1 84 HOOKE'S VIBRATORY THEORY. 



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, for I know not any one experiment 

 or observation that does prove it. And, whereas, it may be 

 objected, That we see the Sun risen at the very instant 

 when it is above the sensible Horizon, and that we see a 

 Star hidden by the body of the Moon at the same instant 

 when the Star, the Moon, and our Eye are all in the same 

 line ; and the like Observations, or rather suppositions, may 

 be urg'd. I have this to answer, That I can as easily deny 

 as they affirm ; for I would fain know by what means any 

 one can be assured any more of the Affirmative than I of 

 the Negative. If, indeed, the propagation were very slow, 

 'tis possible something might be discovered by Eclypses of 

 the Moon ; but though we should grant the progress of the 

 light from the Earth to the Moon, and from the Moon 

 back to the Earth again, to be full two minutes in performing, 

 I know not any possible means to discover it ; nay, there 

 may be some instances, perhaps, of Horizontal Eclypses that 

 may seem very much to favour this supposition of the slower 

 progression of Light than most imagine. And the like 

 may be said of the Eclypses of the Sun, &c. But of this 

 only, by the by. Fourthly, that the motion is propagated 

 every way through an Homogeneous medium by direct or 

 straigJit lines extended ev&ry 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 



