Speed of light

“ While sound travels as vibrations of some material medium, light travels only as vibrations of the ether; solids, liquids, and gases act so as to hinder rather than to assist in its movement. That is, light travels best in a vacuum or in a space devoid of ordinary matter. (b) The speed of light is so great that at ordinary distances on the earth its motion is practically instantaneous. Experiments have shown that its speed is about 186,000 miles to 300,000 kilometers a second. ”

“ No ordinary matter is competent to transmit waves at anything like the speed of light: the rate at which matter conveys waves is the velocity of sound,—a speed comparable to one-millionth of the speed of light. Hence the luminiferous medium must be a special kind of substance; and it is called the ether. ”

“ We see in light a principle which, if it has not its source in the sun, is certainly dependent upon that luminary for its manifestations and powers. From that “fountain of light” we find this principle travelling to us at a speed which almost approaches the quickness of thought itself; yet by the refinements of science we have been enabled to measure its velocity with the utmost accuracy. ”

“ These are situated at different distances, some being so far that light must Velocity and wave-length. be several centuries in reaching us from them. Were there any difference in the speed of light of various colours it would be shown by a change in the colour of the star as its light waxed and waned. The light of greatest speed preceding that of lesser speed would, when emanated during the rising phase, impress its own colour on that which it overtook. The slower light would predominate during the falling phase. ”

“ If the speed with which the bodies increase their distance be sufficiently large, the green may transform into a yellow, the yellow into an orange, the orange into a red; while a still greater velocity is, at all events, conceivable which would cause the undulations to be received with such slowness that the nature of the light could no longer be interpreted by any nerves that the eye contains, and from the mere fact of its rapid motion away from us the star would become invisible. ”

“ Now, if we should soar up from this hall and fly with the velocity of light, we would see continually the very scene that is now before our eyes. But if we should suddenly be transferred to a point distant a little over 3,225,000,000 miles from the earth, and should then speed on with the velocity of light, the earth would appear to us as it looked twenty-four hours ago. If we should then suddenly advance ten, a hundred, or a thousand times as far, we should see what had happened on earth ten, a hundred, or a thousand days ago, and so on. ”

“ Nature may reach the same result in many ways. Like a wave in the physical world, in the infinite ocean of the medium which pervades all, so in the world of organism:, in life, an impulse started proceeds onward, at times, may be, with the speed of light, at times, again, so slowly that for ages and ages it seems to stay; passing through processes of a complexity inconceivable to men, but in ;ill its forms, in all its stages, its energy. ever and ever integrally present. ”

“ But light waves are always at right angle, different from sound waves, and we must come to the conclusion that light does not travel in a straight line, but in a series of curves. While its theoretical speed is 186,000 miles per second, its actual speed is many times that. ”

“ The experimental measures thus far cited have been primarily those of the velocity of light in air, the reduction to a vacuum being derived from theory alone. The fundamental constant at the basis of the whole theory is the speed of light in a vacuum, such as the celestial spaces. ”

“ Contrary to all expectation, and to the profound astonishment of physicists, it was found that light travels at precisely the same speed whether the man who receives it is receding before it with the velocity of the earth or is approaching it at the same velocity. It is an undeniable consequence of this that the ether shares the motion of the earth. ”

“ At higher speeds of rotation some light would reappear, and at lower speeds it would also reappear; by noticing, therefore, the precise speed at which there was constant eclipse the velocity of light could be determined. ”

“ Now, the velocity of wave propagation can be seen, without the aid of any mathematical analysis, to depend on the elasticity of the medium and its density; for we can see that if a medium is highly elastic the disturbance would be propagated at a great speed. ”

“ Far away from this spinning and perilous globe of ours, at the distance of some 95,000,000 miles, stands the sun. A ray of light, starting from his surface at any given moment, takes eight minutes to reach us, although light runs at the speed of 195,000 miles in a second. ”

“ Only in the case of bodies approaching the velocity of light in our frame are the two progressions, that in space and that in time, in any way comparable. Thus, consider our own earth. It moves in relation to the sun with a speed of some eighteen miles a second, which is equivalent to approximately one ten-thousandth part of a unit of space per unit of time. ”

“ Light travels through space at the rate of about 186,000 miles per second. White light is a combination of light of many colors, but the speed of transmission is the same for all colors. The length of waves and the rate of vibrations, however, vary. ”

