I 



WITHIN SOLID AND FLUID BODIES. 119 



reflection ; but in a specimen of yellow Bohemian glass, which gives a copious and 

 brilliant p'een beam by dispersion, the whole of the beam possesses a quaquaver- 

 sus polarisation. 



When we view the dispersed beam in different azimuths, some very interest- 

 ing phenomena present themselves to our notice. In general, the colour of the 

 dispersed light suffers a considerable change, passing, between the azimuths of 

 90° and 180°, from the colour of the dispersed beam to the colour of the trans- 

 mitted beam. This effect is finely seen in the alcoholic solution of tea, where the 

 brilliant red light passes into an olive tint ; but it is still more remarkable in a 

 mixture of Prussian blue and water. The dispersed beam is polarised in the 

 plane of reflection. It is bluish in the azimuth of 90° : pinkish about the azimuth 

 of 100°; greenish in that of 120°; bluish in azimuth 150°; and again innhish in 

 azimuth 170°. These three last tints may be all seen at the same time. 



Such are the general phenomena of internal dispersion, a subject which pro- 

 mises to throw some light on the constitution of those solid and fluid bodies by 

 which it is produced. The apparently superficial dis2)ersion in the quinine solu- 

 tion to which Sir John Herschel has given the name of ejnpolism, is obviously a 

 single case of the general phenomenon in which the ordinates of the curve of dis- 

 persion diminish rapidly after the light has entered the stratum nearest the sur- 

 face ; while the real epipolism which he ascribes to fluor-spar, so far from being 

 an action of the surface, is much less so than that of the quiniferous solution, and 

 entirely similar in its character to that which is produced by the fluids and solids 

 which I have examined. 



The phenomenon of internal dispersion, when considered merely as a case of 

 reflection and polarisation, possesses much novelty and interest. If the exciting 

 beam, as we may caU it, is cylindrical, we have before us an experiment, in which 

 the phenomena of cylindrical reflection, and cylindrical polarisation, are at once 

 exhibited to us. The innumerable reflecting surfaces, receiving the intromitted 

 beam at all possible angles, reflect the incident light in all possible directions, so 

 that the eye, wherever it is placed, sees the beam as if it were self-luminous ; and 

 while the eye is made to revolve in a circle round the cylindrical beam, it receives 

 a pencil of polarised light — polarised in a plane passing through the eye and the 

 axis of the cylinder ; or, what is the same thing, a thousand spectators viewing 

 this beam in the same azimuth, but in directions differently inclined to the hori- 

 zon, would all see exactly the same phenomena of reflection and polarisation ! 



4. On the Causes of the Internal Decomposition and Dispersion of Light. 



In imperfectly crystallized minerals, such as particular specimens of adularia, 

 chrysoberyl, opal, and sapphire, the white and coloured opalescence, and the 

 asterial radiations, have been shewn to arise from minute vacuities, or from open 

 spaces with crystallized sides, or from narrow pipes, or linear spaces parallel to 



VOL. XVI. PABT II. 2 H 



