LIGHT 58,1 light, each star appears to describe in space a circle (not an ellipse) of fixed magnitude in a plane parallel to that of the ecliptic. As seen from the earth, therefore, stars will appear to describe paths which are the projections of these circles on the celestial sphere. These are in general ellipses, but circles for stars at the poles of the ecliptic and straight lines for stars in the ecliptic. This is found to be quite consistent with observation ; and the major axes of these ellipses, the diameters of the circles, or the lengths of the lines subtend equally angles of about 41" at the earth. Hence the velocity of light is to the velocity of the earth as 1 : tan 41"; that is, about 10,000 : 1. Loth these methods depend, for their final result, upon n, true knowledge of the earth s distance from the sun. But the most accurate measurements of this quantity are probably to be obtained from the velocity of light itself, this being independently determined by the physical pro cesses next to be described. Thus the earth s distance from the sun will in future be measured rather by the constant of aberration, or by the acceleration or retarda tion of the eclipses of Jupiter s satellites, than by a transit of Venus, by the moon s motion, or by the parallax of Mars, Thus Homer s and Bradley s processes are now applied to the determination of solar parallax. Fizeau s 3. Fizeau s Direct Measurement of the Velocity of Light. method. x o illustrate the next and by far the most convincing popular proof of the finite velocity of light, suppose a perse n looking at himself in a mirror, before which is moving a screen with a number of apertures, the breadth of each aperture being equal to the distance between any two of them. If the screen be at rest with an aperture before the mirror, the light from the observer s face passes through the aperture and is reflected back, so that he sees himself as if the screen were not present. Suppose the screen to be moving in such a way that, when the light which passed through the aperture returns to the screen after reflexion, ths unpierced part of the screen is in its way, it is evident that the observer cannot see himself in the mirror. If the screen pass twice as fast, the light that escaped by one aperture will, after reflexion, return by the next, so that he will see his image as at first. If three times as fast, the second unperforafeed part of the screen will stop the returning light; so he cannot see his image. To apply this practically a thin metallic disk had a set of teeth cut on its circumference so that the breadth of a, tooth was equal to that of the space between two teeth. This disk could be set in very rapid rotation by a train of wheelwork, and the rate of turning could easily be determined by Savart s method (ses ACOUSTICS, vol. i. p. 108). Light passed between two teeth to a mirror situated at 10 miles distance, which sent it back by the same course, so that when the wheel was at rest the reflected light could be seen. On turning the disk with accelerated velocity the light was observed to become more and more feeble up to a certain velocity, at which it was extinguished ; turning faster it reappeared, growing brighter and brighter till the velocity was doubled ; then it fell off, till it vanished when the velocity was trebled, and so on. It is evident from the first illustration above that the velocity of light in air is to that of the tooth, at the first disappearance of the reflected light, as the distance of the mirror from the disk is to the half breadth of the tooth. It is not to be sup posed tliat the description we have just given embodies all the details of this remarkable experiment. On the con trary, telescopes were used at each station to prevent loss of light as much as possible, and many other precautions were adopted which would be unintelligible without refer ences to Inter parts of this article. This method and its first results were published in 18i9 in the Comptc-s Rendus. The experiments gave, on their very careful repetition by Cornu in 1874, the value 186,700 miles fur the velocity in vacuo (Nature, xi. p. 274). 4. Foucault s Method. This was described in 1850toFou- the Academy of Sciences. It depends upon the principle cault s of the rapidly revolving plane mirror introduced by 111 Wheatstone to demonstrate the non-instantaneous propa gation of an electric discharge. The mirror was made to revolve from 600 to 800 times per second, by means of a siren (see ACOUSTICS) driven by steam. A ray of sunlight fell upon it from a small aperture crossed by a grating of platinum wires. Between the wires and the mirror was placed an achromatic lens the wires being farther from it than its principal focus, but not twice as far so that the rays falling on the mirror were slowly convergent. They formed an image of the wires at a distance of about 4 metres from the mirror. In certain positions of the revolving mirror, the rays fell upon a concave mirror of 4 metres radius whose centre of curvature was at the centre of the revolving mirror. They were, therefore, reflected back directly to the revolving mirror, and, passing again through the lens, formed an image of the wire grating which, when the adjustment was perfect, coincided with the grating itself. This coincidence was observed by reflexion from a piece of unsilvered glass, placed obliquely in the track of the rays, the image in which was magnified by an eye-piece. It is obvious that, when the mirror is made to turn, the light which comes back to it after passing to the fixed mirror, finds it in a position slightly different from that in which it left it. That difference is due to the amount of rotation during the time of passage of the light to and fro along an air-space of 4 metres. Accordingly, as soon as the mirror began to rotate with considerable velocity, the coincidence between the wires and their images was destroyed; and the two were separated more and more widely as the velocity of rotation was increased. It was easy to calculate, from the measured dimensions of the apparatus, the amount of deflexion, and the rate of rotation of the mirror, the velocity of light. The rate of rotation was, of course, given by the pitch of the note produced by the siren. Foucault s early results with this apparatus showed that the velocity of light which had been deduced from the old methods was too large ; and he concludes his first paper by the statement that the determination of the distance of the earth from the sun must now be made by physical instead of astronomical methods. Foucault s process has recently been very considerably improved by Mitchelson, who, in 1879, found for the velocity of light in vacuo 186,380 miles per second (Xature, vol. xxi. p. 226). By interposing a tube filled with water, and having flat Proof glass ends, between the fixed and revolving mirrors, t] ia t Foucault found that (for the same rate of rotation) the g displacement of the image was greater than before in the f aster proportion of the refractive index of water to unity. Thus j n air it was at once evident, by a mode of experimenting exposed Man in to no possible doubt, that light moves faster in air than water - in water, and, therefore, as will be seen later, that the corpuscular theory of light must be abandoned. Other methods of determining the velocity of light in air, and for comparing the velocities of light in air and water (on which depends the most definite proof of Uie erroneousness of the corpuscular theory), and in still and moving water, will be afterwards explained. They give results of very great value, but we cannot introduce then here, as they depend upon somewhat more recondite principles of jihysical optics. It is interesting to observe that, as the nearest fixed star is probably about 200,000 times farther from us than the sun is, we now see such a star by light which left it more than three years ago. If, as is now supposed, -variable stars XIV. - 74
