286 M. W. Haidinger on the Lido-quinine Compound, 



gated in the four or six-sided tabular crystals in two positions, 

 at right angles to each other. There follows as the result — 



1. The substance-colour. — In common light, pale olive-green; 

 in very thin plates, up to y^dth of an inch, colourless. In 

 polarized light, if the light is polarized in the long direction of 

 the crystal, transparent and colourless, at least with thin cry- 

 stals ; if the light is polarized in the transverse direction of the 

 crystal, opake and black, however thin the plates may be. 



An increased interest in the crystals was excited by the follow- 

 ing optical investigations of the metallic surface-colours which 

 Professor Stokes made known at the meeting of the British 

 Association in Belfast, September 1852. Abbe Moigno's Cosmos 

 contains a report of the communication. 



Professor Stokes found that in the same direction of polariza- 

 tion of the reflected ray a vitreous lustre answered to the nearly 

 colourless tint, and the green metallic lustre to the dark or black 

 tint. It follows, therefore, — 



2. Tlie surface-colour. — When polarized in the transverse direc- 

 tion of the crystals, with a tolerably perpendicular incident ray 

 gives a metallic yellowish-green ; with greater angles it varies 

 to a dark steel blue. 



As Professor Stokes discovered in the crystals here mentioned 

 the property of polarized surface-colour*, independently of my 

 observations of analogous crystals, and without having any 

 knowledge of them, so in consequence of the great interest of 

 the phenomenon, he has given in detail the various modifications 

 of the observations and illustrative remarks respecting their 

 nature. 



When I read the report, the circumstance struck me particu- 

 larly, that with similar polarizations a green metallic surface- 

 colour should correspond to a complete black in the substance- 

 colour. The black was in opposition to the perfect transparency 

 of the light polarized perpendicularly to the preceding colours. 

 I had thought, by observations on a tolerable number of bodies, 

 to have satisfactorily established as a law, "that the surface- 

 and substance-colours stand in a complementary relation to 

 each other." To the "green" of the surface a "red" in 

 the colour of the substance should have corresponded. It 

 was now my earnest wish to examine the crystals myself. 

 According to the method given in the Cosmos, I could not suc- 

 ceed in forming them; for through an unfortunate misprint, 

 instead of " solution d'iode dans Valcool" there were the words 

 " solution d'acide dans Valcool. ,i I had quite forgotten, that even 

 in May my honoured friend Wohler had written to me about 



* Dr. Herapath noticed that the metallic green reflected ray was a polar- 

 ized beam, and assigned 41° as the angle necessary to produce polarization 

 by reflexion. — Ed. 



