634 KOVUM onoANUM. [BOOK IL 



times heard at the distance of thirty miles), the sound of those 

 nearest to the spot is heard before that ot the most distant. 

 Even in sight (whose action is most rapid), it is clear that a 

 definite time is necessary for its exertion, which is proved by 

 certain objects being invisible from the velocity of their motion, 

 such as a musket-ball ; for the flight of the ball is too swift to 

 allow an impression of its figure to be conveyed to the sight. 



This last instance, and others of a like nature, have sometimes 

 excited in us a most marvellous doubt, no less than whether the 

 image of the sky and stars is perceived as at the actual moment 

 of its existence, or rather a little after, and whether there is not 

 (with regard to the visible appearance of the heavenly bodies) a 

 true and apparent time, as well as a true and apparent place, 

 which is observed by astronomers in parallaxes. It appeared so 

 incredible to us, that the images or radiations of heavenly bodies 

 could suddenly be conveyed through such immense spaces to the 

 sight, and it seemed that they ought rather to be transmitted in 

 a definite time. 1 That doubt, however (as far as regards any 

 great difference between the true and apparent time), was sub 

 sequently completely set at rest, when we considered the infinite 

 loss and diminution of size as regards the real and apparent 

 magnitude of a star, occasioned by its distance, and at the same 

 time observed at how great a distance (at least sixty miles) bodies 

 which are merely white can be suddenly seen by us. For there 

 is no doubt, that the light of the heavenly bodies not only far 

 surpasses the vivid appearance of white, but even the light ot 

 any flame (with which we are acquainted) in the vigour of its 

 radiation. The immense velocity of the bodies themselves, which 

 is perceived in their diurnal motion, and has so astonished 

 thinking men, that they have been more ready to believe in the 

 motion of the earth, renders the motion of radiation from them 

 (marvellous as it is in its rapidity) more worthy of belief. That 

 which has weighed most with us, however, is, that if there were 

 any considerable interval of time between the reality and the 

 appearance, the images would often be interrupted and confused 

 by clouds formed in the mean time, and similar disturbances of 

 the medium. Let this suffice with regard to the simple measures 

 of time. 



It is not merely the absolute, but still more the relative 

 measure of motions and actions which must be inquired into, 

 for this latter is of great use and application. We perceive that 



Homer, a Danish astronomer, was the first to demonstrate, by con 

 necting the irregularities of the eclipses of Jupiter s satellites with thei? 

 distances from the earth, the necessity of time for the propagation oi 

 light. The idea occurred to Dominic Cassini as well as Bacon, but botb 

 allowed the discovery to slip out oi their hands. Ed. 



