342* 



valuable letters of Mr Hume, the originals of wliicli had not come 

 into the possession of his executors, and that a considerable number 

 of these have appeared in various biographical and periodical publi- 

 cations. 



2. On the Optical Properties of Greenockite, by Sir David 

 Brewster, in a letter to Lord Greenock. 



Greenockite has the form of a regular six-sided prism, with py- 

 ramidal summits, the faces of the pyramid being inclined 36° 20' 

 to their base. The pyramids are sometimes truncated on their 

 summit. 



The crystallization is often composite. 



The index of refraction of Greenockite is 2.6882, corresponding 

 to the middle of the green space, and to the ordinary ray. Hence 

 Greenockite exceeds the Diamond in refractive power, and also 

 chromatc of lead, which I had found to surpass the diamond in 

 this respect. 



The double refraction of Greenockite is comparatively small, 

 which is not usual in substances of a high refractive power. It 

 is so small, indeed, that owing to its great dispersive power it is 

 exceedingly difficult to separate the two images. 



The polarising angle of Greenockite is 68° 36' for the red rays, 

 which corresponds to an index of refraction for that light of 2.5517. 



I found it very difficult to establish the existence of an uniaxal 

 system of rays along the axis of the prism ; but I succeeded in 

 doing this by light of the condensed rays of the sun, by which it 

 can alone be established ; for when in biaxal crystals one of the 

 axes is very weak, as in nitre, its influence on the rays is scarcely 

 visible in crystals of little thickness, such as those we meet with 

 in Greenockite. 



The uniaxal system of rings is negative, as in calcareous spar. 



The light left at the polarising angle is blue and pink. 



Professor Forbes, after reading the foregoing communication, 

 remarked that the uniaxal structure of Greenockite was ascer- 

 tained by himself with the aid of concentrated gas-light, and that 

 his notice on the subject was published in the Philosophical 

 Magazine for July 1840. 



