[ 114 ] 



XIV. On the Action of the Violet and Ultra-violet Invisible Light. 

 By W. Eisenlohr*. 



THE phenomenon described by Stokes under the name fluor- 

 escence, led me to the supposition that this was caused by 

 the interference of the shorter system of waves, blue -violet and 

 ultra-violet (for the sake of shortness, the chemically-acting invi- 

 sible rays of the spectrum may be so designated). I think, with 

 many others, that the eye has the greatest sensibility for a cer- 

 tain duration of vibration (the yellow light), and that it is the 

 more sensitive for longer or shorter waves, the more these differ 

 from the medium light in their depth or height. 



Light itself consists of the visible systems of waves, and 

 besides these, of such as are longer than red and shorter than 

 violet. As the combination of two tones is always deeper than 

 each single one, out of which the compound tone arises, so from the 

 interference of yellowand blue there can result only light of greater 

 length of undulation, and not violet light. Now since red has the 

 longest undulations of the visible light, the combination of red 

 and yellow waves of light can only give a deeper tint than red, 

 and consequently no visible light. A fluorescence in the dark 

 space of the spectrum near the red is not therefore to be expected. 

 It is quite otherwise at the other end of the spectrum. The 

 ultra-violet is the light acting in the dark space of the spectrum 

 near the violet ; its existence could only be shown by its chemical 

 action, before the wonderful discovery of Stokes. It consists of 

 countless systems of undulations, the lengths of which, differing 

 among themselves, have all a shorter duration than the violet 

 light. Through their interference, waves of greater length than 

 their own result ; and by their great variety, tints of combina- 

 tion no less numerous ; hence in many cases all kinds of visible 

 light, or white. 



In other cases a certain colour prevails in the mixture of the 

 tints of combination, which will partly arise from the length of 

 the original waves, and partly from the distance of the reflecting 

 layers of atoms of the fluorescent bodyf. 



Starting from this view, I have made experiments to find 

 sources of light in which high tints prevail, in order to test this 

 idea. Violet and blue glasses, through which the sunlight waa 

 admitted into the room by means of a heliostat, separating 

 single parts of the entire spectrum from the rest, and causing 



* Poggendorff's Annalen, vol. xciii. p. 623. 



t Of course the comparison between the tone of combination, and the 

 light produced by various kinds of ultra-violet or other waves, must not be 

 taken literally, for otherwise the number of vibrations of the resulting 

 colour must be equal to the difference in the number of vibrations of the 

 original rays. 



