ON LIGHT. 263 



coloured glasses, crystals, resins, and liquids depend 

 upon the greater or less facility with which the several 

 coloured rays are transmitted through their substance. 

 There is no medium known, not even air or the purest 

 water, which allows all the coloured rays to pass through 

 it with equal facility. Independent of the partial re- 

 flexion which takes place at the surfaces of entry and 

 emergence, a portion greater or less according to the 

 nature of the medium, is always stifled, or as it is called 

 in optical language, " absorbed ." and this absorptive 

 action is exerted unequally on the differently refrangible 

 rays ; so that when a beam of white light is incident on 

 any such medium, it will be found at its emergence 

 deficient in some one or more of the elements of colour, 

 and will therefore have a tint complementary to that of 

 the absorbed portion. Supposing, as is most probable 

 in itself, and agrees with the general tenor of the facts, 

 that an equal per-centage of the light of any specified 

 colour which arrives at any depth within the medium is 

 absorbed in traversing an equal additional thickness of 

 it, the intensity of the coloured ray so circumstanced 

 would diminish in geometrical, as the thickness traversed 

 increases in arithmetical progression. The more absorb- 

 able any prismatic colour, then, the more quickly will it 

 become so much reduced in proportion to the rest as to 

 exercise no perceptible colorific action on the eye. And 

 thus it is found that in looking through different thick- 

 nesses of one and the same coloured glass or liquid, the 

 tint does not merely become deeper and fuller, but 

 changes its character. Thus a solution of sap-green, or 



