Curiosities of Light and Sight

CHAPTER V.. Curiosities of Vision 165

Curiosities of Vision 165

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.

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.

But, indeed, the elastic solid theory of the ether has failed to give a consistent explanation of some of the most important points in observational optics; and, in spite of the exalted position which it has held, it can now hardly be regarded as representing a physical reality. The famous researches of Hertz have established upon a secure experimental basis the hypothesis of Maxwell that light is an electro-magnetic phenomenon. Such electrical radiations as can be produced by suitable instruments are found to behave in exactly the same manner as those to which light is due. They travel through space with the same speed; they can be reflected, refracted, polarised, and made to exhibit interference effects. No fact in physics can be much more firmly established than that of the essential identity of light and electricity. It follows then that the displacements of the ether which constitute light-waves are not necessarily of the same gross mechanical nature as those which we see on the surface of water, or which occur in the air when sound is transmitted through it. The displacements which the ether undergoes are not mechanical--primarily at all events--but electrical. Every one knows what a simple mechanical displacement is. If we push aside the bob of a suspended pendulum, that is a mechanical displacement. But if we electrify a stick of sealing wax by rubbing it with flannel, the surrounding ether undergoes electric displacement, and no one understands what electric displacement really is. Ultimately, no doubt, it will turn out to be of a mechanical nature, but it is almost certainly not a simple bodily distortion such as is caused, for example, when one presses a jelly with the finger.

Since, then, it is no longer necessary to assume that the exceedingly rare and subtile ether is a jelly-like solid in order to account for the manner in which it transmits light, one of the most serious difficulties in the way of its acceptance is removed. It is true that nothing is definitely known concerning the mechanism which takes the place of the simple transverse vibrations formerly postulated, but every one will admit that it is far easier to believe in what we know nothing about than in what we know to be impossible.

All scientific men are in fact agreed in recognising the real and genuine existence throughout space of an ether capable, among other things, of transmitting at the speed of 186,000 miles per second disturbances which, whatever their precise nature, are of the kind which mathematicians are accustomed to call waves. How an ether wave is constituted will probably be known when we have found out exactly what electricity is: and that may be never.

The sensation of light results from the action of ether waves upon the organism of the eye, but the old belief that the sensation was primarily due to a series of mere mechanical impulses or beats, just as that of sound results from the mechanical impact of air-waves upon the drum of the ear, cannot any longer be upheld. The essential nature of the action exerted by ether waves is still undetermined, though many guesses at the truth have been hazarded. It may be electrical or it may be chemical; possibly it is both. Ether-waves, we know, are competent to bring about chemical changes, as in the familiar instance of the photographic processes; they can also produce electric phenomena, as, for example, when they fall upon a suitably prepared piece of selenium; but there is no evidence that they can exert any direct mechanical action of a vibratory character, and indeed it is barely conceivable that any portion of our organism should be adapted to take up vibrations of such enormous rapidity as those which characterise light-waves.

Of the multitude of ether-waves which traverse space it is only comparatively few that have the power of exciting the sensation of light. As regards limited range of sensibility there is a very close analogy between hearing and seeing. No sensation of sound (at least of continuous sound) is produced when air-waves beat upon our ears unless the rate of the successive impulses lies within certain definite limits. It is just so with vision. If ether-waves fall upon our eyes at a less rate than about 400 billions per second, or at a greater rate than 750 billions per second, no sensation of light is perceived. There is another and more generally convenient way of stating this fact. Since all waves found in the ether travel through space at exactly the same speed--186,000 miles a second--it follows that the length of each of a series of homogeneous waves must be inversely proportional to their frequency, that is, to the rate at which they strike a fixed object, such as the eye. Instead, therefore, of specifying waves by their frequency we may equally well specify them by their length. Waves whose frequency is 400 billions per second have a length of about 1/34000 inch, this being the one four hundred billionth part of 186,000 miles; and those whose frequency is 750 billions have a wave-length of 1/64000 inch. Waves, then, of a length greater than 1/34000 inch or less than 1/64000 inch have no effect upon our organs of vision.

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