as it revolves, that the beam of light reflected from it meets the concave mirror M; consequently the image at a will disappear and reappear once in each revolution; but as the speed is increased to more than about 10 revolutions per second the eye no longer detects the intermittence, but sees a continuous image of the slit. As the speed of rotation increases, it is noticed that the image of the slit is no longer at a, but is displaced toward b, the amount of the displacement being proportional to the speed of rotation of the mirror. This displacement is due

FIG. 479.-Velocity of light measured by rotating mirror.

to the fact that while light is traveling from m to M and back again, the mirror has turned forward through a small angle, and consequently the returning light is reflected slightly upward, as shown in the above figure, and not back along its original path, causing the image of the slit to be displaced to b. The displacement ab may be measured by a micrometer, and from it the angle through which the returning beam is turned upward may be determined. But this angle will be twice the angle through which the mirror turns while light is traveling from m to M and back ($817). All that remains therefore is to determine the distance mM and the speed of revolution of the mirror; the velocity of light may then be easily calculated.

In Foucault's experiment the distance mM was only 4.12 meters, and the greatest displacement ab was about 0.3 mm. when the rotating mirror was making 800 turns per second.

Consequently his result was not very accurate. But he tried

the very important experiment of introducing a long tube of water between m and M through which the light was sent, and was able to show that the velocity of light in water was less than in air, a result of the greatest significance in determining the nature of light ($839).

806. Michelson's Modification.-In 1879 Michelson, then at the United States Naval Academy, modified Foucault's method by substituting for the concave mirror M a lens through which the light passed to a very distant mirror where it was reflected back. In his first experiments the distance from the revolving mirror to the fixed mirror was 605 meters. This great increase in the distance between the mirrors caused a correspondingly greater displacement which could be measured with far less. percentage of error. His experiments in 1879-82 and those conducted according to his method by Newcomb in 1882 are the most accurate determinations of this important constant that have been made.

In some of Michelson's experiments the displacement to be measured by the micrometer was 13.3 cm., or 400 times that obtained by Foucault.

The results obtained by this improved method are as follows:

807. Velocity Same for All Colors.-In these various determinations of the velocity of light, the light employed was either sunlight, starlight, or light from the electric arc. But although these lights are complex, there was not found in any case a perceptible difference in velocity between light of different colors, when measured in air or in interplanetary space.

WAVE THEORY.

808. Mode of Propagation.-Only three methods are known by which energy may be transmitted from one point of space to

another. First, by the movement as a whole of some medium reaching from one point to the other, as in case of ropes, belts, or shafting.

Second, by projectiles, as in case of a shot from a gun or a ball thrown.

Third, by waves, as in case of sound or water waves.

We have found that light is communicated from one point to another with a velocity of about 300,000,000 meters or 186,400 miles per second. By which of the above processes is it propagated? We may evidently reject the first as inconceivable. The second was advocated by so great a philospher as Sir Isaac Newton, while Huygens, the celebrated Dutch physicist, urged the claims of the third. While much of the most convincing evidence will be found in phenomena that must be taken up later in our study, there are some considerations which even at this point may help us to reach a tentative conclusion.

The velocity with which a projectile travels depends on the initial impulse. If light is communicated by means of particles shot out from the luminous body we should expect to find the velocity depending on the source and that particles emanating from the sun would have a different velocity from those from an electric light.

On the other hand, the velocity of a wave depends only on its wave length and the nature of the wave (whether compressional or transverse, etc.) and the properties of the medium of which it is a disturbance. Sound waves from fiddle, pipe, or drum advance with the same speed through air. If light is a wave motion we may expect to find light waves, whatever their source, traveling with the same velocity through space, and this is precisely what experiment shows to be the case. This consideration therefore points to its being a wave motion.

809. The Ether. On the other hand, if light is propagated by waves, they are waves of what? Light passes through interstellar space and through the most perfect artificial vacua that can be produced. If there are light waves, they must be in some medium which extends throughout space as far as the most distant star from which we receive light, it must fill all vacua and permeate all bodies through which light can pass. And yet no resistance to the motion of earth or planets through

this medium has ever been detected. Yet in spite of these objections such a medium must be supposed to exist if light is communicated by waves, and it has been named the luminiferous ether or simply the ether.

810. Other Evidence for the Ether.-It is remarkable that there is independent evidence for the existence of such a medium. obtained from the study of electricity and magnetism. Electric

and magnetic forces act through vacua, and may be produced as Faraday supposed by tensions and pressures in a surrounding medium. When a magnet draws a piece of iron to itself we may in imagination see it pushed up to the magnet by the stresses in the ether.

But far more important is the direct evidence of Hertz' experiments; for electric waves have been proved to exist and are found to have the same velocity as light. There is, therefore, a medium in which waves may exist and travel with the velocity of light.

811. Electromagnetic Theory.-And since the velocity of a wave depends both on the properties of the medium and on the kind of wave motion, it is highly probable that the vibrations in light waves are exactly the same as in electric waves; or, in other words, that light waves are electric waves.

This theory of the nature of light waves is known as the electromagnetic theory of light, it was proposed and developed by Maxwell in 1865. Its conception and establishment, next to that of the conservation of energy, is the most remarkable achievement of physical science in the nineteenth century.

In our further study we shall endeavor to test the probability of the hypothesis that light is a wave motion by inquiring whether it affords simple and natural explanations of the various phenomena as they arise.

812. Form of Light Waves.—If light comes from a source as a series of waves, the form of a wave, as it advances in all directions with equal velocity must be spherical, and the direction of advance being radial is at right angles to the wave front.

813. Beams and Rays.-When light shines through a small opening, the stream of light is called a beam, and a very narrow beam is called a ray. When the beam comes from a very distant source, the rays of which it may be conceived as made up are

parallel, and it is called a parallel beam; in that case the wave

fronts are planes.

When light comes from a point, the rays diverge radially from the source and the wave fronts are spherical segments having the source as their center. Such a beam is divergent, and its waves enlarge as they advance.

FIG. 480.-Parallel beam with

plane waves.

By means of a lens or curved mirror, a beam of light may be made to converge toward a point which is called the focus, in which case the wave fronts must be concave spherical surfaces which contract as they approach the focus.

814. Geometrical and Physical Optics.-The study of light is also called optics. That method of treating the subject which

FIG. 481.-Divergent beam with convex expanding waves.

FIG. 482.-Convergent beam with concave contracting waves.

ignores the existence of waves and treats a beam of light as a bundle of rays is called geometrical optics, while the other method which investigates the dependence of the various phenomena of light on the properties of waves is known as physical optics.

REFLECTION OF LIGHT AND MIRRORS.

815. Reflection: Regular and Diffuse-When light reaches a surface where there is a change of medium, some is reflected or turned back into the first medium while some penetrates into the second medium.

When the reflection takes place at a flat polished surface, light comes to the eye as though directly from the distant objects themselves, and if the polish is perfect none of the light seems to come from the reflecting surface, but we seem to be looking through an opening at objects beyond. This is known as regular reflection.