46 FUESXEL OX TIIK COLOrruS PRODUCED IX 



towards a luminous point, before which I had placed a scries of 

 glass plates to polarize the incident light. At the other extre- 

 mity of the tube I placed, at the angle of complete polarization, 

 two plates of glass not silvered and very slightly inclined towards 

 each other, so as to produce fringes of sufficient breadth. Then 

 observing with a magnifying glass the light thus reflected, I dis- 

 covered the existence of three systems of fringes which touched 

 and mixed slightly with each other, in consequence of the tube 

 not being sufficiently long. 



The middle system, proceeding from the superposition of the 

 fringes produced by the meeting of the rays which had suf- 

 fered the same refraction, was much more intense than the two 

 others, which resulted from the coincidence of the rays oppositely 

 refracted. The light was not sufficiently intense to enable me 

 easily to discover in these the position of the dark bands of the 

 first order; but it appeared to me, as far as I could judge, that 

 the distance of the centre of each of the systems on the right 

 and left from the centre of that in the middle was the breadth of 

 seven fringes. Another more precise experiment, detailed at 

 the end of this memoir, shows that the feeble colours produced 

 by this tube belong to the sixth order. 



Although the existence of double refraction in the oil of tur- 

 pentine establishes a great analogy between the phaenomenon of 

 its colorization and that presented by crystalline laminae cut pa- 

 rallel to the axis, yet nevertheless they differ essentially in many 

 respects. In the crystalUne laminae, the rotation of the rhom- 

 boid of calcareous spar produces a variation in the intensity of 

 the tint without changing its nature ; in the oil of turpentine, 

 on the contrary, the same motion of the rhomboid changes the 

 nature of the tint without diminishing its intensity. Lastly, the 

 tube containing this liquid may be made to turn upon its axis 

 without producing any change either in the nature or in the 

 vividness of the colours ; whilst on turning the crystalline laminae 

 in its plane, the colours are augmented or lessened until they 

 are reduced to a pure white. 



The singular modification which double total reflexion at 

 an azimuth of 45° impresses on polarized light, and which im- 

 parts to it the appearances of complete depolarization when ana- 

 lysed by a rhomboid of calcareous spar, does not deprive it, as is 

 known, of the properties of colouring crystallized laminae. These 

 tints have even as much vividness as those produced by ordi- 



