WITHIN SOLID AND FLUID BODIES. 117 



by the first stratum is by no means very abundant, and the intromitted beam, 

 even after passing through one or 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 passing through the coloured medium. 



When the phenomena of internal dispersion ai-e exhibited in coloured fluids 

 and solids, the influence of absorption upon the dispersed light is very interesting. 

 Previous to its dispersion the light has the same colour as the transmitted light, 

 were it to emerge at that point of its path, and when viewed at an azimuth 

 above 90°, a portion of the dispersed light has that colour. The quantity of light 

 possessing this colour increases between the azimuth of 90° and 180°. In order 

 to see this effect disembarrassed from another influence, we must make the in- 

 tromitted 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 dis- 

 persion. 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 becoming yellowish green and whitish, 

 while that of the latter becomes whitish yellow. 



3. On the Polarisation of Disjyersed Light. 



As the dispersed light is turned from its path by reflection, and is reflected 

 at angles proper for polarising it, its partial polarisation at least might have been 

 anticipated. Sir John Herschel viewed it through a tourmaline, and states that 

 no signs of polarisation were perceived in it ; but his method of obtaining the 

 blue line from light diverging fi-om a large area of the sky, and therefore reflected 

 at various angles far above and far below the polarising angle, rendered it im- 

 practicable to detect its state of polarisation. The method which I adopted, of 

 using a narroAV cylindrical beam of strong light, affording a bright dispersed beam 

 more than an inch in length, enabled me to discover its polarisation, and to in- 

 vestigate its peculiarities. 



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 polarised in the plane of reflection, 

 while the more refi-angible of its rays, constituting an intensely blue beam, had a 

 different state of polarisation. 



This insulation of the bluer rays greatly increased the beauty of the pheno- 

 menon, and promised to throw some light upon its cause. I was therefore anxious 

 to ascertain their state of polarisation, which was not indicated by the analysing 

 rhomb. With this viewl transmitted through the solution a strong beam of polarised 



