POLARISATION OF LIGHT. 



53 



and grind it into the form of a wedge, as 

 shewn at A B,y/v. GO, and then give it 



Fig. 60. 



C E 



the doubly refracting structure, both its 

 positive and negative tints will increase 

 gradually from B to A, so that if the 

 maximum tint near A, is yellow of the 

 first order, it will shade off gradually, 

 and terminate in white near B. In a 

 wedge of this kind, two inches long, and 

 having an angle of 8, the highest tint 

 is between the blue and the white of the 

 first order, corresponding to 2.20 in 

 Newion's scale of colours, and the lowest 

 tint is between the black and the blue, 

 corresponding to about 0.8. We have 

 consequently a scale nearly tw r o inches 

 long to measure a variation in the 

 tints amounting to 2.2 0.8 = 1.4. The 

 method of using this wedge is shewn in 

 fig* 60. If it is required to ascertain 

 very exactly the tint of a plate of crys- 

 tallised glass C D, it must be held as in 

 the figure, and moved gradually from 

 A to B. When it has the position C D, 

 the intersectional figure m is opened ho- 

 rizontally, which proves that the tints of 

 the wedge A B, at the point m, are higher 

 than those of C D. In the position GH, 

 the figure at o is open vertically, and 

 therefore the tints of the wedge at o are 

 lower than those of the plate. But in 

 the intermediate position E F, a dark 

 cross is produced, which indicates the 

 perfect equality between the tints of the 

 wedge at n and those of the plate E F. 

 By a scale of equal parts, one of which 

 may be the one tenth or the twentieth 

 part of an unit in Newton's scale, 

 all tints may be compared with each 

 other, and referred to their exact place in 

 the scale of colours. This wedge is par- 

 ticularly suited as a vernier for the chro- 

 matic thermometer already mentioned. 



8. On the production of the doubly 

 refracting structure by evaporation and 

 gradual induration. In the beginning 

 of 1814, Dr. Brewster discovered that: 



the structure which produces double re- 

 fraction could be communicated to soft 

 substances by gradual and unequal indu- 

 ration ; and he afterwards published 

 some of his results in the Phil. Trans. 

 for 1816. 



When isinglass is dried in a circular 

 glass trough, and is placed in the pola- 

 rising apparatus Jig. 39, it exhibits the 

 black cross and four luminous sectors 

 like negative crystals with one axis of 

 double refraction. When a thin cylin- 

 drical plate of isinglass is indurated at 

 its outer edge, it gives the black cross 

 and four luminous sectors like positive 

 crystals with one axis of double refrac- 

 tion: 



A thin cylinder of isinglass, with a 

 hole in its centre like the glass tube 

 shown in Jig. 57, gives exactly the ap- 

 pearance there represented, but both 

 the structures are, in this case, positive. 



When jelly is evaporated in rectan- 

 gular troughs of glass, as A B C D, Jig. 6 1 , 



Fig. 61. 



B c 



the induration commences at the surface 

 a b, and fringes m n are formed parallel 

 to the surface, having the same structure 

 (viz. positive) as the external fringes in 

 plates of unannealed glass. The surface 

 a b sinks as the induration advances, 

 but at last the jelly adheres so firmly at 

 a and b to the sides of the trough, that the 

 surface a b becomes fixed. Hence, as 

 the induration proceeds, the softer jelly 

 about rs is expanded or drawn, as it 

 were, towards a b, in consequence of its 

 moisture escaping slowly through a b, 

 and its adherence to the more indu- 

 rated structure above it. The conse- 

 quence of this is, that an opposite or 

 negative structure is developed at r s, 

 and this structure is necessarily sepa- 

 rated from the negative structure at 

 m n by a black neutral line M N, in 

 which there is no double refraction. 



If the glass trough is open at its bot- 

 tom B C, so as to allow the induration 

 to take place there also as in Jig. 62, 

 positive fringes are formed at op, and 

 negative ones at r s, and these are sepa,- 



