exhibited by certain Non-metallic Substances, 397 



I hope to be able to add something to what has been already 

 done by M. Haidinger and Sir David Brewster, and it seemed 

 not out of place to mention the observations in which those 

 views originated. 



It appears, then, that certain substances, many of them of 

 vegetable origin, have the property of reflecting (not scattering) 

 light which is coloured and has a metallic aspect. The sub- 

 stances here referred to are observed to possess an exceedingly 

 intense absorbing action with respect to rays of the refrangibilities 

 of those which constitute the light thus reflected, so that for 

 these rays the opacity of the substances is comparable with that 

 of metals. Contrary, however, to what takes place in the case of 

 metals, this intense absorbing action does not usually extend to all 

 the colours of the spectrum, but is subject to chromatic varia- 

 tions, in some cases very rapid. The aspect of the reflected light, 

 which itself alone would form but an uncertain indication, is not 

 the only nor the principal character which distinguishes these 

 substances. In the case of transparent substances, or those of 

 which the absorbing power is not extremely intense (for example, 

 coloured glasses, solutions, &c), the reflected light vanishes, or 

 almost entirely vanishes, at a certain angle of incidence, when it 

 is analysed so as to retain only light polarized perpendicularly 

 to the plane of incidence*, which is not the case with metals. 

 In the case of the substances at present considered, the reflected 

 light does not vanish, but at a considerable angle of incidence 

 the pencil polarized perpendicularly to the plane of incidence 

 becomes usually of a richer colour, in consequence of the removal, 

 in a great measure, of that portion of the reflected light which 

 is independent of the metallic properties of the medium; it com- 

 monly becomes, also, more strictly related to that light which 

 is absorbed with such great intensity. The reflexion from a 

 transparent medium is weakened or destroyed by bringing the 

 medium into optical contact with another having nearly or exactly 

 the same refractive index. Accordingly, in the case of these 

 optically metallic substances, the colours which they reflect by 

 virtue of their metallicityf are brought out by putting the 

 medium in optical contact with glass or water. A remarkable 



* I do not here take into account the peculiar phenomena which have 

 been observed by Sir David Brewster with reference to the influence of the 

 double refraction of a medium (such as calcareous spar) on the polarization 

 of the reflected light, which, indeed, are not very conspicuous unless the 

 medium be placed in optical contact with a fluid having nearly the same 

 refractive index. 



t I use this word to signify the assemblage of optical properties by 

 which a 'metal differs from a transparent medium, or one moderately 

 coloured, suchja-s a coloured glass. 



