POLARISATION OF LIGHT. 



23 



phenomena, Dr. Brewster took a plate of 

 the Spath Calcaire basee, which has a na- 

 tural face perpendicular to the axis, and 

 having; cut it into two pieces, and placed 

 them parallel to one another, with a suf- 

 ficient interval between them to admit 

 the interposition of different films, he was 

 enabled to see in succession all the modi- 

 fications which they underwent, but of 

 which it is not easy to convey any idea 

 without numerous drawings. 



CHAPTER VII. 



Double System of Rings produced by 

 Crystals, with two axes of Double 

 Refraction Phenomena exhibited by 

 them Table of the Inclination of the 

 resultant Axes of Crystals General 

 Law of the Tints. 



THE double system of coloured rings was 

 discovered in 1817, by Dr. Brewster, 

 who described them in the Philosophical 

 Transactions, as seen in Topaz, where they 

 appear with unusual splendour ; but the 

 axes or lines along which each system is 

 seen, are so much inclined to one 

 another, that we shall first describe them 

 as seen in Nitre, where they were soon 

 after discovered by the same author. 



Nitre or Saltpetre crystallizes in six- 

 sided prisms, with angles of about 120, 

 and its principal axis of double refraction 

 is coincident with the axis of this prism. 

 Having detached with the edge of a knife 



and the assistance of a smart blow of a 

 hammer, a small piece from the end of the 

 prism, grind it down upon a file or 

 coarse freestone till it is reduced to the 

 eighth or tenth of an inch ; and having 

 smoothed its parallel faces, which should 

 be as nearly as possible perpendicular to 

 the axis of the prism, wet each of them 

 slightly with the tongue, and having dried 

 them quickly, place the plate thus formed 

 between two plates of glass, so that a 

 thin layer of Canada balsam may cement 

 them to the glass. This will produce the 

 effect of the most perfect polish, and will 

 preserve the plate of nitre for years with- 

 out injury. 



Let the nitre thus prepared be placed 

 in the apparatus of Jig. 21, the eye look- 

 ing into the plate C, and seeing the black 

 undefined space already mentioned. When 

 the plane of the axis of nitre is parallel or 

 perpendicular to the plane of primitive 

 polarisation, there will be seen the beau- 

 tiful system of rings shown in Fig. 27. 

 This system is intersected with a black 

 cross, one of w r hose arms passes through 

 the centres of the two systems of curves, 

 while the other arm of the cross, which 

 is always less defined, is at right angles 

 to the former, and equi-distant from the 

 centres of the two systems. 



If we now turn the plate of nitre round 

 its axis, the revolution commencing at 

 0, the black cross will open, as shown in 

 Fig. 28. As the revolution advances, it 



Fig. 27. 



Fig. 28. 



D 



