300 WRITINGS OF JOSEPH HENRY. [1851 



to different properties at different ends. But now the original 

 theory of emission, at first a simple mechanical conception, 

 becomes so loaded with supplementary hypotheses that as a 

 whole it is unwieldy, and we are induced to look for some 

 other possible hypothesis which shall equally well connect 

 the phenomena in accordance with known mechanical prin- 

 ciples, and not be subject to the same charge of complexity. 

 Such an assumption is found in the present received undu- 

 latory theory of light. 



In reviewing the foregoing sketch of the rise, growth, and 

 abandonment of the theory of emission we see that an hypoth- 

 esis, though not absolutely true, may serve an important 

 purpose in the way of the definite conception of old phe- 

 nomena, and in the discovery and prediction of new; and 

 indeed in some cases, paradoxical as it may appear, a false 

 hypothesis, from its ease of application, may be of more use 

 than one which is absolutely, true. Man, with his finite 

 faculties, cannot hope in this life to arrive at a knowledge 

 of absolute truth; and were the true theory of the universe, 

 or in other words, the precise mode in which Divine Wis- 

 dom operates in producing the phenomena of the material 

 world, revealed to him, his mind would be unfitted for its 

 reception ; it would be too simple in its expression, and too 

 general in its application, to be understood and applied by 

 intellects like ours. 



It may be asked why theories, so apparently different as 

 those of emission and undulation, should both lead to the 

 discovery of new truths? The answer is that the former 

 is involved in the latter, and that all the supplementary 

 hypotheses we have mentioned have their representation in 

 the different phases of wave-motion. Thus an undulation 

 is reflected in the same manner as an elastic ball ; a change in 

 velocity also takes place in the undulation on entering a new 

 medium ; and the fits of Newton are represented by lengths 

 of waves, and the polarization of Malus by transverse vibra- 

 tions reduced to the same or parallel planes. The undulatory 

 theory is a more general expression, and contains truths 

 which are not to be logically deduced from the theory of 



