LIGHT. 57 



of observation and calculation, and he soon discovered that the error 

 was always between certain limits and depended upon the distance of 

 the planet i'lom the earth. These facts were inexplicable on the sup- 

 position that the transmission of light was instantaneous, but the phe- 

 nomenon was fully explained by allowing 16| minutes as the time re- 

 quired by light to traverse the earth's orbit. Its velocity therefore was 

 determined to be 200,000 miles per second. 



Did the particles of light possess the ponderable properties of mat- 

 ter even in the most minute degree, their immense velocity would give 

 them a momentum so great that nothing could withstand their force ; 

 the delicate mechanism of the eye would be infallibly destroyed, and 

 even those bodies, which were capable of opposing tlie greatest resist- 

 ance, would, by the long continued action of such power, be subjected 

 to decomposing influences far greater than any others which nature fur- 

 nishes. If, for example, a single molecule of light be supposed to weigh 

 one grain, its momentum would equal that of a ball weighing 300 

 pounds, and moving at the rate of 150 miles per hour. Since, then, the 

 infinite number of molecules that enter the eye in a single second do 

 not by their united influence produce any injury to its exquisitely deli- 

 cate structure, it becomes difficult to resist the conclusion that light 

 must be absolutely imponderable, and consequently devoid of one of 

 the ordinary properties of matter. 



The undulatory theory does not suppose any actual transmission of 

 particles, but only a rapid vibration of the luminiferous ether by which 

 motion is propagated without any progressive movement in the parti- 

 cles, in the same manner as sound is produced by the vibrations of the 

 atmosphere. 



Not content witli the discovery of the velocity of light, philosophers 

 next turned their attention to the measurement of the rapidity of the un- 

 dulations and the amplitudes and altitudes of the luminous waves, and 

 for the solution of this refined problem we are indebted to Newton 

 himself. Having placed a slightly convex lens in contact with a pane 

 of glass, and allowed a beam of decomposed light from a prism to fall 

 exactly upon the centre, he observed that luminous concentric rings 

 were produced of such dimensions as to be susceptible of measurement, 

 and their diameters, with that of the sphere of which the lens was a 

 part, gave the requisite data from which to calculate the distances be- 

 tween the glass surfaces. From these calculations he found that if the 

 distance at the first ring was represented by 1, at the second it would 

 be 2, at the third 3, &c. These distances he supposed to correspond to 

 the amplitude of the luminous waves ; that at the first ring the space 

 8 



