Albert Abraham Michelson

“ The first of a series of luminous vibrations emanating from a disturbed atom have a greater amplitude, just as the first vibrations from a musical instrument of percussion have the greater amplitude.
As spherical waves pass out from their source, they have work to do on a larger scale, therefore their amplitude decreases. Imagine that an atom of sodium in a flame suffers a collision which sets it in motion. It continues to vibrate, and like a piano string, with lessening amplitude; these vibrations are propagated outwards in all directions. As they pass outwards their amplitude decreases. ”

“ This requirement is that a sufficient number of waves shall produce a length which may be reproduced with such a degree of accuracy that the difference between the new standard and the one now serving as the standard cannot be detected by the microscope.
The process is, in principle, an ideally simple one, and consists in counting the number of waves in a given distance. However, in counting such an enormous number, of the order of several hundred thousands, one is liable to make a blunder—not an error in a scientific sense, but a blunder. ”

“ If, therefore, an apparatus is so constructed as to permit two pencils of light, which have traveled over paths at right angles to each other, to interfere, the pencil which has traveled in the direction of the earth's motion, will in reality travel of a wave-length farther than it would have done, were the earth at rest. ”

“ But since the displacement is proportional to the square of the velocity, the relative velocity of the earth and the ether is probably less than one sixth the earth's orbital velocity, and certainly less than one-fourth.
In what precedes, only the orbital motion of the earth is considered. If this is combined with the motion of the solar system, concerning which but little is known with certainty, the result would have to be modified; and it is just possible that the resultant velocity at the time of the observations was small though the chances are much against it. ”

“ In our work our standard is first placed in an ice-box, in front of which is a refractometer. There we measure the fraction and count the whole number. When we have had it in there as long as may be required, we transfer it to our comparer, which is already in an icebox; so there are no changes of temperature suffered by our standard between the time when we measure its length in wave lengths and the time when we stop off its length for comparison with, or verification of some other standards. ”

“ The reply was: "Yes, ether is the only form of matter about which we know anything at all." In fact, the moment we begin to inquire into the nature of the ultimate particles of ordinary matter, we are at once enveloped in a sea of conjecture and hypotheses—all of great difficulty and complexity.
One of the most promising of these hypotheses is the "ether vortex theory," which, if true, has the merit of introducing nothing new into the hypotheses already made, but only of specifying the particular form of motion required. ”

“ A motion of one plane by the hundred thousandth of an inch produces all the gradations from blackness to maximum light and blackness again. It is hardly possible to err in judgment by one tenth of this quantity, or the millionth of an inch. ”

“ The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote. Nevertheless, it has been found that there are apparent exceptions to most of these laws, and this is particularly true when the observations are pushed to a limit, i. e., whenever the circumstances of experiment are such that extreme cases can be examined. ”

“ Suppose that an ether strain corresponds to an electric charge, an ether displacement to the electric current, these ether vortices to the atoms—if we continue these suppositions, we arrive at what may be one of the grandest generalizations of modern science—of which we are tempted to say that it ought to be true even if it is not—namely, that all the phenomena of the physical universe are only different manifestations of the various modes of motions of one all-pervading substance—the ether. ”

“ It is, at any rate, not at all inconceivable that such a medium should at the same time possess both properties. We know that the air itself does not possess such properties, and that no matter which we know possesses them in sufficient degree to account for the propagation of light. Hence the conclusion that light vibrations are not propagated by ordinary matter, but by something else. ”

“ The visibility is not a quantity that can be measured, as we can a distance or an angle—unless, to be sure, we first define it. After defining it properly, we can produce, in accordance with that definition, interference fringes that shall have any desired visibility. By the use of fringes which have a known visibility we can educate the eye in estimating visibility, or we may have these standard fringes before us for comparison at the time of observation, and may then determine when the two systems are of the same clearness ”

“ We find that if we gradually increase the width of the source, the fringes grow less and less distinct, and finally disappear entirely. If we note the instant when the fringes disappear, we can calculate from the dimensions of the apparatus the width of the source. Or, if we alter the dimensions of the apparatus and observe when the fringes cease to be visible in our observing telescope, we have the means of measuring the diameter of the source, which may be a double star, or the disc of one of Jupiter's satellites, or one of the minor planets. ”

“ We know that light vibrations are transverse to the direction of propagation, while sound vibrations are in the direction of propagation. We know also that in the case of a solid body transverse vibrations can be readily transmitted. Thus, if we have a long cylindrical rod and we give one end of it a twist, the twist will travel along from one end to the other. If the medium, instead of being a solid rod, were a tube of liquid, and were twisted at one end, there would be no corresponding transmission of the twist to the other end, for a liquid cannot transmit a torsional strain. ”

“ Notwithstanding this, the medium which propagates light waves, and which was supposed to resist after the fashion of an elastic solid, must offer no appreciable resistance to such enormous velocities as those of the planets revolving in their orbits around the sun. The earth, for example, moves with a velocity of something like twenty miles in a second, has been moving at that rate for millions of years, and yet, as far as we know, there is no considerable increase in the length of the year, such as would result if it moved in a resisting medium. ”

