





E. B. Hunt on the Dispersion of Light. 371 



ly unproved, unsustained by analyses and mechanics, and hence 

 inadmissible. 



Again the possibility of spherical waves of sensible intensity 

 in all directions, like those imagined as secondary waves, is by no 

 means clear. An impulse received by a surface atom or particle 

 may give it a vibratory motion in one direction, hut not in all. 

 How can an equal intense wave be thrown off perpendicular to 

 this line of vibration? These atoms and particles seem to be 

 mere media of transmission for previously existing impulses, aion_ 

 their lineal direction of impact. Instead of spherical secondary 

 waves of ether, the case seems to demand vibrating ethereal col- 

 umns, much like sonorous vibrations in pipes. The rectilinear 

 propagation of light, is only explicable on the wave theory by 

 supposing the lateral cscillaticn a mere nullity. The exterior 

 rays of a beam should otherwise create by their lateral action, 

 new luminous waves not destroyed by interference. This cause 

 could destroy only those lateral waves formed in the body of the 

 beam. Besides, why should secondary waves be produced at 

 surfaces and not equally through the whole mass of media? Or 

 if all particles do produce such waves, why is not the side of a 

 beam of rays in a material medium as luminous as its end? If all 

 do not produce them, where are the secondary waves in atmos- 

 pheric refraction 1 



But the objection drawn from dispersion has still greater force. 

 The explanation of refraction, must from the nature of the case, 

 contain that for dispersion when the difference between colors is 

 given. That of Huyghens entirely fails of this. It does what 

 is worse * admit it, and it proves that dispersion cannot exist ; for 

 facts show that t\u> different colored rays have velocities sensibly 

 equal in the same medium, and must therefore be equally refract- 

 ed along the same line of media. 



Huyghens 7 demonstration seems to me defective, not in its 

 mathematics, but in its conflict with mechanical principles and 

 observed facts. Geometrical construction and physical mechan- 

 ics are independent provinces : the admission of a mathematical 

 element not entering nature or the exclusion of one actually en- 

 tering it, may lead to results wholly foreign to fact. The admis- 

 sion of the element of velocity as the cause of refraction, seems 

 the fundamental error which has created the whole difficulty 

 with dispersion. Change of rdocity is not change of direction, 

 and even though it could produce it, dispersion would demand 

 an inequality of chromatic velocities which astronomy, from its 

 celestial throne, emphatically denies. A position of dissent from 

 generally received opinions is not to be coveted. Yet in matters 

 of natural science the honest question is, Not, what says Paul 

 or Aristotle, but what is the fact. Respect for great names, in- 

 volves no obligation to cloak error, however honorable its pater- 



