THE PROOF OF THE EARTH'S MOTION 283 



tempted to solve the problem as to whether or not the trans- 

 mission of light is instantaneous. Every schoolboy has observed 

 the difference in the propagation of light and sound. The flash 

 of a rifle from a distance is seen long before we hear the report. 

 It required tests of no great delicacy to approximately measure 

 the speed of sound. The phenomena of light had been studied 

 from the most ancient times ; Ptolemy of Alexandria had left 

 a treatise upon the subject. The laws of reflection were known 

 to his day ; refraction, the bending of light as it passes through 

 media of varying density, had been observed closely by 

 Poseidonius, by some of the Arabians, and by Friar Bacon. 

 With the tremendous awakening of physical investigation which 

 followed the discovery of America, and especially with the in- 

 vention of the telescope, the subject was taken up anew by 

 scores of eager minds. Lord Bacon fumbles with it, as with 

 so many other of the current ideas of the time. There is a 

 passage in the Novum Organum that may be worth the quoting, 

 alike as an excellent example of the later Bacon's stumbling 

 logic and confused thought, and of the notions prevalent in 

 his day. It runs : 



" . . . The flight of the musket 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 ex- 

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

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

 of its evidence, 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 ap- 

 peared so incredible to us that the images or radiations of 

 heavenly bodies could suddenly be conveyed through such im- 

 mense spaces to the sight, and it seemed that they ought rather 

 to be transmitted in a definite time. 



" That doubt, however (as far as regards any great difference 

 between the true and apparent time), was subsequently com- 

 pletely 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 



