1814.] New Properties of Light, IPS 



place them upon one another in any position but that which they 

 had in the original ciystal, they will be found to depolarise light in 

 every position, like caoutchouc, from the want of coincidence 

 among their neutral axes. 



It follows, therefore, from this theory, that every layer of caout- 

 chouc and gum arabic and horn is a doubly refracting and a 

 polarising crystal ; and that the agate when cut in one direction 

 gives two bright images ; whereas in another direction it is known 

 to give only a bright and a nebulous image. 



III. On the coloured Rings produced ly common and polarised 



Light. 



When a ray of common light falls upon a plate of topaz at an 

 angle of 60° 38' in a direction parallel to either of the longest 

 diagonals of its primitive rectangular prism,* and is reflected from its 

 posterior surface, it exhibits, when viewed through a bundle of glass 

 plates, a series of twelve or fourteen brilliantly coloured eliptical 

 rings, each of the first three or four containing all the prismatic 

 colours, and the rest being composed only of pink and blue. The 

 incident pencil is in this case polarised by reflection from the 

 posterior surface of tlie topaz. The rings are therefore produced 

 during the return of the ray from the posterior to the anterior sur- 

 face. The only use of the bundle of glass plates is to extinguish 

 the light reflected from the anterior surface, and prevent it from 

 overpowering the coloured rings. 



When the incident ray is polarised the rings are seen without 

 the bundle of glass plates, and the transmitted light displays rings 

 with colours complimentary to those which form the rings seen by 

 reflection. These rings undergo many beautiful transformations by 

 varying the circumstances under which they are produced, all of 

 which I have endeavoured to represent in coloured drawings. 



Whh a plate of topaz -jL^ of an inch thick, the conjugate 

 diameter of tlie fourtli red ring (the two central spots not being 

 reckoned) subtends an angle of 18° 30' ; and with a plate -^^ of 

 an inch tliick, the angle subtended by the same ring is 8° 25'. 

 Hence the diameters of the rings are inversely as the thickness of 

 the plates. 



I have discovered the same rings in the agate, in rock crystaly 

 mica, ice, amber, tartrate of potash and soda, sulphate of poiashi 

 prjLssiate of potash, nitrate oj potash, and in acetate of lead, melted 

 and crystallized between two plates of glass. 



By comparing the magnitude of the rings produced by topaz, ice, 



• If thii i> not the primitive form of the topaz, which there is reason to believe, 

 from »orne recent ohtervationv of llaiiy, the two positions of the incident ray may 

 be tlcjcribed in reference to the gener.il form of the crystals of topaz. They are 

 in a plan* which cuts at equal angles the two faces that contain an angle of 1S4* 

 t«', and they ure inclined 66* 38' lu the axis of the prism, or '29* V'2' to the surface 

 •f thr laminz. 



Vol. hi. N° III. N 



