398 Prof. Stokes on the Metallic Reflexion 



character of metallic reflexion consists in the circumstance, that 

 as the angle of incidence increases from 0°, the phase of vibra- 

 tion of light polarized in the plane of incidence is accelerated 

 relatively to that of light polarized in the perpendicular plane. 

 Accordingly, the same change takes place, with the same sign, 

 in the case of these optically metallic substances ; but the amount 

 of the change is subject to most material chromatic variations, 

 being considerable for those colours which are absorbed with 

 great energy, but insensible for those colours for which the 

 medium is comparatively transparent, so that the absorption 

 may be neglected which is produced by a stratum of the medium 

 having a thickness amounting to a small multiple of the length 

 of a wave of light. If the medium be crystallized, it may happen 

 that one only of the oppositely polarized pencils which it trans- 

 mits suffers, with respect to certain colours, an exceedingly in- 

 tense absorption ; or, if that is the case with both pencils, that 

 the colours so absorbed are different. It may happen, likewise, 

 that the absorption varies with the direction of the ray within 

 the crystal. In such cases the light reflected by virtue of the 

 metallicity of the medium will be subject to corresponding varia- 

 tions, so that the medium is to be regarded as not only doubly 

 refracting and doubly absorbing, but doubly metallic. 



The views which I have just explained are derived from a com- 

 bination of certain theoretical notions with some experiments. 

 They have need of being much more extensively verified by ex- 

 periment ; but, so far as I at present know, they are in confor- 

 mity with observation. 



To illustrate the effect of bringing a transparent medium into 

 optical contact with an optically metallic substance, I may refer 

 to safflower-red. If a portion of this substance be deposited on 

 glass by means of water, and the water be allowed to evaporate, 

 a film is obtained which reflects on the upper surface a yellowish- 

 green light, but on the surface of contact with the glass a very 

 fine green inclining to blue. A green of the latter tint appears 

 to be more truly related to the colours absorbed with greatest 

 energy. Similar remarks apply to the light reflected by Hera- 

 pathite, according as the crystals are in air or in the mother- 

 liquor. If a small portion of Quadratite (platino-cyanide of 

 magnesium) be dissolved on glass in a drop or two of water, and 

 the fluid be allowed to evaporate, the tints reflected by the upper 

 and under surfaces of the film of crystals are related to one 

 another much in the same way as in the case of safflower-red. 

 For a fine specimen of the salt last mentioned I am indebted to 

 the kindness of M. Haidinger. I may mention in passing, that 

 the platinocyanides as a class are of extreme optical interest. 

 The crystals are generally at the same time doubly refracting, 



