M. W. Haidinger on the Colours of Mausite. 217 



It is true the lighter olive-gveen is sometimes rather more intense, 

 more yellow, but instead of the red tint a perfectly black one 

 appears ; in the direction of its axis the crystal is opake. 

 It would be possible to use plates of Mausite, about a line in 

 thickness, as we use the best tourmaline if they were cut parallel 

 to the axis, and placed between two polished glass plates. These 

 small prisms and plates are not so thick, but the tints may be 

 best compared through a dichroscopic lens, as shown in the 

 above figure. When the thickness is less, however, the hyacinth 

 becomes clearer, it passes into a liver-colour, which exhibits more 

 and more yellow, and passing through an actual oUve-green^ 

 indistinguishable from the colour of the axis in other specimens, 

 attains a still clearer tint. If a crystal be split with a knife, and 

 the splinter be magnified sixty times in order to examine it, the 

 two yellow tints are found almost to coincide, except that the 

 one is nearly colourless. Microscopes are now often constructed 

 for polarized light. But when this is not the case, it suffices to 

 hold' a crystal of Iceland spar between the ocular and the eye. 

 On the other hand, however, the above-mentioned broader, table- 

 like groups of crystals are themselves so dark as to have reached 

 the liver-coloured tint immediately preceding hyacinth-red. 

 Hence the series of colours for the ordinary and extraordinary 

 rays are identical, — white, olive-green, liver-colour, hyacinth-red, 

 black. But the ordinaiy ray is more absorbed, the darker tints 

 making their appearance at less thicknesses. At present the 

 proportion of these thicknesses corresponding to the same tint 

 for both rays, may be stated as being about 1 : 8 or 1 : 10, until 

 future experiments on larger and more complete crystals shall 

 have furnished numerical data; then, too, it may be decided 

 whether the more absorbed ray is also most refracted, as required 

 by the general law which embraces both Babinet's laws. 



The actual object of the present connnunication, however, is 

 to direct attention to an indubitable consequence of the above 

 observations; that is, the proof of the fact that there are 

 strong dichromatic crystals in which the differences of tint in 

 different directions do not arise from absolute diflFcrences of 

 colour, but depend upon the greater and less absorption of the 

 light in passing through such crystals. 



Certainly this difference arising from greater and less absorp- 

 tion could not account for the change of the dark or clear blue 

 tint of Cordierite into a yellowish-white, or, in darker crystals, 

 even to a honey-yellow tint ; neither can the blackish-blue (ink- 

 blue, purple), beautiful violet-blue, and asparagus-green colours 

 of diasporc arise, one from the other, by greater or less absorption. 

 Their theoretical consideration, in reference to the position and 

 form of the colouring bodies, or to the general arrangement of 



Phil. Mag. S. 4. Vol. 7. No. 44. March 1854. Q 



