260 On the Optical and Physiological Discoveries 



tention of our readers. When Mr Brooke attempted to put 

 down a general law * of nature on the single authority of a 

 hasty observation on the sulphato-tri-carbonate of lead, which 

 he made out to be rhombohedral, he erected that little fabric 

 of his imagination against a principle deduced from hundreds 

 of facts, and now admitted by every mineralogist who has ex- 

 amined it. Although it was hinted to Mr Brooke, that a law 

 without any exception required a number of facts, and these 

 well established, to bring it into doubt, and that it was possible 

 that his solitary observation might be wrong, or at least that 

 some explanation of the anomaly might be found if his obser- 

 vations were right, — yet this gentleman again presented him- 

 self at the bar of the public with fresh asseverations of the truth 

 of his observations, and committing the most unheard of tres- 

 passes on the fields of double refraction and polarisation, on 

 which he had no qualification to sport. The point at issue was 

 now taken up by Mr Haidinger, who examined a great varie- 

 ty of the finest crystals of the sulphato-tri-carbonate of lead. 

 This skilful mineralogist speedily found that the mineral in 

 question was a compound and not a simple crystal as Mr 

 Brooke had supposed, and, by measuring its angles with great 

 nicety, he proved, by actual calculation and experiment, that 

 its primitive crystal was not a rhombohedron, but belonged to 

 the hemiprismatic system of Mohs. Although Mr Haidinger 

 pointed out the very re-entering angles of the composition, and 

 though the combination is so plain to the dullest eye that we 

 have observed it in polarised light in more than an hundred 

 crystals without a single exception, yet, will our readers be- 

 lieve it, Mr Brooke again came forward, asserting the accura- 

 cy of his results, and questioning the demonstrations of Mr 

 Haidinger.-}- This dying struggle for error neither merited 

 nor has received any reply ; and those who have on this occasion 

 combated for the truth, have (to use the sentiment of Laplace) 



* We allude to the optical law of primitive forms, of which we shall 

 give an account in an early number. See Edinburgh Encyclopaedia, 

 Art. Optics, vol. xv. p. 572, &c. and Beudant's Traite Element. Mineral. 

 Paris, 1824, p. 167. 



t The able and elaborate paper of Mr Haidinger will be found in the 

 Edinburgh Transactions, vol. x. p. 21 7- See also this Journal, vol- ii. p. 73. 



