72 LECTURES TO SCIENCE TEACHERS. 



which would be -propagated from it in all directions. This 

 I conceive to be a rough dynamical illustration of what 

 takes place in this actual phenomenon, namely, that the 

 incidence of etherial waves causes a certain agitation in the 

 ultimate molecules of the body, and causes them to be in 

 their turn centres of agitation to the ether ; in fact that 

 the incident light renders the medium so to speak self- 

 luminous, so long as it is under the excitement of the 

 incident light. That is the view which I maintained from 

 the first, and which is clearly expressed in my original 

 memoir, which was published in the Philosophical Trans- 

 actions of 1852. There is one phenomenon, that of phos- 

 phorescence, which I felt from the first to be exceedingly 

 analogous to that which is now known by the name of 

 fluorescence, a word I suggested in that original memoir, 

 derived from fluor spar, which was one of the first minerals 

 in which the phenomenon had been observed, as the 

 analogous term, opalescence, is derived from the name of 

 the mineral opal. I am unable to draw any sharp line of 

 demarcation between fluorescence and phosphorescence. So 

 far as I had observed, the effect was only of instantaneous 

 duration, although, as I have expressly stated, I had not 

 made experiments on a revolving mirror to determine 

 whether a finite duration could be perceived With regard 

 to the explanation of the law which I believed to be 

 universal, that in this phenomenon the refrangibility is al- 

 ways lowered, that is to say, the light coming out is always 

 of lower refrangibility than the incident light, I offered 

 a certain conjecture, which I did not hold to very tightly, 

 and I have somewhat changed my views in that respect ; 

 but I held from the first that the effect is not a direct . 

 but an indirect one ; that the light is not simply reflected 

 from the ultimate particles of bodies. It is curious that 

 some two or three writers have attributed to me the notion 

 that in this phenomenon the light reflected from the mole- 

 cules of the body was changed in refrangibility. They have 

 attributed that notion to me, and then contended against it ; 

 but if you will allow me to read a short passage from my 

 original paper, it will show that I am not responsible for 

 that. I wrote these words : " In considering the cause of 

 internal dispersion, we may, I think, at once discard all 

 supposition of reflections and refractions of the vibrations 





