LIGHT. 



233 



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 minuteness and pro- 

 digious 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 measu- 

 rable rapidity with which they succeed each other, were computed by Doctor 

 Young, and are exhibited in the table previously shown. 



On a cursory view, it must appear singular that two hypotheses, founded on 

 assumptions so essentially different, should concur in affording the means of 

 { explaining so great a number of facts with equal precision and almost equal 

 ; facility. This, however, is the case with respect to the corpuscular and uudu- 

 ' latory theories of light, from both of which the mathematical laws to which the 

 phenomena are subject may be deduced, though not in all cases with the same 

 degree of facility. So far as the corpuscular doctrine is available for the pur- 

 poses of deductive explanation, it possesses all the characteristics of a good 

 theory. It supposes the operation of a force with which we are in some 

 measure familiar. We are accustomed to contemplate the effects of attraction 

 in the grand phenomena of astronomy ; we perceive them at every instant in 

 the downward tendency of all heavy bodies ; and, though they disappear in the 

 small bodies of nature, they are reproduced in the phenomena of electricity, 

 magnetism, capillary attraction, and various chemical actions, where they can 

 be not only distinctly traced, but reduced to mathematical formulae, and sub- 

 mitted to accurate calculation. The undulatory hypothesis is not seized by the 

 mind with th* 1 name facility ; yet it also possesses some of the least equivocal 

 characteristics of philosophical truth. No phenomenon has yet been discovered 

 decidedly at variance with any of its principles. On the contrary, most of the 

 phenomena follow from those principles with remarkable ease ; and in numer- 

 ous instances, consequences deduced from the theory by a long and intricate 

 analysis, and where no sagacity could possibly have divined the result, have 

 been found to be accurately true when brought to the test of experiment. Hence 

 this hypothesis begins to be generally adopted by philosophers, and, in recent 

 times, by far the most illustrious names in the annals of optical discovery are 

 included in the list of its supporters. 



That the sensation of light is produced by the vibrations of an extremely 

 rare and subtle fluid, is an idea that was maintained by Descartes, Hooke, and 

 some others ; but it is to Huygens that the honor solely belongs of having re- 

 duced the hypothesis to a definite shape, and rendered it available to the pur- 

 poses 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 inge- 

 nuity has seldom been equalled, Dr. Young was conducted to some very re- 

 markable numerical relations among some of the apparently most dissimilar 

 phenomena of optics to the general laws of diffraction, and to the two princi- 

 ples of coloration of crystallized substances. Malus, so late as 1810, made 

 the important discovery of the polarization of light by reflection, and success- 

 fully explained the phenomenon by the hypothesis of an undulatory propaga- 



