Colours. 213 



frangible rays, so as to afford the appearance of 

 green. These principles applied to many other 

 of the phenomena of nature will serve to explain 

 their causes ; and if they excite you but to use 

 your own understandings, and to think for your- 

 selves, this sketch of the phenomena of light and 

 colours may be of as essential service to you as 

 the most laboured detail. 



Since the former editions of this work were 

 published, philosophers have entered into a new 

 field of investigation in the region of optics. Be- 

 sides the properties of light indicated by the 

 words reflection, refraction, and Inflection, there 

 has recently been discovered another, denomi- 

 nated polarization. Dr. Sebeck in Germany, Dr. 

 Brewster in Scotland, and M. M. Malus and 

 Biot in France, are the philosophers to whom we 

 owe the principal discoveries in this new track of 

 inquiry. 



When the particles of light traverse crystal- 

 lized bodies, endowed with a double refraction 

 (such, for example, as Iceland spar), they expe- 

 rience about their centre of gravity divers mo- 

 tions, which depend upon the nature of the forces 

 which the particles of the crystal exercise upon 

 them. Sometimes the effect of these forces is 

 limited to disposing all the moleculae of the same 

 ray similarly the one to the other, in such manner 

 that their homologous faces are turned towards 

 the same parts of space. This is the phenomenon 

 to which Malus gave the name of polarization, 



