760 Prof. R. W. Wood 



on 



a solution of photo-rhodamine formed by the action of 

 light on a solution of rhodamine of similar concentration. 

 In this way the fluorescent light was spread out into a 

 spectrum and the possible masking effect was eliminated. 

 It was found that the green fluorescence was enormously 

 stronger in the case of the photo-rhodamine than in the 

 case of the original solution. This makes it seem certain 

 that we have an intermediate coloured body, which is 

 fluorescent. Perrin has recorded similar results. 



A number of other fluorescent substances were examined, 

 with the following results : — 



Phloxine : Bleached by the action of light, the absorption 

 band remaining in its original position and gradually fading 

 away. 



Resorufme : Similar to phloxine. 



Acridine Orange : Solution yellow by transmitted light, 

 becomes reddish after long illumination, with broadened 

 absorption band. Much like fluorescein. 



Napthaline Red : Alcoholic solution shows orange fluor- 

 escence. Bleached by illumination, with no trace of other 

 coloured compound. Solution in hot water : Illumination 

 causes the precipitation of a fine suspension which gives a 

 bluish colour by transmitted light, The precipitate is 

 soluble in alcohol and shows fluorescence similar to that 

 of the alcoholic solution of the original substance, but less 

 bright. 



Influence of Temperature on Fluorescence. 



Perrin states in his paper {Annates de Physique, 1918) 

 that the intensity of the fluorescent light is independent of 

 temperature, and draws a comparison between fluorescence 

 and radio-activity. 



I have found, however, that rhodamine is almost non- 

 fluorescent in a water solution at 100°. The observation 

 was first made in the case of a small tube containing about 

 1 c.c. of the solution, illuminated with sunlight focussed 

 by the large lens. The solution was raised to the boiling- 

 point in about a minute and the fluorescence disappeared, 

 reappearing if the solution was cooled. Fluorescein 

 appeared to be slightly brighter at zero than at room- 

 temperature, and slightly less at 100°. Eosine showed no 

 temperature effect. On account of the peculiar temperature 

 effect in the case of rhodamine, it was necessary to keep 

 the solutions cool by a water jacket during the trans- 

 formation experiments recorded in the previous section. 



