276 The Founders of Geology LECT. 



minerals might be readily determined ; but where they 

 became fine-grained, nothing more could be said about the 

 nature and association of their constituents than might be 

 painfully deciphered with the help of a hand-lens. Hence 

 though not actually at a standstill, petrography continued 

 to make but slow progress. In some countries indeed, 

 notably in Britain, it was almost entirely neglected in 

 favour of the superior attractions of fossils and strati- 

 graphy. But at last there came a time of awakening and 

 rapid advance. 



In order to trace the history of this petrographical 

 resuscitation, I must ask you to accompany me to the 

 workshop of an ingenious and inventive mechanician, 

 William Nicol, who was a lecturer on Natural Philosophy 

 at Edinburgh in the early part of this century. Among 

 his inventions was the famous prism of Iceland spar that 

 bears his name. 1 Every petrographer will acknowledge 

 how indispensable this little piece of apparatus is in his 

 microscopic investigations. He may not be aware, how- 

 ever, that it was the same skilful hands that devised the 

 process of making thin slices of minerals and rocks 

 whereby the microscopic examination of these substances 

 has become possible. 



In the course of his experiments, Nicol hit upon the 

 plan of cutting sections of fossil wood, so as to reveal 

 their minutest vegetable structures. He took a slice from 

 the specimen to be studied, ground it perfectly flat, polished 

 it, and then cemented it by means of Canada balsam to a 

 piece of plate-glass. The exposed surface of the slice was 



1 See Nicol's original account of his prism in Edin. New Phil. Journ. 

 vol. vi. (1829), p. 83. 



