412 On the Decomposition and Dispersion of Light. 



dispersive power, or if the difference consists merely in the 

 unequal length of certain portions of the two spectra, then in 

 all these cases light will be dispersed by the extraneous ele- 

 ment. If, for example, we place a film of oil of cassia between 

 two prisms of flint glass, the light reflected from the film will 

 be blue. The index of refraction for certain of the red rays is 

 the same in the glass and in the oil, and consequently none 

 of these rays enter into the reflected pencil, which must there- 

 fore be blue, whatever be the inclination of the incident rays. 

 If we now suppose this film of oil to be solidified, and dissemi- 

 nated in infinitely small atoms through flint glass, or a fluid 

 that has the same action as the glass upon light, we should 

 have the phjenomenon of a blue disj^ersion*. 



A beam of blue light thus produced should be polarized at 

 the polarizing angle, and partially polarized at other angles ; 

 and if this is not its character, we must look for some cause 

 by which it has been counteracted. We have already seen 

 that, in the Bohemian yellow glass, none of the light is polar- 

 ized by reflexion ; and that in the quiniferous solution only a 

 part of it is so polarized, the whole pencil in the one case, and 

 the residual pencil in the other, having a quaquaversus polar- 

 ization. This effect cannot be the result of an opposite po- 

 larization by the refraction of the dispersed light at the surfaces 

 of the reflecting particles, because such an action would only 

 reduce the amount of polarization by reflexion ; and I have 

 found by direct experiment, namely, by making the blue light 

 pass through different thicknesses of the fluid, that such an 

 effect is not produced. Unless, therefore, we suppose that 

 this quaqiiaversus polarization is a new property of light, pro- 

 duced by a peculiar action of certain solid and fluid bodies, 

 we are driven to the conclusion, no less remarkable, that it is 

 produced by an infinite number of doubly refracting crystals, 

 liaving their axes of double refraction lying in every possible 

 direction, and therefore reflecting from their posterior surfaces 

 a pencil of light with quaquaversus polarization. 



St. Leonard's College, St. Andrews, 

 January 30, 1846. 



* In the experiment with jim-'^sinn blue, which is a very splendid one, 

 the particles are mechanically suspended in the water; so that we have 

 here an ocular demonstration that the particles are tiie canse of the disper- 

 sion and the quaquaversus polarization. 



