126 On Negative Fluorescence and Phosphorescence. 



of various wave-lengths. Now, if the period of oscillation of the 

 resulting rays is not too great for them to be perceptible by the 

 eye, the body must phosphoresce ; but if it is too great, the 

 rays, which of themselves are invisible, might during the period 

 of exposure combine with the impinging rays (of course after 

 these have been reflected), and combination-colours might be so 

 produced, which would be directly cognizable by the eye. The 

 rate of vibration of the combination-ray (this is expressly stated) 

 is supposed equal to the difference of the rates of vibration of the 

 two conspiring rays. 



24. If we are to accept the theoretical conceptions of M. 

 Eisenlohr and, particularly, those of M. Lommel, the possibility, 

 or rather the necessity, of an increase of refrangibility — that is, of 

 a negative fluorescence, at the same time as the positive fluores- 

 cence, would be thereby conceded. The two physicists just 

 named speak only of those combination-vibrations which were 

 earliest discovered in acoustics, and are often spoken of as Tar- 

 tar's combination-tones. Professor Helmholtz*, however, has 

 shown that in addition to these deeper combination-tones, or 

 " difference-tones," there are also higher combination-tones, 

 " summation-tones," whose rate of vibration is the sum of the 

 rates of vibration of the primary tones. If we extend this dis- 

 covery to the oscillations of the sether, a ray of much greater re- 

 frangibility may be generated by the conjoint action of two rays 

 of smaller refrangibility. For instance, from two ultra-red rays 

 of but slightly different refrangibilities, light of about half the 

 time of vibration (that is, indigo or violet light) might be pro- 

 duced. If vibrations of various kinds are produced in the mole- 

 cules of the fluorescent body by the action of the exciting rays, 

 but all or most of them of long period, it is not only possible 

 but necessary that, if these vibrations are ultra-red and therefore 

 of themselves invisible, visible light should be generated, even 

 without the aid of the reflected rays, by the production of " sum- 

 mation-vibrations." 



25. The experimental proof of negative fluorescence can be 

 certainly given only by bringing some appropriate substance 

 successively into the various parts of a very pure spectrum and 

 carefully analyzing the resulting fluorescent light. Chlorophyll, 

 guaiacum, and some other substances exhibit fluorescence even 

 when exposed to light of low refrangibility, as the yellow or 

 orange of the spectrum. I am not aware whether the red flu- 

 orescent light of chlorophyll and the so-called violet light of 

 guaiacum have been accurately analyzed, attention being speci- 

 ally directed to ascertain whether this compound light does not 

 contain rays of greater refrangibility than those constituting the 



* Poggendorff's Annalen, vol. xcix. p. 497 (518). 



