XIX. On certain new Phenomena of Colour in Labrador Fel- 

 spar, with Observations on the nature and cause of its Change- 



able Tints. By DAVID BREWSTER, LL.D. F.R.SS. L. & E. 



, ff 



(Read May 20. 1829.) 



SIR ISAAC NEWTON'S theory of the colours of natural bodies, is 

 perhaps the most ingenious and lofty of all his speculations. It 

 was devised, however, at a time when the doctrine of light had 

 made comparatively but little progress, and before the disco- 

 very of various principles on which the colours of bodies must 

 depend, or by which, at least, they must be extensively modified. 

 The different dispersive powers of transparent substances ; the 

 irrationality of the spectrum ; the action of striated surfaces ; 

 the decomposition of polarised light ; the reflection of coloured 

 light at the confines of equally refracting media ; and the ab- 

 sorption of common and of polarised rays, are principles which 

 embrace within their individual range a great variety of facts to 

 which the Newtonian theory of colours bears no relation. In 

 that theory, indeed, we recognise more the flight of a transcen- 

 dant genius, than the patient and anxious step of inductive re- 

 search ; and so firmly has it entrenched itself among the strong- 

 holds of modern science, that no regular attempt has been made 

 to unsettle it, or even to submit to a rigorous analysis the va- 

 rious phenomena of colour, as displayed in mineral and vegetable 

 bodies, and in the artificial combinations of the laboratory. Such 

 a task I should not have presumed to undertake ; but in the 

 course of an extensive examination of minerals, the subject has 

 been forced upon my attention, and having extended the inquiry 

 to vegetable bodies, as well as to chemical combinations, I pro- 



