RECENT PROGRESS IN OPTICS. 121 



for comparing the variation of refractive index of dilute solutions with 

 variation of concentration. The fact of solution brings about a change 

 of molecular constitution, affecting both the electric conductivity and 

 the refractive index ; and the changes in optical density are measurable 

 in terms of the number of interference fringes which cross the field of 

 view for a given variation of dilution. 



LUMINESCENCE. 



While all work on the visible spectrum is confessedly optical, we can 

 no longer make an arbitrary division point, and declare that one part 

 of the spectrum belongs to the domain of optics and the other not. 

 Since the days of Brewster and the elder Becquerel fluorescent solu- 

 tions have enabled us to bring within the domain of optics many wave 

 lengths that were previously invisible. Stokes's explanation of this, 

 as a degradation of energy quite analogous to the radiation of beat 

 from a surface on which sunlight is shining, has been generally accepted. 

 But whether the phenomena of fluorescence and phosphorescence are 

 in general physical or chemical, has for the most part remained unknown, 

 or at least very uncertain. E. Wiedemann, who suggested the term 

 luminescence to include all such phenomena, published in 1895 (Anna- 

 len der Physik und Ohemie, p. G01, April, 1895), in conjunction with 

 Schmidt, a part of the outcome of an extended investigation under- 

 taken with a view to clearing up these uncertainties. He has shown 

 that it is often possible to distinguish between cases in which the 

 emission of light springs from physical processes and those in which it 

 is due to chemical action, or at least invariably accompanied by this. 

 We have here, as in photography, a transformation of radiant into 

 chemical energy, to which is superadded the retransformation of chem- 

 ical into radiant energy of longer period, and this either at the same 

 time or long after the action of the exciting rays. Indeed, between 

 this process and that of photography in colors, the analogy is quite 

 striking. What has generally been called jmosphorescence is well 

 known to be the effect of oxidation in the case of phosphorus itself and 

 in that of decaying wood or other organic matter, which under certain 

 conditions shines in the dark. 



Wiedemann has shown that the shining of Balmain's luminous paint, 

 and generally of the sulphides of the alkaline earths, is accompanied 

 with chemical action. A long period of luminosity after the removal 

 of the source renders highly probable the existence of what he now 

 calls chemi-luminescence. A large number of substances, both inor- 

 ganic and organic, have been examined both by direct action of light 

 and by the action of cathode rays in a controllable vacuum tube 

 through which sparks from a powerful electric influence machine were 

 passed. Careful examination with appropriate reagents before and 

 after exposure was sufficient to determine whether any chemical change 

 had been produced, Thus the neutral chlorides of sodium and potas- 



