82 dove's experiments on the 



then will WJ — m, = (o — e) ( ^ — - I- 



As c — - is a constant quantity for a definite substance, the dif- 



ference m — >«; will be proportional to the quantity o—e. Hence it 

 follows, That when for one definite colour the light is circularly pola- 

 rized by an interposed crystallized lamina, it may for the other colours 

 be linearly and oppositely circidarly polarized, and that the difference 

 between the single colours increases ivith the thickness of the lamina and 

 with the intensity of the double refraction. 



If the incident light is circular for the centre of the spectrum, when 

 the diiference of path is \ for tliis centre, the light is not yet linear for 

 the extreme limits of the spectrum. If it is here linear in the red, with 

 a 4- undulation difference of path, in the blue it is circular. With 4 dif- 

 ference of path in the red, it will, if it is circular to the right, be linear 

 in the blue, and circular to the left in the extreme violet. Linear light 

 in the red, with difference of 'path 1 , gives on the left in the green a 

 circular light, in the indigo a linear light perpendicular thereto and 

 approaching the circular on the right in the extreme violet ; finally, on 

 the left, circular in the red, with difference of path 4, will give linear in 

 the yellow, circular on the right \A'here the blue passes into the indigo, 

 and perpendicular to it linear at the commencement of the violet, and 

 so forth. In order to prove this by experiment an equilateral jJrism of 

 Guinand's flint glass was placed upright, so that after the remoA^al of 

 the condensing-lens p the red end of the spectrum fell exactly upon the 

 aperture e of the Nicol's polarizing prism. The cube had by gradual 

 heating exhibited the phaenomena which corresponded to a diflTerence 

 of path of \, \, \ undulation, and the other coloured rays were brought 

 into the axis of the polarizing-apparatus, and the alteration of the Iceland 

 spar figure examined. This might easily be accomplished without re- 

 volving the prism, as the height of the instrument may be altered at 

 pleasure by means of the sliding-tube, as may its inclination by means 

 of the motion of the prismatic rod. Mica plates of various thicknesses 

 were examined in the same manner as the heated cube. The changes 

 may be seen most beautifully when, beginning with the violet, the instru- 

 ment is slowly lowered in the sliding-tube through the single colours 

 of the spectrum. The gradual transitions are, in respect to the diff"erence 

 of colours from one end of it to the othei", exactly the same as those 

 which are obtained by the heating and cooling of the cube. 



In the same manner the phaenomena, when the incident light is cir- 

 cularly analysed by a mica plate inserted before the Iceland spar, are 

 throughout similar to those before described. Instead of the homo- 

 geneous rays of the spectrum, we can of course also employ in these 

 experiments a monochromatic lamp or absorption by coloured glasses. 

 It is only when the light has been circularly polarized in one colour 



