XIII. 



ON THE GENERAL EQUATIONS OF MONOCHROMATIC LIGHT 

 IN MEDIA OF EVERY DEGREE OF TRANSPARENCY. 



[American Journal of Science, ser 3, vol. XXV, pp. 107-118, February, 1883.] 



1. THE last April and June numbers of this Journal* contain an 

 investigation of the velocity of plane waves of light, in which they 

 are regarded as consisting of solenoidal electrical fluxes in an indefi- 

 nitely extended medium of uniform and very fine-grained structure. 

 It was also supposed that the medium was perfectly transparent, 

 although without discussion of the physical properties on which 

 transparency depends, and that the electrical motions were not 

 complicated by any distinctively magnetic phenomena. 



In the present paper t the subject will be treated with more 

 generality, so as to obtain the general equations of monochromatic 

 light for media of every degree of transparency, whether sensibly 

 homogeneous or otherwise, which have a very fine-grained molecular 

 structure as measured by a wave-length of light. There will be no 

 restriction with respect to magnetic influence, except that an oscil- 

 lating magnetization of the medium will be excluded. J 



In order to conform as much as possible to the ordinary view of 



*See pages 182-194 and 195-210 of this volume. 



tThis paper contains, with some additional developments, the substance of a com- 

 munication to the National Academy of Sciences in November, 1882. 



Where a body capable of magnetization is subjected to the influence of light (as 

 when light is reflected from the surface of iron), there are two simple hypotheses which 

 present themselves with respect to the magnetic state of the body. One is that the 

 magnetic forces due to the light are not of sufficient duration to allow the molecular 

 changes which constitute magnetization to take place to any sensible extent. The other 

 is that the magnetization has a constant ratio to the magnetic force without regard to 

 its duration. We might easily make a more general hypothesis which would embrace 

 both of those mentioned as extreme cases, and which would be irreproachable from a 

 theoretical stand-point ; but it would complicate our equations to a degree which would 

 not be compensated by their greater generality, since no phenomena depending on such 

 magnetization have been observed, so far as the writer is aware, or are likely to be, 

 except in a very limited class of cases. 



For the purposes of this paper, therefore, it has seemed better to exclude media 

 capable of magnetization, except so far as the first mentioned hypothesis may be appli- 

 cable. But it does not appear that this requires us to exclude cases in which the medium 

 is subject to the influence of a permanent magnetic force, such as produces the 

 phenomenon of the magnetic rotation of the plane of polarization. 



