2 Transactions of the 



ances. I hold the play of colour in the two cases proceeds from 

 two very different causes ; and that this opinion is correct I shall 

 now be able to show by an experiment. I have here a bottle con- 

 taining water stained blue by a little indigo from my water-colour 

 paint-box : to this transparent blue liquid I have added a little 

 vermilion in impalpable powder ; and straightway I have obtained 

 all the fluorescent qualities of Phycocyan. That is, two colouring 

 matters, one opaque the other transparent, dissolved out of the 

 plant (according to another theory developed from the plant), are 

 represented in one artificial liquid by indigo and vermilion. Now 

 hold the bottle to the light, the transparent indigo alone is visible, 

 the grains of vermihon showing only their shaded sides, which 

 appear grey or black by contrast with the strong light, and there- 

 fore, do but subtract a little brightness from the blue. Now change 

 the position of the bottle, and by so doing change the illumination ; 

 the "intensiv karminroth" of Cohn at once shows itself. The 

 transparent blue throws back no coloured rays, but the opaque red 

 particles are exhibited in quantity sufficient to make " the hlue one 

 red " (Shakspeare). Seeing then that the fluorescence of quinine and 

 this fluorescence of Phycocyan spring from different causes, I think 

 it will be proper to distinguish them, and I propose for shortness 

 to name this play of colour described and exhibited to-night — 

 Pseudo- dichro ism . 



I am indebted to my friend Mr. Sheppard for the ingenious 

 experimentum crucis above described. It is the exact counterpart 

 of nature's operation in the production of Phycocyan, which consists 

 of opaque red particles in a transparent blue fluid. Thanks to my 

 friend for his solution of the mystery, and also for enabling me to 

 present this account of it for the consideration of our Fellows. 



" Omne ignotum pro magnifico." 



But the mystery ceases when we know all about it. 

 BisHOPSBouRNE Rectory, Nov. 7, 1870. 



