THE \^ 



MONTHLY MICROSCOPICAL JOURNAL. 



JANUAEY 1, 1871. 



I.— Note on FLUORESCENCE v. PSEUDO-DICHROISM. 

 By the late Kev. J. B. Eeade, F.E.S., P.E.M.S. 



(Bead Iff ore the Eoyal Microscopical Society, Nov. 9, 1870.) 



At several meetings during tlie last three years I have exhibited 

 to the Society a coloured liquid, prepared by Mr. Sheppard from 

 jEresh-water algae, species undecided, found in a spring in Kent. 

 An exactly similarly-coloured solution was described by Dr. 

 Ferdinand Cohn, of Breslau, in the number of Schultze's ' Archiv,' 

 published about the same time, early in 1867. 



The characteristic qualities of the English liquid will be found 

 detailed at length in a letter from ]\Ir. Sheppard to myself, read 

 before the Society on the 8th of May, 1867 ; and I will here quote 

 Dr. Cohn's description, which so exactly tallies with that of Mr. 

 Sheppard, that we may assume that the liquids are exactly similar, 

 and that the phenomena of colour are the same in both. 



Dr. Cohn writes : — " Another most startling optical peculiarity 

 of the Phycocyan is its fluorescence ; for seen only by transmitted 

 Hght on a clear background, the Phycocyan appears indigo-blue; 

 but with reflected light and a dark background it shows an intense 

 carmine red," This account of the play of colour is exactly that 

 given by Mr. Sheppard, except that he compares the colour seen by 

 reflected light with that of the carnehan ; and I think that this 

 simile is more exact than that of Dr. Cohn, seeing that the hquid 

 shows an opaque dull intense scarlet, rather than the bright trans- 

 parent crimson of carmine. 



Now this phenomenon of fluorescence (for Dr. Cohn's phrase 

 has been generally adopted) has attracted less attention than might 

 ^lave been expected. Other solutions, for instance quinine, exhibit 

 .cnfluorescence, or faint iridescent gleams of blue and pink; but in the 

 ' case of Phycocyan the violent change from all blue to all red by 

 '"altered illumination goes far beyond what is seen in any of these 

 chemical compounds, and leads one to believe that the one name 

 OEluorescence is not a proper description of the two sets of appear- 



Q VOL. V. B 



