Page:The American Cyclopædia (1879) Volume X.djvu/443

LIGHT 437 weighed ^^5- of a grain, it would have a mo- mentum equal to that of an ounce ball moving with a velocity of 1,000 ft. a second; and if it be assumed that the light molecules are many millions of times lighter than this, it may also be assumed that as many millions of molecules may be made to act together. It may be an- swered, on the other hand, that if the luminif- erous ether is composed of particles so small as to be able to easily pass between the particles of ponderable matter, as some suppose they do, they have not the power to impart momentum. But this position fails when we consider the immensely greater velocity of light than that of any ponderable bodies that are supposed to be traversed by the ethereal particles in conse- quence of the motion of such ponderable bod- ies. It must be admitted, however, that the experiments of Mr. Bennett, although they add difficulty to the emission theory, do not amount to a demonstration; that is, after all, more nearly reached by the perfect competency of the undulatory theory to explain all the various phenomena of radiation. But the velocity and momentum of the luminous particles are held to be no greater objection to the emission theory than that offered by the fact that the velocity of light in a homogeneous medium is uniform ; which, it is maintained, could not be the case if the luminous particles were emitted by the force of the self-luminous body, which must be admitted to be a variable force. Velocity of Light. The velocity of light is so great that no sensible space of time is occupied in its passage between any two points on the surface of the earth. The first determinations of this velocity were made from observations on the heavenly bodies. Roemer, a Danish astrono- mer, in 1675 made the first estimation by means of observations on the eclipses of Jupiter's first satellite. This body passes into the planet's shadow at equal intervals of 42 h. 28 m. 36 s. Let w, fig. 2, represent the satellite, e and e' the earth, and 8 the sun. When the earth is travelling in that part of its orbit between a and Z>, the distance between it and the satellite varies but little, and therefore the intervals between successive occupations do not sensi- FIG. 2. Eoemer's Observations. bly change ; but when the earth has reached the opposite part of its orbit, as at e', a retar- dation has taken place in all the occultations amounting to about 16 m. 40 s., the precise time varying as the points e and e' vary with re- spect to the elliptical orbit. Calculations from data furnished in this way show the velocity of light to be about 190,000 m. per second. Another method which has been employed is that of aberration, or an apparent change in the position of the fixed stars, caused con- jointly by the velocity of light coming from them and that of the earth in its orbit. (See ABERRATION.) The velocity of light has also been determined by observations upon distan- ces between places on the surface of the earth, machinery being used to mark the otherwise insensible periods of time. In 1849 M. Fizeau measured the time it took for light to travel from Suresnes to Montmartre and back again. A toothed wheel, having the teeth and the in- tervals between them of equal width, is made to revolve rapidly in a plane at right angles to a beam of light, with such a velocity that the beam, having passed through an interval, and having been reflected back by a mirror placed at the other station, will be intercepted by a tooth which has in the mean time taken the place of the interval. The main apparatus was placed at Suresnes and the mirror at Mont- martre, a distance of 28,516 ft. With a cer- tain velocity of the wheel the beam of light would be intercepted and no reflection ob- served; with twice the velocity it would re- appear; and with three times the velocity it would again be obscured. The velocity of light deduced from recent observations with this ap- paratus is 185,000 m. a second, which accords pretty closely with results of astronomical ob- servations. Another method only requires a distance of less than 12 ft., and employs the principle of the revolving mirror first used by Wheatstone in his experiments on the dura- tion of the electric spark. (See ELECTRICITY.) This method was proposed by Arago, and car- ried out independently by Foucault, Fizeau, and Breguet. An important feature in the experiment is the passing of a beam of light through a long tube containing a liquid, in which its velocity is found to be retarded ; a fact which is strongly confirmatory of the truth of the undulatory theory, one of whose consequences is that light has a greater ve- locity in air and gases than in liquids, while the emission theory leads to an opposite con- clusion. Intensity. The intensity of light may be estimated by an amount which is re- ceived on a unit of surface, and depends upon the degree of luminosity, upon the distance from the source, and upon the obliquity to the rays of the surface illuminated. The intensity of light emanating from a point is inversely proportional to the square of the distance. This agrees with a similar law in regard to heat, and is proved by considering the internal surfaces of two spheres to be illuminated from points at the centre. If the sources of light are equal to the surfaces, they will each receive the same amount of light ; but if one has a ra- dius twice as great as the other, it will have four times the area, and therefore an equal area would receive only one fourth as much