MAGNITUDE OF ITS WAVES. 



exact results of the physical investigations which have been 

 carried on, on this subject, are given. 



7. Whichever theory we adopt to explain the phenomena oi 

 light we are led to conclusions that strike the mind with astonish- 

 ment. According to the corpuscular theory, the molecules of 

 light are supposed to be endowed with attractive and repulsive 

 forces, to have poles to balance themselves about their centres 

 of gravity, and to possess other physical properties which we 

 can only ascribe to ponderable matter. In speaking of these 

 properties, it is difficult to divest oneself of the idea of sensible 

 magnitude, or by any strain of the imagination to conceive that 

 particles to which they belong can be so amazingly small as 

 those of light demonstrably are. If a molecule of light weighed 

 a single grain, its momentum (by reason of the enormous velocity 

 with which it moves) would be such that its effect would be 

 equal to that of a cannon-ball of one hundred and fifty pounds, 

 projected with a velocity of one thousand feet per second. How 

 inconceivably small must they therefore be, when millions of 

 molecules, collected by lenses or mirrors, have never been found 

 to produce the slightest effect on the most delicate apparatus 

 contrived expressly for the purpose of rendering their materiality 

 sensible ! 



If the corpuscular theory astonishes us by the extreme minute- 

 ness and prodigious velocity of the luminous molecules, the 

 numerical results deduced from the undulatory theory are not 

 less overwhelming. The extreme smallness of the amplitude of 

 the vibrations, and the almost inconceivable but still measurable 

 rapidity with which they succeed each other, were computed by 

 Dr. Young, and are exhibited in the above table. 



8. That the sensation of light is produced by the vibrations of 

 an extremely rare and subtle fluid, is an idea that was main- 

 tained by Descartes, Hooke, and some others ; but it is to Huygens 

 that the honour solely belongs of having reduced the hypothesis 

 to a definite shape, and rendered it available to the purposes of 

 mechanical explanation. Owing to the great success of Newton 

 in applying the corpuscular theory to his splendid discoveries, 

 the speculations of Huygens were long neglected ; indeed, the 

 theory remained in the same state in which it was left by him 

 till it was taken up by our countryman, the late Dr. Young. 

 By a train of mechanical reasoning, which in point of ingenuity 

 has seldom been equalled, Dr. Young was conducted to some 

 very remarkable numerical relations among some of the appa- 

 rently most dissimilar phenomena of optics to the general laws 

 of diffraction, and to the two principles of coloration of crystal- 

 lised substances. 



205 



