514 NOTES TO BOOK IX. 



end, they are right-handed at the other end : but there 

 is a different kind of plagihedral form, which occurs in 

 some other crystals, for instance, in Apatite : in these 

 the plagihedral faces are right-handed at the one ex- 

 tremity and left-handed at the other. For the sake of 

 distinction, we may call the former homologous plagihedral 

 faces, since, at both ends, they have the same name ; and 

 the latter heterologous plagihedral faces. 



The homologous plagihedral faces of Quartz crystals 

 are accompanied by homologous circular polarization of 

 the same name. I do not know that heterologous circu- 

 lar polarization has been observed in any crystal, but it 

 has been discovered by Dr. Faraday to occur in glass, &c. 

 when subjected to powerful magnetic action. 



Perhaps it was presumptuous in me to attempt to 

 draw such comparisons, especially with regard to living 

 persons, as I have done at the end of this Book. Having 

 published this passage, however, I shah 1 not now sup- 

 press it. But I may observe that the immense num- 

 ber and variety of the beautiful optical discoveries which 

 we owe to Sir David Brewster makes the comparison 

 a very imperfect representation of his triumphs over 

 nature; and that, besides his place in the history of 

 the Theory of Optics, he must hold a most eminent 

 position in the history of Optical Crystallography, when- 

 ever the discovery of a True Optical Theory of Crystals 

 supplies us with the Epoch to which his labours in this 

 field form so rich a Prelude. I cordially assent to the 

 expression employed by Mr. Airy in the Phil. Trans. 

 for 1840, in which he speaks of Sir David Brewster as 

 " the Father of Modern Experimental Optics." 



