138 Prof. Stefan on the Dispersion of Light by Quartz, 



larization may be exhibited in the following manner : — Polarized 

 light is passed through the rotating medium, made to fall upon 

 a conical mirror, which serves as an analyzer, and projected upon 

 a screen placed perpendicular to the axis of the mirror. The 

 white light falling upon the cone appears spread out into a 

 coloured fan. Or a plate of calcareous spar is introduced into a 

 polarizing apparatus, so that the ring-like figures appear small 

 and near the centre of the field, whilst the black cross is spread 

 over the whole field. If the pencil of rays, where it consists of 

 parallel rays, be passed through a plate of quartz cut perpen- 

 dicular to the axis, then the black cross just mentioned will be 

 transformed into a coloured fan. 



The occurrence of dispersion through refraction, or through 

 alteration of the plane of polarization, leads to the conclusion 

 that in the one case the refractive index, and in the other the 

 angle of rotation is a function of the length of the undulations 

 of a colour. Each colour is determined by the length of the 

 undulations, also by the refractive index, or by the angle of rota- 

 tion in a given substance. There must therefore be a connexion 

 between the two last-named quantities. This connexion may 

 be disclosed by a prismatic analysis of the light as it leaves the 

 polarizing apparatus. 



The rotation of the plane of polarization is proportional to 

 the thickness of the plate of quartz. When the latter is con- 

 siderable, then the amount of rotation for the different colours 

 is equal to several complete revolutions. When the polarizer 

 and analyzer are placed parallel, the latter removes from the 

 light coming through the quartz all coloured rays which have 

 undergone rotations which are odd multiples of 90°. In the 

 places of these colours, dark bands appear in the spectrum. 

 In order to arrive at the number of the bands, the thickness of 

 the plate in millimetres should be multiplied by £ and | ; the 

 number of odd integers between the two products is the number 

 of bands. 



For the purpose of getting the bands as sharply defined as 

 possible, the following rule may be given : — Place the prism so 

 that it gives with a mean ray a minimum of deviation, and the 

 quartz plate so that the bands in the fixed spectrum have the 

 maximum deviation. The latter is the sign that the rays pass 

 through the quartz parallel to the optic axis 



On making the analyzer rotate, the bands pass from the red 

 towards the violet end, or the reverse, according as the analyzer 

 moves in the sense of the rotation of the plane of polarization 

 or the contrary. Thereby the number of bands may be altered 

 by a unit. 



The relative position of the bands is dependent upon the 



