12 The Theory of the Aether 



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 commu- 

 nicated 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 Eays from the centre of a Sphere. 



" Fifthly, in an Homogeneous medium this motion is propa- 

 gated 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 Kays 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-swrface, or locus at any instant of a disturbance gene- 



