418 Prof. Lorenz on the Theory of Light, 



deduced from our fundamental equations. Here, as previously 

 in the theory of double refraction, we have arrived at one of the 

 stations from which the formal part of the theory takes its start, 

 and, thanks to the great development which this part of the 

 theory has attained, a whole section of the doctrine of light 

 again lies before us (without our needing to take one step fur- 

 ther) as a simple consequence of our theory. If the theory of 

 reflexion and refraction had not already been developed as it has 

 been, we should not endeavour to express the limiting equations 

 by means of any other auxiliary magnitudes than our compo- 

 nents; for both the limiting equations and the law of double 

 refraction, when expressed by means of these, assume their sim- 

 plest form, and hence also the calculation with them would be 

 simpler and more elegant. 



In the above calculation we have indeed taken into conside- 

 ration the immediate effect of the periodic part of the compo- 

 nents, which must accompany every wave of light in the periodi- 

 cally heterogeneous body and is dependent upon it; there arises, 

 however, also a secondary effect of the periodic motions of the 

 two media, a mutual reflex action, which cannot be without influ- 

 ence upon the visible light-motion. Here, as throughout all 

 nature, we meet with a perturbation; and this small departure 

 from the results obtained will probably be capable of confirma- 

 tion by experiment. Were both media homogeneous, both 

 the calculation and the principle of the maintenance of the in- 

 tensity would be exactly true ; not so, however, in the opposite 

 case. This also may be made directly evident; for a part of the 

 original quantity of light must necessarily be extinguished in 

 the production of periodic wave-motions within the body. I will 

 here pass over another perturbation, to which I have previously 

 directed attention*, and which depends upon the fact that the 

 two media are not separated by a perfectly sharp mathematical 

 plane. 



The loss of visible light above mentioned must not be con- 

 founded with that which arises from simple absorption. The 

 latter can be easily calculated. Modern investigations into the 

 reflexion and refraction of imperfectly transparent and metallic 

 bodies have, in fact, led to the remarkable result, that the same 

 laws which apply in the case of transparent bodies apply here 

 also, with the single difference, that the refractive ratio now 

 assumes the complex form a + b */ — 1. From this fact we can 

 draw the important conclusion that our differential equations 

 hold good not only for transparent bodies, but for all bodies 

 without exception. It is moreover apparent, if we endeavour to 



* Poggendorff's Annalcn, vol. cxi. p. 111. 



