288 M. W. Haidinger on the Iodo-quinine Compound, 



hyacinth-red, dark blood-red, or black, according to the thickness 

 of the plates or crystals polarized perpendicularly to the axis. 



In the one, as with the other species, perfectly transparent 

 pale greenish crystals or plates polarize, when crossed, to a com- 

 plete black. If the crystals are thin, the crossing produces only 

 red, just like the colour in the examination through the calca- 

 reous spar of the dichroscopic lens. Very thin plates of andalu- 

 site have a pale but evident red colour. If two such plates are 

 laid in a parallel position on each other, the red vanishes, it is 

 absorbed, and the thicker pale green aggregate is in a surprising 

 degree lighter coloured than each single plate. Just so, also, 

 the exceedingly thin crystal plates of the Herapathite are evi- 

 dently pale red ; indeed there are many crystals, which, being 

 unequally thick, exhibit both red and green parts. The more 

 evident the red, so much the more in the examination by polar- 

 ized light does the tint polarized ill the direction of the axis 

 approach to a perfect absence of colour, so that the crystals are 

 not at all seen, while the image polarized perpendicularly to it 

 appears more or less deep red. 



The shades of colour of the " Herapathite " may be represented 

 in the following form conformably to the above observations. 



1. Substance-colour. — In common light in very thin crystal 

 plates, pale red, between rose and brick-red ; in thicker crystals, 

 pale olive-green or greenish -gray, which with increased thick- 

 ness is sometimes yellowish. 



In polarized light, according to the thickness, the image polar- 

 ized in the direction of the axis, colourless to a pale green ; the 

 image polarized perpendicularly to the axis, blood-red to black ; 

 black even with a thickness of 3-^dth of an inch. 



2. Surface-colour. — Polarized perpendicularly to the axis, with 

 an almost perpendicular incidence of the light grass-green, with 

 a greater angle of incidence in the plane of the axis, verdigris, 

 " drake v blue ; dark steel-blue. 



When polished on glass (auf Glas aufpolirt) the substance- 

 colour is a reddish-brown, there appears no metallic green ; but 

 in all azimuths polarized perpendicularly to the plane of inci- 

 dence, a not very bright surface blue. 



"Wohler's green hydrochinon, which I examined some time 

 since, gives when polished in all azimuths from a metallic 

 pinchbeck-brown to a brass-yellow, in connexion with a violet- 

 blue substance-colour. 



In crystals, the pinchbeckbrown is polarized perpendicularly 

 to the axis, corresponding to the tint of least transparency in the 

 substance-colour. With greater inclinations polarized perpendi- 

 cularly to the plane of incidence, blue is added, so that the colour 

 in the lower image of the dichroscopic lens changes eventually 

 into dark steel-blue. 



