118 SIR DAVID BREWSTER ON THE DECOMPOSITION AND DISPERSION OF LIGHT 



light, and was surprised to find that the blue beam which it yielded by disper- 

 sion, retained the same intensity in every position of the analysing prism, and 

 therefore possesses a qitaquarersus polarisation, such as that which light receives 

 when transmitted through a congeries of minute doubly refracting crystals having 

 their axes in all possible directions. 



In making the same experiment with other dispersing fluids and solids, I 

 found some in which the Avhole beam was completely polarised in the plane of 

 reflection, and others in which it exhibited solely a quaquaversus polarisation ; but 

 as these experiments indicate new processes in the decomposition and polarisa- 

 tion of light, which require a more extended analysis, I shall resume the subject 

 in a separate communication, contenting myself at present with a general account 

 of the more important facts, and the results to which they lead. 



Having transmitted a condensed beam of light through an alcoholic solution 

 of the leaves of the Common Laurel, or of Tea, either green or black, I found that the 

 hnght red beam which it dispersed, possessed, like the blue one in the quiniferous so- 

 lution, a quaquaversus polarisation, a small portion of the light being polarised in the 

 plane of reflection. The green beam dispersed by the preparation of orcine, has the 

 same properties, the white portion of it disappearing and reappearing during the re- 

 volution of the analysing rhomb. In the aqueous solution of esculine,* the dispersed 

 pencil consists of two finely-contrasted pencils, the one whitish and polarised in 

 the plane of reflection, and the other a vertf deep blue, having quaquaversus pola- 

 risation. The white pencil is more intense than the blue one, which is the very 

 reverse of what takes place in the solution of quinine. The alcoholic solution of 

 the seeds of the Colchicum autumnale gives a bright and copious green beam of dis- 

 persed light, which consists of two pencils, one whitish and polarised in the plane 

 of reflection, and the other bright green, with a quaquaversus polarisation. The 

 same property is possessed by a solution of guiacuvi in alcohol, which disperses, 

 by the stratum chiefly near its surface, a beautiful violet light ; and also by an 

 alcoholic solution of sidphate of strychnine^ which disperses a green light, after it 

 has stood for some days. The same property is possessed by almost aU the oils, 

 in some of which the dispersed light is exceedingly beautiful, varying from a pale 

 green to a blue tint. 



The polarisation of the dispersed beam in one plane, namely, in the plane of 

 reflection, is exhibited in several fluids and solids. It is very marked in the bile 

 of the ox, which disperses an olive-green light ; in a solution of gum-myrrh in 

 alcohol, diluted with water, which disperses a bright white beam ; and in an 

 orange-coloured glass, which disperses a pale greenish beam. 



In many fluid solutions, the beam with a quaquaversus polarisation is very 

 intense, when compared with the faint pencil which is polarised in the plane of 



* In the alcoholic solution of Esculine, itie faint-blue approaches to violet. The polarisation is 

 like that in quinine. 



