216 On Absorption and the Colours of lliin Plates. 



rious substances *, and placing their axes in different azi- 

 muths to the plane of primitive polarization, we obtain ex- 

 tremely singular spectra, in which the bands approximate to 

 those of absorbing media. 



But there is another result of this class of experiments to 

 which I would especially call the attention of philosophers. 

 The colours of the bands thus produced have no resemblance 

 to those of the original spectrum, so that the spectrum has 

 actually been analysed by dissection. This effect is so de- 

 cided, that even by a single subdivision of a banded spectrum 

 I have succeeded in insulating a band nearly white, and of 

 course incapable of being decomposed by the prism. 



Hence we deduce from the phenomena of thin plates, and 

 polarized tints, the existence of a new property of light, in 

 virtue of which the reflecting force selects, as it were, out of 

 differently coloured rays of the same refrangibility rays of a 

 particular colour, allowing the others to pass into the trans- 

 mitted beam ; or to use the language of the undulatory theory, 

 the colour produced by the interference of homogeneous pen- 

 cils reflected from the first and second surfaces of thin plates, 

 is different from the colour produced by the interference of 

 the transmitted light with that which has suffered two inter- 

 nal reflexions within the plate. If, for example, we use the 

 greenish yellow light of the spectrum between the lines D and 

 E, the system of reflected rings will be more yellow than the 

 transmitted rings towards E, and more green than the same 

 rings towards D ; a result, which, in so far as the transmitted 

 tints are concerned, is seen in the colours of smalt blue glass. 



Here then we have a principle not provided for in either 

 of the theories of light to which the phaenomena of absorption, 



* I have constructed apparatuses of this kind made out of composite 

 crystals of calcareous spar, including one and more thin plates of its own 

 substance. The beautiful and apparently capricious tints which such cry- 

 stals exhibit when properly cut into prisms, or when prisms are applied to 

 their surface, are nothing more than the luminous bands of the spectrum 

 subdivided by one or more dissections. I have now before me such a cry- 

 stal, in which a prism cemented externally brings out the spectrum, which 

 would otherwise have suffered total internal reflexion. A virtual prism 

 forming part of the rhomb polarizes the incident light, an included hemi- 

 trope plate affords the polarized tints, and a second virtual prism analyses 

 the light which the plate transmits. In some parts of the rhomb there are 

 plates of different thickness, by which the luminous bands are beautifully 

 subdivided. In this manner, by the slight aid of an applied prism, we are 

 •furnished with a complicated optical apparatus. Such a combination, 

 which it is easy to make artificially by inclosing thin doubly refracting 

 plates between prisms of calcareous spar, affords an ocular explanation of 

 those beautiful forms of the system of' polarized rings which are produced 

 in composite crystals of calcareous spar. These subdivided bands, indeed, 

 are portions of that system seen obliquely by prismatic refraction. 



