THE UNDULATORY THEORY. 497 



to be in the polarization plane ; the main requisite 

 being, that light polarized in planes at right angles 

 to each other, should also have the vibrations at 

 right angles. Accordingly, for some time, this 

 point was left undecided by Young and Fresnel, 

 and, more recently, some mathematicians have 

 come to the opinion that ether vibrates in the 

 plane of polarization. The theory of transverse 

 vibrations is equally stable, whichever supposition 

 may be finally confirmed. 



We may speak, in the same manner, of the 

 suppositions which, from the time of Young and 

 Fresnel, the cultivators of this theory have been 

 led to make respecting the mechanical constitution 

 of the ether, and the forces by which transverse 

 vibrations are produced. It was natural that va- 

 rious difficulties should arise upon such points, for 

 transverse vibrations had not previously been made 

 the subject of mechanical calculation, and the 

 forces which occasion them must act in a different 

 manner from those which were previously con- 

 templated. Still, we may venture to say, without 

 entering into these discussions, that it has appear- 

 ed, from all the mathematical reasonings which 

 have been pursued, that there is not, in the con- 

 ception of transverse vibrations, anything incon- 

 sistent either with the principles of mechanics, or 

 with the best general views we can form, of the 

 forces by which the universe is held together. 



I willingly speak as briefly as the nature of my 

 VOL. IL K K 



