EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 159 



phenomena of light. The rainbow had always struck 

 the attention of the most careless observers, and there 

 was no difficulty in perceiving that its conditions of 

 occurrence consisted in rays of the sun shining upon 

 falling drops of rain. It was impossible to overlook the 

 resemblance of the ordinary rainbow to the comparatively 

 rare lunar rainbow, to the bow which often appears upon 

 the spray of a waterfall, or even upon beads of dew 

 suspended on grass and spiders' webs. In all these cases 

 the uniform conditions are rays of light and round drops 

 of water. Boger Bacon had noticed these conditions, as 

 well as the analogy of the rainbow colours to those pro- 

 duced by crystals a . But the knowledge was empirical 

 until Descartes and Newton showed how the phenomena 

 were connected with all the other facts concerning the 

 refraction of light. 



There can be no better instance of an empirical truth 

 than that detected by Newton concerning the high re- 

 fractive powers of combustible substances. Newton's 

 chemical notions were almost as vague as those prevalent 

 in his day, but he observed that certain ' fat, sulphureous, 

 unctuous bodies/ as he calls them, such as camphor, oils, 

 spirit of turpentine, amber, &c., have refractive powers 

 two or three times greater than might be anticipated from 

 their densities b . The enormous refractive index of dia- 

 mond, led him with great sagacity to regard it as also 

 of the same unctuous or inflammable nature, so that he 

 may be regarded as predicting the combustibility of the 

 diamond, afterwards demonstrated by the Florentine 

 Academicians in 1694. Brewster having entered into a 

 long investigation of the refractive powers of different 

 substances, confirmed Newton's assertions, and found that 



a 'Opus Majus.' Edit. 1733. Cap. x. p. 460. 



k Newton's ' Opticks/ Third edit. p. 249. Leslie's 'Dissertation/ 

 Encyclopaedia Britaunica, p. 550. 



