408 Sir D. Brewster om the Decomposition and Dispersio7i 



more undispersing strata, is dispersed nearly as copiously as 

 before. In the glasses and in the vegetable solutions there 

 are no peculiarities which require explanation, excepting those 

 which arise from the absorption of the dispersed beam in pass- 

 ing through the coloured medium. 



When the phasnoniena of internal dispersion are exhibited 

 in coloured fluids and solids, the influence of absorption upon 

 the dispersed light is very interesting. Previous to its di- 

 spersion, the light has the same colour as the transmitted light; 

 were it to emerge at that point of its jiath, and when viewed 

 at an azimuth above 9QP, a portion of the dispersed light has 

 that colour. The quantity of light possessing this colour in- 

 creases between the azimuth of 90^ and 180°. In order to 

 see this effect disembarrassed from another influence, we must 

 make the intromitted beam parallel to the surface of the fluid 

 or solid, so as just to graze it. In this way the dispersed light 

 is not changed in its passage to the eye after dispersion. When 

 the beam passes through the coloured medium without this 

 precaution, it again suffers absorption proportional to the 

 thickness of the coloured substance through which it has 

 passed, and sometimes disappears altogether. This effect is 

 finely seen in the darker solutions, which disperse a brilliant 

 red, or a brilliant green light ; the colour of the former be- 

 coming yelio-iioish-green and ixihitish, while that of the latter 

 becomes 'whitish-yellow. 



3. On the Polarization of Dispersed Light. 



As the dispersed light is turned from its path by reflexion, 

 and is reflected at angles proper for polarizing it, its partial po- 

 larization at least might have been anticipated. Sir John Her- 

 schel viewed it through a tourmaline, and states that no signs of 

 polarization were perceived in it ; but his method of obtaining 

 the blue line from light diverrring from a large area of the sky, 

 and therefore reflected at various angles far above and far 

 below the polarizing angle, rendered it impracticable to detect 

 its state of polarization. The method which I adopted, of 

 using a narrow cylindrical beam of strong light, affording a 

 bright dispersed beam more than an inch in length, enabled 

 me to discover its polarization, and to investigate its pecu- 

 liarities. 



Upon examining the blue beam in the quiniferous solution 

 with an analysing rhomb of calcareous spar, I found that a 

 considerable part of it, consisting chiefly of the less refrangible 

 portion of its rays, was polarized in the plane of reflexion; 

 while the more refrangible of its rays, constituting an intensely 

 blue beam, had a different state of polarization. 