“ At a very modest reckoning the motion of the earth in the interval might alter the light-time by several days. Actually, however, any speed of the earth through the aether up to the velocity of light is admissible, without affecting anything observable. ”

“ It is precisely because its speed through water is reduced that the velocity of the water will affect the velocity of the light, thereby permitting us to test out the relativistic law of the composition of velocities. ”

“ Now it is found that when the speed approaches that of light, 186,000 miles or 3 x 1010 centimetres per second, the energy is higher than it should be if it followed the usual rule, viz. energy is equal to half the mass multiplied by the square of the velocity. ”

“ We must remember that it is only light in vacuo which is propagated with the speed , hence which possesses an invariant velocity. Light passing through a transparent medium moves with a velocity which is less than ; hence in this case its velocity ceases to be invariant and is affected by the velocity of the medium, as also by that of the observer. ”

“ Light gives precisely the same kinds of information, and hence it is only natural to infer that light also is a wave motion. We know, in fact, that it is so; but before giving the evidence to prove it, it will be well to make a little preliminary study of the chief characteristics of wave motion.
FIG. 1 One of the difficulties encountered in studying wave motion is the rapidity of the propagation of the waves. A fairly moderate speed is attained by the waves propagated along a spiral spring. ”

“ It has not been possible to measure their speed with absolute accuracy, but their rapidity is such that their transit appears to be instantaneous. The latest researches seem to have proved that their speed is superior to that of light, which is 300,000 kilometres a second. ”

“ This cause is the reflection suffered by the light in this experiment. In fact, it is possible to show that when there is a difference in the velocity of two rays, this difference is changed when the rays are reflected by a mirror in motion. A separate calculation of the effects in Babinet's experiment shows that they have fairly equal values, but with opposite signs.
This explanation renders even more probable the hypothesis of the change of speed, and it has seemed to me that an experiment made in moving water should help to decide the question with certainty. ”

“ In two or three cases the speed exceeds one hundred miles per second, but these are quite exceptional. By taking several thousand stars having a given proper motion, we may form a general idea of their average distance, though a great number of them will exceed this average to a considerable extent. The conclusion drawn in this way would be that the stars having an apparent proper motion of 10" per century or more are mostly contained within, or lie not far outside of a sphere whose surface is at a distance from us of 200 light-years. ”

“ What is it that these rays of light striking our retina convey to our brain, and from our brain to whatever is ourselves, in the seat of consciousness above this? What is this mysterious correspondence set up between something within and something without, that at times sends such a clamour of harmony through our whole being? Why do certain combinations of sound in music and of form and colour in art affect us so profoundly? What are the laws governing harmony in the universe, and whence do they come? It is hardly trees and sky, earth, or flesh and blood, as such, that interest the artist ”

“ Don't burden a line drawing with heavy half tones and shadows; keep them light. The beauty that is the particular province of line drawing is the beauty of contours, and this is marred by heavy light and shade. Great draughtsmen use only just enough to express the form, but never to attempt the expression of tone. ”

“ And in art we have the same play of opposing factors, straight lines and curves, light and dark, warm and cold colour oppose each other. Were the balance between them perfect, the result would be dull and dead. But if the balance is very much out, the eye is disturbed and the effect too disquieting. It will naturally be in pictures that aim at repose that this balance will be most perfect. In more exciting subjects less will be necessary, but some amount should exist in every picture, no matter how turbulent its motive ”

“ But now that the whole field of vision has been discovered, and that the subtlest effects of light and atmosphere are capable of being represented, it has become necessary to decide how far complete accuracy of representation will help the particular impression you may intend your picture or drawing to create. The danger is that in producing a complete illusion of representation, the particular vitality of your medium, with all the expressive power it is capable of yielding, may be lost. ”

“ The intense individualism of the English sought out every detail, every leaf and flower for itself, painting them with a passion and intensity that made their painting a vivid medium for the expression of poetic ideas; while the more synthetic mind of the Frenchman approached this search for visual truth from the opposite point of view of the whole effect, finding in the large, generalised impression a new world of beauty. And his more logical mind led him to inquire into the nature of light, and so to invent a technique founded on scientific principles. ”

“ At every point, through every point, along every line, athwart every line, myriads of light-waves are at all times rushing with the inconceivable velocity just mentioned. It is from such waves that we have learned all we know about the universe outside our own earth. ”

“ It might be very difficult to test theories in which the velocity of light is assumed to change on passing through narrow slits or near large masses in motion, or to suffer permanent change in velocity on passing through a lense. ”