Elements of natural philosophy (Volume 2-3) . eousmedia; Geometric*!illustration. Wave front notspherical inheterogeneousmedia. Fig. 1. § 5. Waves of light, like those of sound, proceed fromany disturbed molecule as a centre, with a constant velocityin all directions, through media of homogeneous front of the luminous wave in such media is, there-fore, always on the surface of a sphere whose centre isat the place of primitive disturbance, and whose radiusis equal to the velocity of propagation multiplied intothe time since the wave began. Thus, if a moleculeof ether be disturbed at
Elements of natural philosophy (Volume 2-3) . eousmedia; Geometric*!illustration. Wave front notspherical inheterogeneousmedia. Fig. 1. § 5. Waves of light, like those of sound, proceed fromany disturbed molecule as a centre, with a constant velocityin all directions, through media of homogeneous front of the luminous wave in such media is, there-fore, always on the surface of a sphere whose centre isat the place of primitive disturbance, and whose radiusis equal to the velocity of propagation multiplied intothe time since the wave began. Thus, if a moleculeof ether be disturbed at (7, and thevelocity of propagation be denotedby V, and the time elapsed sincethe disturbance by £, then will thefront of the wave at the expirationof this time be upon the surface ofa sphere whose centre is at C andradius C A = V. t. If the medium through-which thewave moves be not homogeneous, theshape of the wave front will not be spherical, but willvary from that figure in proportion as the medium de-parts from perfect § 6. The circumstances attending the propagation ofluminous and sonorous waves are similar. The intensity ELEMENTS OF OPTICS. 169
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