412 Prof. Lorenz on the Theory of Light. 



but that, on the other hand, this assumption brings with it not 

 only the explanation of circular polarization, which had necessi- 

 tated its being made, but also the explanation of double refrac- 

 tion and chromatic dispersion — phenomena which had given rise 

 to the fundamental hypotheses of the molecular forces — these 

 latter hypotheses become quite superfluous ; and an hypothesis 

 which is superfluous is false. The whole complicated apparatus 

 of hypotheses becomes merely an arbitrary appendage to the 

 theory as soon as all can be explained by periodicity. 



Thus, as in other branches of mathematical physics, so also in 

 optics, we are obliged to give up the common conceptions con- 

 cerning the nature of molecular forces. It would be of very 

 little use to try to construct the theory of light upon new phy- 

 sical hypotheses, or upon new conceptions of what goes on in the 

 interior of bodies, concerning which we are probably quite unable 

 to form any conception whatever. The science of our day takes 

 a totally different direction, and seeks to free itself from all such 

 conceptions, which are only iynes fatui, and perhaps no better 

 guides than the conceptions of Bacon's time were in their day. 



In the theory of light we do not require to know any magni- 

 tudes but such as we can directly or indirectly take cognizance 

 of. These magnitudes are, the Intensity, the Velocity, the Direc- 

 tion of emission, the Colour, the Phase, and lastly the position of 

 the Plane of Polarization. Light, as is well known, does not 

 make a momentary impression upon our eye ; and the intensity 

 which we measure is therefore a mean intensity, or the sum of 

 all the impressions made during a short interval of time ; but if 

 we suppose the eye capable of perceiving the intensity of every 

 colour, at every single moment of time, and at every point in 

 space, it would then be able to determine not merely the mean 

 intensity, but also the velocity, the direction of emission, the 

 colour, and the phase; for every alteration of any one of these 

 would modify the impression. We may therefore comprehend 

 all these magnitudes under one idea — that of Intensity in a 

 wide sense. In addition to this function of the time and of the 

 coordinates of space, we only require two more in order to be 

 able to determine the position of the plane of polarization ; light 

 is thus at every point and at every moment fully characterized 

 by three magnitudes dependent upon the time (t) and the coor- 

 dinates of the point [x, y, z). 



The problem of the theory now becomes to And three partial 

 differential equations which hold good for all media, and express 

 the dependence of these three magnitudes upon the independent 

 variables (x, y, z, t). From these equations we must then be 

 able to deduce all the phenomena of light, those only excepted 

 which depend upon unknown forces, such as electrical and che- 



