LIGHT. 



second more for every 198,000 miles that the earth's distance 

 from Jupiter is increased, the reason of which obviously is, that 

 light takes one second to move over that space ; hence it is 

 apparent that the velocity of light is at the rate, in round 

 numbers, of 200,000 miles per second. 



Such was the discovery which has conferred immortality upon 

 the name of Koemer ; a discovery to which, as we have shown, 

 he was accidentally led when seeking to determine the velocity 

 of one of the moons of Jupiter. The velocity of light thus 

 determined would, in the corpuscular theory, be regarded as 

 that with which the particles of light issuing from the surface of 

 a visible object move through space. In the undulatory theory, 

 however, which is more generally received, this velocity is that 

 with which the waves are propagated through space, in the 

 same sense as waves appear to move on the surface of water 

 if a pebble be dropped in to form a centre round which they 

 are propagated. 



It is necessary to remember when considering any system of 

 undulations, no matter through what medium they may be pro- 

 pagated, that the progressive motion which belongs to them is a 

 motion of form merely, and not of matter. The waves which are 

 propagated round a centre when a pebble is dropped into calm 

 water, present an appearance to the eye as though the water 

 which formed the wave really moved outward from the centre of 

 the undulations. Such is, however, not the case. No particle 

 of the fluid has any progressive motion whatever, of which many 

 proofs may be offered. If any floating body be placed on the 

 surface of the water, it will not be carried along by the waves, 

 and if similar waves be formed, as they might be, by giving a 

 peculiar motion to a sheet or cloth, they would have the same 

 appearance of progressive motion, although the parts of the sheet 

 or cloth, as is evident, would have no other motion than the up- 

 and-down motion that would form the apparent undulations. 

 The waves of the sea appear to the eye to be endowed with a 

 progressive motion. A slight reflection, however, on the con- 

 sequences of such a motion, will soon convince us that it can 

 have no reality. The. ship which floats upon the waves is not 

 carried forward with them ; they pass beneath her, now lifting 

 her on their summits, and now letting her sink into the abyss 

 between them. Observe a sea-fowl, or a nautilus floating on 

 the water, and the same effect will be presented. If, how- 

 ever, the water itself partook of the motion as well as its 

 waves, the ship and the fowl would be carried forward in 

 the direction of that motion. Once on the summit of a wave, 

 there they would continually remain, and their motion would 

 200 



