DOUBLE REFRACTION. 151 



Thus, then, the two rays on emerging from the first 

 crystal, instead of being changed in their nature, -are ex- 

 actly alike ; it suffices, to show them {indistinguishable 

 from each other, to make one of these rays turn round 

 the line of its own direction through 90. Thus we are 

 brought by the phenomena of double refraction to distin- 

 guish in rays of light different sides endowed with dif- 

 ferent properties. We are brought by observation to 

 acknowledge that the extraordinary ray emerging from a 



rhomb revolve continuously, we have four images (as at c) unequally 

 bright by pairs: at 45 four equally bright (D), the other pair now be- 

 come faint (E), until at 90 they are reduced to two, (F). The same 

 changes are repeated at G and H; when at 135 the four are equally 

 bright, till, after two become faint at i, we arrive at 180, where, as 

 at K, the two brighter coalesce into one. The same changes then 

 take place in reverse order; four images at 225, two at 270, four at 

 315, and lastly two at 360 \ 



To give any idea of the analysis of these phenomena, it is necessary 

 in the first instance to observe accurately the form of the crystal, and 

 obtain a distinct idea of the terms the axis and the principal section of 

 the crystals, which will be understood at once by the aid of the an- 

 nexed diagram; where taking the short diagonal of two of the oppo- 

 site faces of the crystal, as A D, the plane passing through it A D x, is 

 the principal section, and the diagonal of that plane A x, the axis of 

 the crystal. The double refraction of a ray R is represented by its 

 division into two rays, o the ordinary, and E the extraordinary. 



Whatever theory we adopt as to the nature of light, the phenomena 

 can only be explained by supposing a section of each of the rays 



