Device for Demonstrating Non-achromatism of the Eye | [108] |
| [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.
The ether is a thing of immeasurable tenuity, being many millions of times rarer than the most perfect vacuum of which we have any experience: it offers no sensible obstruction to the movements of the celestial bodies, and even the flimsiest of material substances can pass through it as if it were nothing. Yet we have been taught that this same ether is an elastic solid with a great degree of rigidity, its resistance to distortion being, in comparison with the density, nearly ten thousand million times greater than that of steel: thus was explained the prodigious speed with which it propagates transverse vibrations.
A few years ago, a distinguished leader in science endeavoured in the course of a lecture to illustrate these apparently incompatible properties with the aid of a large slab of Burgundy pitch. He showed that the pitch was hard and brittle, yet, as he said, a bullet laid upon the slab would, in the course of a few months, sink into and penetrate through it, the hard brittle mass being really a very viscous fluid. The ether, it was suggested, resembled the pitch in having the rigidity of a solid and yet gradually yielding; it was, in fact, a rigid solid for luminiferous vibrations executed in about a hundred-billionth part of a second, and at the same time highly mobile to bodies like the earth going through it at the rate of twenty miles in a second.
This illustration, felicitous as it is, would, however, scarcely avail to force conviction upon an unwilling mind, even if it were admitted that the period of an ether wave is necessarily no more than a hundred-billionth of a second or thereabouts, which is probably very far from the truth.