which exhibit the phenomena of Fluorescence. 85 



tube, held in the ray behind it, appeared perfectly free from 

 colour. Another test-tube, containing the solution of ferro- 

 cyanide of iron in oxalic acid, was placed in the same posi- 

 tion ; the internal dispersion was much diminished, though 

 not altogether destroyed, and it was of a greener hue than 

 when the sulphate of quinine was not interposed. This effect 

 did not take place on the interposition of a similar thickness 

 of distilled water. From this it was concluded that there was 

 true, as well as false, internal dispersion. 



2d, Thinking that if the solution under examination were 

 itself fluorescent, it ought to cut off the rays capable of pro- 

 ducing fluorescence, just as the quinine salt does, I filled two 

 glass vessels of similar form and size, the one with a solution 

 of ammoniacal sulphate of copper, the other with a solution 

 of prussian blue in oxalic acid. They were almost identical 

 in colour and appearance when viewed by transmitted light ; 

 when examined by reflected light, the iron solution appeared 

 dull and slightly green, and the result proved that the light 

 which passed through them was really very different in its 

 properties. When a tube containing bisulphate of quinine 

 was placed in a ray of light which had passed through the 

 copper solution, its course was marked by a beautiful fluores- 

 cent blue ; but when placed in a ray that had traversed the 

 blue iron solution, it remained colourless. When a solution 

 of the prussian blue was itself substituted for the quinine salt 

 in the tube, a like result was obtained ; the ray that had 

 passed through the ammoniacal copper solution produced a 

 fine fluorescence, while that through the iron solution exhi- 

 bited little or no internal dispersion ; and so it was with other 

 fluorescent substances. This difference of action of the two 

 blue solutions equally took place when the ferrocyanide was 

 diluted so as to be far lighter in colour than the copper solu- 

 tion. 



On casting the prismatic spectrum into a vessel containing 

 prussian blue dissolved in oxalic acid, it was found that the 

 solution transmitted green light in those parts of the spec- 

 trum which are ordinarily yellow, and blue in those portions 

 which are usually green or blue ; while nothing was trans- 

 mitted in the violet portion of the spectrum, nor did any lu- 



