investigated in its Optical Relations. 287 



the substance, and even communicated a letter from M. Kindt 

 in Bremen respecting it ; also that an experiment was made, 

 which, however, but imperfectly succeeded, and produced crystals 

 far too small. Wohler had also sent me some very small crystals, 

 but I neglected to examine them with a high magnifying power. 

 I now wrote afresh to Stokes and Wohler. The first kindly sent 

 me some crystals on a leaf of mica, and again communicated the 

 process of their formation, as given above, and as it nowhas yielded 

 distinct crystals to Dr. Ragsky likewise in our chemical labora- 

 tory. I afterwards compared with them Dr. Herapath's treatises. 



Through the dichroscopic lens I now saw the dichroism, but 

 for an accurate examination the crystals were quite too small. 

 But when I brought the Herapathic crystals on the stage of the 

 microscope with a magnifying power of ninety, everything was 

 clear at a glance. The different pale olive-green, pale red, deep 

 blood-red, black colours lying among one another were distin- 

 guishable in their true character, as well in ordinary as in po- 

 larized light, which with a compound microscope is very easily 

 produced by simply laying a rhomboid of doubly refracting spar 

 on the eye-piece ; you have then the two images oppositely po- 

 larized close to each other, as in the dichroscopic lens. In order 

 to have perfectly smooth surfaces to the spar, pieces of (thin) 

 plate-glass are cemented on the faces of the rhomboid in order 

 completely to restore the polish, which from the softness of the 

 spar is so soon injured. 



Now every single crystal was distinguished: you saw how 

 some of them in different places were unequally thick ; how the 

 image in the direction in which the light is more absorbed was 

 for the thicker parts quite black ; a black as midnight," says 

 Herapath, "even when the thickness of the crystal does not 



amount to j^dth °^ an mcn - -^ ut M was a l so seen tnat 

 thinner places of the same crystal were dark blood-red ; and that 

 consequently the black itself showed this absence of all colour only 

 because even the last red ray is absorbed by the thick crystal. 

 For the effect on the surface, therefore, the colour of the sub- 

 stance must always be considered as red, and to such the metallic 

 green polarized perpendicularly to the axis observed by Professor 

 Stokes is its true complementary colour. The crystals of the 

 iodo-quinine compound form, therefore, a fresh confirmation of 

 the validity of the position (which is favoured by all the before- 

 mentioned ones), that the surface-colour is connected with the 

 substance-colour as its complement. 



You could not easily have two kinds of crystals, which, in 

 relation to the substance-colour, would be more similar to each 

 other than the Herapathite of which we are now speaking, and 

 the transparent Brazilian Andalusite, — the same pale olive- green 

 polarized in the direction of the axis of the crystal, the same 



