834 



REFRACTION. 



Hence it appears that the deviation of the extraor- 

 dinary ray is 0, when the light passes parallel to 

 the nxis, hut increases gradually, until the incident 

 light lx- perpendicular to that axis, where that 

 ,!. \ i. ui. in i- a maximum. 



lliiyens, by very careful observations on the 

 positions of the ordinary ami extraordinary rays at 

 different incidences, arrived at the general law, 

 that the reciprocal of the index of refraction of the 

 extraordinary ray was measured by the radius of 

 an eclipse, whose lesser axis is to its greater as the 

 reciprocals ot the greatest and least indices of ex- 

 traordinary refraciwn. 



Among the crystals with one axis, that being a 

 positive axis, qtiariz or rock crystal may be 

 selected as an instance. The index of refraction 

 for the ordinary ray in this crystal is 1-5484, and 

 this is also the index of the extraordinary ray when 

 the light passes along the axis ; but if the incident 

 rays changes its direction with respect to the axis, 

 it will be round that the index of the extraordinary 

 ray continually increases, until the ray be at right 

 angles, where it is a maximum, being 1/5582. 



Among the crystals which have two axes of 

 double refraction, may be mentioned glauberite and 

 sulphate of iron. M. Fresnel discovered the im- 

 portant law, that in crystals of this class, both the 

 rays follow the law of extraordinary refraction. 

 There is another interesting family of crystals, i. e., 

 those which have one axis for the most refrangeable 

 rays, and another axis for those rays which are least 

 refrangeable. An example of this occurs in glau- 

 berite, which has two resultant axes, inclined to 

 one another at an angle of 5. The power of 

 double refraction may be given artificially, either by 

 compression, or the rapid transmission of heat. 



Under our article Polarization of Light, we 

 presented to the reader's notice some curious phe- 

 nomena connected with the doctrine of colours. See 

 page 605, volume 5th. 



Biot thought that the colours obtained by 

 passing polarized light through a doubly refracting 

 crystal, were the same as those in Newton's scale ; 

 but Sir D. Brewster showed this not to be the case. 



In crystals where the displacement of the 

 coloured rings is very considerable, the two oval 

 central spots are drawn out into long spectra or 

 feather like tails of red, green, and violet, exhibit- 

 ing the appearance represented by the annexed 

 wood engraving. A curious phenomena will be 

 observed in these spectra, if we examine them 

 through coloured media, fitted to absorb the several 

 tints ; they will be found to consist of well defined 

 spots of the elementary colours, arranged on each 

 side of the principal section of tlie spectra, begin- 

 ning at the bottom with 

 violet, and ending at the 

 top with red. The ex- 

 periments on this branch 

 of the science of light 

 by Sir J. Herschel are 

 completely confirmatory 

 of Sir David Brewster's 

 theory, that all the tints 

 are related to two rectan- 

 gular axes, and that the 

 apparent axes passing 

 through the systems of 

 rings, are merely axes of 

 compensation. When Ro- 

 chelle salt is the crystal 

 employed, the length of 

 the spectra shown in the 

 figure above, is not less 

 than ten degrees. 



A new species of double refraction has been dis- 

 covered in experimenting with the icosatetrahe- 

 ilnniiil crystal Analcime, or Cubizite, represented 

 in the figure below, with its planes of double re- 



frrction, and polarization, and the tints of the 

 intermediate solids. The tints polarized by this 

 crystal are those of Newton's scale, and are nega- 

 tive in relation to each of the four axes, and there 

 is a distinct separation of the ordinary and extraor- 

 dinary rays when the light from any minute lumi 

 nous object passes through any pair of the four 

 planes adjacent to any one axis of the crystal. In 

 all other doubly refracting crystals, the refracting 

 axis has no fixed position in the crystal ; in this 

 the axes are fixed. 



As before observed, the doubly refracting pro- 

 perty may be given by the transmission of heat 

 through glass, and many curious phenomena may 

 thus be obtained. Our limits will not permit us to 

 enter into minute details on this subject ; but we 

 may give one example. Let there be two glass 

 plates made doubly refracting by heat, made to 

 pass from the surface to the centre of its structure, 

 and let the plates be crossed, as shown in the figure ; 



they will exhibit the appearance here shown. The 

 tints are raised in the square of intersection, A, B, 

 C, D, when the negative structure crosses the 

 positive, but depressed when the crossing struc- 

 tures are of the same kind. The curves of the 

 tints are hyperbolar. For an account of an inge- 

 nious application of this branch of science to the 

 measurement of heat, see Thermometer. 



