CIRCULAH POI.ARIZATIOX OF LIGHT. !S;> 



through a plate of definite thickness that we can determine, whether the 

 difference of path of the two rays be or — - — undulation. 



If, however, the same plate is examined in the different parts of the 

 spectrum, we obtain by the experiments just mentioned n itself. It is 

 manifest that if we wish to obtain by refraction phaenomena of circular 

 polarization in white light, it is advisable so to determine the thickness 

 of the plate or the temperature of tlie glass that the difference of path 

 for the central rays will become ^ undulation. For this purpose 1 use 

 the flame of alcohol coloured yellow by common salt or nitrate of soda. 



5. Phcencymena of Colours of combined Crystals in WJiite Light. 



It now becomes easy tp account for the complicated phaenomena of 

 colours obtained by the insertion of a crystallized plate parallel to the axis 

 and of any given thickness behind a crystallized lamina cut perpendicular- 

 ly to the axis. For as the light is circularly polarized for one colour on 

 the right, for the other on the left, and rectilinearly for an intermediate 

 one, the black tufts on their two sides assume different colours : the 

 phaenomena in the even quadrants differ essentially from those in the 

 odd ones, but the rings of colours in both are essentially different froni 

 the succession of colours in Newton's rings. The phaenomenon may be 

 previously determined from the known values of the indices of refraction, 

 the length of waves for the homogeneous rays of the spectrum, and the 

 thickness of the plate ; but it may also be experimentally exhibited by 

 adjusting the condensing-lens p of the apparatus so that the spectrum 

 in the aperture of the Nicol's polarizing prism e be concentrated to white; 

 a confirmation, the frequent repetition of which, however, is not advis- 

 able, on account of the intensity of the light of the apparatus. 



6. PhcEiiomena of Colours in Tvnn-Crystals. 

 In passing from the artificial combinations of two crystals to natural 

 twin-crystals ve have to distinguish them into three classes: namely, 

 the axes of the united individual crystals are either perpendicular or pa- 

 rallel to each other, or they are inclined at some angle with one another. 

 The section is always to be made perpendicular to the axis of one of 

 the individual crystals. Though the first case may immediately give 

 the phaenomena just mentioned, yet, as far as I am aware, it does not 

 occur with transparent crystals, whilst the second case may occasion the 

 phaenomena of colours with biaxal crystals only. Thus, if (as for in- 

 stance in arragonite,) a very thin crystal is so united with another that 

 its crystallographic axis lies parallel to the axis of the crystal which is 

 divided by it into two parts, these two parts (since the optical axes of 

 this lamella render perceptible, however small, angles with the bounding 

 planes,) will operate as double-refracting prisms upon the light passing 

 through these axes, because their optical axes du not lie in the plane uf 



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