“ It was naturally considered absurd to say that in the very center of a geometrical shadow there should not only be light, but that the brightness should be fully as great as though no obstacle were present. ”

“ If they were as large as the sun, the angle they would subtend would be about one-hundredth of a second. A forty-inch telescope can resolve angles of approximately one-tenth of a second, so that, if we were to attempt to measure, or to observe, a disc of only one-hundredth of a second, it would require an objective whose diameter is of the order of forty feet—which, of course, is out of the question. It is, however, not altogether out of the question to construct an interference apparatus such that the distance between its mirrors would be of this order of magnitude. ”

“ Also, if the medium is dense the propagation would be slower than if it were rare. It can easily be shown that if the elasticity were represented by E, and the density by D, the velocity would be represented by the square root of E divided by D. So that, if the density of the medium which propagates light waves were as great as the density of steel, the elasticity, since the velocity of light is some 60,000 times as great as that of the propagation of sound in a steel wire, must be 60,000 squared times as great as the elasticity of steel. ”

“ In the case of the blue radiations, however, when there is a magnetic field and only horizontal vibrations are allowed to pass through, the line is double. The doubling is very distinct, and the separation is so wide that it should be easily seen by means of the spectroscope. When the vertical vibrations alone are allowed to pass through, there is a very much more complicated effect. In all cases we can see that the line is double, as in the case of red cadmium light, but in this case each component of the double lines is at least quadruple, or even more complex. ”

“ What will be the effect on the interference phenomena if our source of light sends out two homogeneous trains of waves of slightly different periods? It is evident that each train will independently produce its own set of interference fringes. These two sets of fringes will coincide with each other when the difference in the lengths of the two optical paths in the interferometer is zero. When, however, this difference in path is increased, the two sets of fringes move across the field of view with different velocities, because they are due to waves of different periods. ”

“ In all probability, it not only exists where ordinary matter does not, but it also permeates all forms of matter. The motion of a medium such as water is found not to add its full value to the velocity of light moving through it, but only such a fraction of it as is perhaps accounted for on the hypothesis that the ether itself does not partake of this motion. ”

“ ANALYSIS OF THE ACTION OF MAGNETISM ON LIGHT WAVES BY THE INTERFEROMETER AND THE ECHELON
A little over a year ago the scientific world was startled by the announcement that Professor Zeeman had discovered a new effect of magnetism on light. The experiment that he tried may be briefly described in the following way: If we place a sodium flame in front of the slit of a spectroscope, we get in the field of view a bright double line. If the flame is placed between the poles of a powerful electro-magnet, it is found that the lines are very much broadened ”

“ To these more distant bodies our own little family of planets is probably invisible; even the sun itself is a second-rate star. If, however, Jupiter were sufficiently bright, then the sun and Jupiter together would form what is called a "double star," and to an inhabitant of a distant planet which might be traveling about this distant star it would appear as a double star with a separation of about one second, which may be expressed as the angle subtended by two luminous points about one-half inch apart when at a distance of three miles. ”

“ Strung on a piece of wire are a number of iron washers—rings of iron about an inch in diameter. When these are shaken they produce vibrations whose wave length is even shorter than that produced by the whistle just sounded. On shaking the rings you perceive the immediate response of the flame, and on shaking the rings behind the obstacle the flame responds still, but much more feebly. I take a new set of rings one-half inch in diameter. On shaking these the flame responds as before, but when I place the rings behind the obstacle the flame scarcely responds at all. ”

“ We are thus confronted with the apparent inconsistency of a solid body which must at the same time possess in such a marked degree the properties of a perfect fluid as to offer no appreciable resistance to the motion of bodies so very light and extended as the comets. ”

“ It is hard to realize that such observations concern bodies that are distant millions of miles from us; in fact, the distance is so great that it can be more readily expressed by the time light takes to reach us from these bodies. In some cases this may be as much as several years. We can compare this distance with the circumference of the earth, by considering that light or a telegram will go around the earth seven times in a second, while from these bodies it would take several hours for light to reach us. ”

“ COMPARISON OF THE MICROSCOPE AND TELESCOPE WITH THE INTERFEROMETER
One of the principal objections which have been urged against the wave theory of light is the fact that light appears to travel in straight lines, whereas sound, which is known to be a wave motion, does not cast a shadow; in other words, the sound waves are capable of bending around an obstacle in the path of the waves. ”

“ Reversing the water current should produce a shifting of one-half a fringe in the opposite direction, so that the total shifting would actually be of the order of one interference fringe. But we can easily observe one-tenth of a fringe, or in some cases even less than that. Now, one fringe would be the displacement if water is the medium which transmits the light waves. ”

“ A number of independent courses of reasoning lead to the conclusion that the medium which propagates light waves is not an ordinary form of matter. Little as we know about it, we may say that our ignorance of ordinary matter is still greater. ”