Light, Heat, and Electricity . 213 



as the "calorific wave^''), "the vibrations of the second wave 

 being transversal and unaccompanied by any change of density 

 in the medium/^ The latter is therefore my " luminiferous 

 wave/^ 



Again^ according to Cauchy's investigations, " a ray entering 

 any medium will be subdivided into three rays, and, when the 

 elasticity of the aether in this medium is the same in all direc- 

 tions, the three will have a common direction and two of them a 

 common velocity, being thus reduced to two, viz. a single ray 

 and a double ray — the vibrations of the former [my calorific ray] 

 being in the direction of propagation of the wave, and those of 

 the double ray transversal,^^ constituting (on my view) common 

 or unpolarized light. 



It may be observed also that FresneFs theory admits that the 

 vibrations of polarized light may he either parallel or perpendi- 

 cular to the plane of polarization; but, finding that the vibra- 

 tions of the "ordinary ray '^ (by double refraction) are perpen- 

 dicular to the principal plane of the crystal, which is the plane 

 of its polarization also, he drew the conclusion that the vibrations 

 of all polarized light must be feryendicular to the plane of pola- 

 rization. On the other hand, Cauchy^s researches led him to the 

 conclusion that the vibrations of a polarized ray are in the plane 

 of its polarization. This apparent difference in the results of 

 these two eminent analysts may, I should hope, be reconciled 

 (on my view) by supposing that Cauchy's investigations refer to 

 the " ordinary ray '^ polarized by reflection ; for its vibrations I 

 consider to be in the plane of its polarization. If I am right, 

 therefore, I think we shall have obtained a most satisfactory 

 general accordance between the able researches of the three 

 great French mathematicians. 



Although the disturbance of the rether associated with the 

 molecules of a body, by a luminiferous wave, is very trifling, I 

 believe nevertheless that it is to this cause we must look for the 

 explanation of the phenomena of phosphorescence and also of 

 fluorescence. Molecular vibrations being, as we know, compa- 

 ratively very sluggish, are required to maintain the continuance 

 of the eetherial (phosphorescent) vibrations, which must other- 

 wise be expected to subside almost instantaneously (on the 

 withdrawal of the exciting cause) if not sustained by the reaction 

 of the associated molecules, in like manner as heat-waves (in the 

 conduction of temperature) are maintained after the source of 

 the heat has been withdrawn. 



With regard to the nature of caloritic waves, long before I had 

 arrived at any definite views I felt compelled to reject the idea 

 of their consisting in molecular motion, for the following rea- 

 sons : — 1st. Because they arc propagated through space, wlicre 



