Concluding, then, that the condition associated with the emission of light by a luminous body is a rapid vibratory motion of very small amplitude, Hooke next inquires how light travels through space. “The next thing we are to consider," he says, “is the way or manner of the trajection of this motion through the interpos'd pellucid body to the eye: And here it will be easily granted—
"First, that it must be a body susceptible and impartible of this motion that will deserve the name of a Transparent; and next, that the parts of such a body must be homogeneous, or of the same kind.
"Thirdly, that the constitution and motion of the parts must be such that the appulse of the luminous body may be communicated or propagated through it to the greatest imaginable distance in the least imaginable time, though I see no reason to affirm that it must be in an instant.
"Fourthly, that the motion is propagated every way through an Homogeneous medium by direct or straight lines extended every way like Rays from the centre of a Sphere.
"Fifthly, in an Homogeneous medium this motion is propagated every way with equal velocity, whence necessarily every pulse or vibration of the luminous body will generate a Sphere, which will continually increase, and grow bigger, just after the same manner (though indefinitely swifter) as the waves or rings on the surface of the water do swell into bigger and bigger circles about a point of it, where by the sinking of a Stone the motion was begun, whence it necessarily follows, that all the parts of these Spheres undulated through an Homogeneous medium cut the Rays at right angles."
Here we have a fairly definite mechanical conception. It resembles that of Descartes in postulating a medium as the vehicle of light; but according to the Cartesian hypothesis the disturbance is a statical pressure in this medium, while in Hooke's theory it is a rapid vibratory motion of small amplitude. In the above extract Hooke introduces, moreover, the idea of the wave-surface, or locus at any instant of a disturbance gene-
