and Phosphorescence. 125 



allowed to make use of a mode of expression which is becoming 

 somewhat obsolete) . This statement makes no claim to novelty ; 

 I have myself made it as a passing remark a good while ago*. 

 The phenomena of incandescence at the focus of pure rays of 

 heat, rays of light and probably rays of heat of high refrangi- 

 bility being excluded, amount to no more than a new argument 

 in support of this old truth. 



23. It is possible to give theoretical reasons in favour of the 

 existence of a negative fluorescence — that is to say, of a process 

 which increases the refrangibility of rays in the same way as it 

 is diminished in the phenomena of fluorescence as hitherto 

 observed. 



Professor Stokes assumes that the vibrations of the aether 

 which constitute the exciting light produce oscillations in the 

 molecules of the fluorescent body which are not infinitely small ; 

 and hence it is concluded that their periods are longer than that 

 of the exciting light. And as molecular movements give rise to 

 vibrations of the aether of equal period with themselves, the dis- 

 persed light comes to be of less refrangibility than the exciting 

 rays. 



Professor Angstrom makes the same assumption as Professor 

 Stokes, but draws from the circumstance of the resulting mole- 

 cular movements not being infinitely small the opposite conclu- 

 sion f — namely, that oscillations of a higher order (that is, the 

 octave) are thereby imparted to the sether. This would conse- 

 quently come to be an increase of refrangibility, or a negative 

 fluorescence. 



M. W. EisenlohrJ considers that fluorescence is produced by 

 the interference of the bluish-violet and ultra-violet rays. This 

 interference is supposed to give rise to light of smaller refrangi- 

 bility, just as combination-tones are of lower pitch than the two 

 interfering tones. But we must remember, in opposition to this 

 view, that, according to it, fluorescence could never be observed 

 in a pure spectrum, since there rays of only one degree of refran- 

 gibility are to be found at the same place. 



M. Lommel§ has adopted a combination of the theories of 

 Stokes and Eisenlohr. The exciting rays are supposed to throw 

 the particles of the body exposed to them into oscillations the 

 rate of which depends upon the nature of the body. The rays 

 emitted by the body in consequence of this action are supposed, 

 like the rays given out by most luminous or radiating bodies, to 

 be in general not homogeneous, but to consist of elementary rays 



* Henle and Pfeufer's Zeitschrift fur rationelle Medicin, ser. 3. vol. vim 

 p. 234. 



t Pogg. Ann. vol. xciv. p. 162. % Ibid. vol. xciii. p. 623. 



§ Ibid. vol. xcvii. p. 642. 



