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Our next task, therefore, will be, where possible, to fill these concepts with new meaning, or else to replace them by other concepts read from the actual phenomena. Once this is done the way will be free for the development of the picture of the spectrum phenomenon which is in true accord with the Goethean conception of Light and Colour.
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Now, it is a common feature of all these experiments that by necessity they are based on an arrangement whereby a light-beam can be made to appear and disappear alternately. In this respect there is no difference between the first primitive attempts made by Galileo and the Academicians, and the ingeniously devised experiments of the later observers, whether they operate with a toothed wheel or a rotating mirror. It is always a flash of light – and how could it be otherwise? – which is produced at certain regular intervals and used for determining the speed of propagation.
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Let us now see what we are really told by the number 186,000 miles a second, as the measure of the speed with which a light-impulse establishes itself spatially. In the preceding chapter we learnt that the earth’s field of gravity offers a definite resistance to our visual ray. What is true for the inner light holds good equally for the outer light. Using an image from another dynamic stratum of nature we can say that light, while appearing within the field of gravity, ‘rubs’ itself on this. On the magnitude of this friction depends the velocity with which a light-impulse establishes itself in the medium of the resisting gravity. Whereas light itself as a manifestation of levity possesses infinite velocity, this is forced down to the known finite measure by the resistance of the earth’s field of gravity. Thus the speed of light which has been measured by observers such as Fizeau and Foucault reveals itself as a function of the gravitational constant of the earth, and hence has validity for this sphere only.1 The same is true for Roemer’s and Bradley’s observations, none of which, after what we have stated earlier, contradicts this result. On the contrary, seen from this viewpoint, Roemer’s discovery of the light’s travelling with finite speed within the cosmic realm marked by the earth’s orbit provides an important insight into the dynamic conditions of this realm.
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If one sets in the path of a luminous cone a glass-walled trough filled with water, then, if both water and surrounding air are slightly clouded, the cone is seen to make a more acute angle within the water than outside it (Fig. 13). Here in an external phenomenon we meet the same weakening in the light’s tendency to expand that we recognized in the shortening of our experience of depth on looking through a dense medium. Obviously, we expect the externally observable narrowing of the light-cone and the subjectively experienced change of optical depth to show the same ratio.
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The first to be brought in this sense under our examination is the concept of the ‘light-ray’.
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Evidently what in all these cases is measured is the speed with which a beam of light establishes itself in space. Of what happens within the beam, once it is established, these observations tell nothing at all. The proof they are held to give of the existence of a finite speed of light, as such, is a ‘proof of a foregone conclusion’. All they tell us is that the beam’s front, at the moment when this beam is first established, travels through space with a finite velocity and that the rate of this movement is such and such. And they tell us nothing at all about other regions of the cosmos.
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Among the experiments undertaken with the aim of establishing the properties of the propagation of light by direct measurements, quoted earlier, we mentioned the Michelson-Morley experiment as having a special bearing on Einstein’s conceptual edifice. It is the one which has formed the foundation of that (earlier) part of Einstein’s theory which he himself called the Special Theory of Relativity. Let us see what becomes of this foundation – and with it the conceptual edifice erected upon it – when we examine it against the background of what we have found to be the true nature of the so-called velocity of light.
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In order to compare the rate of expansion of a luminous cone inside and outside water, we must measure by how much less the width of the cone increases within the water than it does outside. (To be comparable, the measurements must be based upon the same distances on the edge of the cone, because this is the length of the way the light actually travels.) In Fig. 13 this is shown by the two distances, a-b and a’-b’. Their ratio is the same as that by which the bottom of a vessel appears to be raised when the vessel is filled with water (4:3).
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In present-day optics this concept signifies a geometrical line of infinitely small width drawn, as it were, by the light in space, while the cone or cylinder of light actually filling the space is described as being composed of innumerable such rays. In the same way the object producing or reflecting light is thought of as composed of innumerable single points from which the light-rays emerge. All descriptions of optical processes are based upon this conception.
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That we have to do in these observations with the speed of the light-front only, and not of the light itself, is a fact fully acknowledged by modern physical optics. Since Lord Rayleigh first discussed this matter in the eighties of the last century, physicists have learnt to distinguish between the ‘wave-velocity’ of the light itself and the velocity of an ‘impressed peculiarity’, the so-called ‘group-velocity’, and it has been acknowledged that only the latter has been, and can be, directly measured. There is no possibility of inferring from it the value of the ‘wave-velocity’ unless one has a complete knowledge of the properties of the medium through which the ‘groups’ travel. Nevertheless, the modern mind allows itself to be convinced that light possesses a finite velocity and that this has been established by actual measurement. We feel reminded here of Eddington’s comment on Newton’s famous observations: ‘Such is the glamour of a historical experiment.’ (Chapter XIV.)3