358 RICHARDSON— DYNAMICAL EFFECTS OF [April 22, 



wave-length side being more deviated than that on the short wave- 

 length side, although in each group the colors are in the normal 

 order. This is the so-called anomalous dispersion which was dis- 

 covered by Kundt and which is exhibited by all transparent colored 

 bodies, like the aniline dyes, wdiich possess a metallic shimmer. 

 Immediately on the short wave-length side of A,,, we have seen that 

 /u,- has a negative value in the case we are contemplating, /x in this 

 region has therefore what mathematicians call an imaginary value. 

 It can be shown that this imaginary value means that the waves are 

 incapable of entering the medium. When a train of waves of this 

 wave-length falls on the medumi they are not absorbed, properly 

 speaking, but are completely reflected. The substance would appear 

 to be opaque to light of this wave-length not because it absorbs the 

 light which falls on it but because it reflects it completely. If 

 mixed light which contained some of this particular wave-length 

 were made to undergo a sufficient number of successive reflections 

 from plates of the substance, only light of this particular region of 

 frequency would ultimately be left over, since a certain percentage 

 of the other wave-lengths always gets through. This principle has 

 been utilised by Rubens to isolate radiations of definite wave-length 

 in the infra-red part of the spectrum. These radiations are called, 

 very appropriately, residual rays. 



The foregoing discussion does not touch the very interesting 

 questions of the absorption of light by insulating media. There will 

 be no absorption, properly speaking, unless there are forces acting 

 on the moving electrons which tend to dissipate the energy of the 

 light. Such forces must in general exist and it is usually assumed 

 that there is a retarding force proportional to the velocity of the 

 moving electrons, chiefly because this is the simplest assumption 

 wdiich can be made which is not in contradiction with fact. The 

 existence of forces of this kind modifies the foregoing conclusions 

 to a considerable extent in detail but it does not afl^ect their general 

 character. 



Planck has pointed out that it follows from the principles of the 

 electromagnetic theory of light that the radiation from the moving 

 electrons gives rise to a retarding force which may be taken to be 

 proportional to their velocity. Such a force must un(|uestionably 



