FORMATION OF AGGREGATES 267 



spectrum, the radiation is rendered so intense that 

 in a very short while the parts upon which the 

 rays fall cease to be luminous, and the screen 

 becomes marked with a number of dark bands 

 corresponding to the luminous ones in the spectrum. 

 According to Stokes, the combined action of the 

 more refrangible and less refrangible rays is neces- 

 sary. He also believed that it was not a case of 

 thermoluminescence, because during the phenomenon 

 the refrangibility of the phosphorescent light under- 

 goes a further change from higher to lower. 



There is a chemical change of some sort ac- 

 companying this phenomenon, as well as that of 

 ordinary fluorescence and phosphorescence. 



This appears at first sight to be a contradiction 

 of Stokes' law that a transformation when it does 

 occur is from higher to lower, whereas here there 

 seems to be a change from slower waves to more 

 rapid ones. There appears, however, to be a change 

 in this direction, although the excited luminosity is 

 of higher frequency than that in the infra-red that 

 excites it to so great an extent. 



Some instances are to be found in which this 

 efi"ect is even more marked. For example, a ray of 

 deep red light will cause a solution of naphthalene- 

 red to emit an orange-yellow light. This is an 

 illustration of Tyndall's so-called fluorescence re- 

 versed, or calorescence. But it seems not at all 

 improbable that the fluorescence in this case, as in 

 that of chlorophyll, is due to the emission of 

 energy, which the substance received by exposure 



