POLARISATION OF LIGHT. 



suffer no change by turning the plate 

 round its axis. 



Let us now suppose th? plate of quartz 

 to be placed, at D F E G, in the polarising 

 apparatus shown in Jig. 39, p. 36, and 

 let us suppose that to an eye at E the 

 colour in the centre of the rins:s is red. 

 Let the analysing plate C be turned 

 round from right to left, keeping its in- 

 clination to A C invariable, and the red 

 colour will change successively to orange, 

 yellow, green and violet, the plate C ac- 

 quiring, as it were, by its rotation, the 

 power of reflecting these colours in suc- 

 cession, a result which is perfectly ex- 

 plained by supposing that the rays of 

 each of those colours are polarised in 

 different planes. - Upon trying various 

 specimens of quartz, M. Biot found seve- 

 ral in which the very same phenomena 

 were produced by turning the plate C 

 from left to right. Hence in reference 

 to this property some specimens of 

 quartz are right handed and others left 

 handed. 



In order to analyse this remarkable 

 property we must use homogeneous light. 

 When this is done, we find that the ray, 

 yellow, for example, is reflected by the 

 plate C, but when C is turned round, the 

 yellow ray becomes more and more faint, 

 and after a certain angle of rotation it 

 disappears. The homogeneous red dis- 

 appears at a less angle of rotation, and 

 the homogeneous violet at a greater 

 angle. 



By employing plates of quartz of 

 various thicknesses, M. Biot found that 

 for the same ray the arcs of rotation, 

 after which it disappeared, were pro- 

 portioned to the thicknesses of the 

 plates ; and that in the same plate they 

 were reciprocally proportional to the 

 squares of the lengths of their fits or to 

 the squares of the length of an undula- 

 tion. 



Supposing the thickness of the quartz 

 to be one millimetre or 1-2 5th of an 

 inch, the following were the arcs of ro- 

 tation for the different rays. 



Arc of rotation for one 

 millimetre of quartz. 

 Extreme red . . 17. 4964 



Mean red . . . 18.9881 



Limit of red and orange . 20.4798 



Mean orange . . 21 .3968 



Limit of orange and yellow 22 . 3 1 38 



Mean yellow . . 23.9945 



Limit of yellow and green 25 .6752 



Mean green . . 27.8606 



Limit of green and blue . 30.0460 



Mean blue . . 32.3088 - 



Arc of rotation for one 

 millimetre of (ju-irtz. 



Limit of blue and indigo . . 34.5717 



Mean indigo . . . 36.1273 



Limit of indigo and violet . 37.6829 



Mean violet . . . 40.8828 



Extreme violet . . . 44.6827 



From these curious facts it follows that 

 polarised light transmitted along the axis 

 of qliavtz comports itself as if the planes 

 of polarisation of its different rays re- 

 volved in the interior of the crystal, in 

 some crystals from left to right, and in 

 others from right to left. 



If we combine two plates of right- 

 handed or two of left-handed quartz, 

 the deviation of the plane of polarisation 

 of any ray will be equal to that which 

 would be produced by a plate whose 

 thickness is equal to the sum of 

 their thicknesses ; but if we combine a 

 plate of right-handed with a plate of 

 left-handed quartz, the effect will be 

 equal to that of a plate whose thickness 

 is equal to the difference of their thick- 

 nesses, and the deviation will be to 

 the riffht if the right-handed plate is 

 the thickest, and to the left if the left- 

 handed plate is the thickest. If the two 

 plates are equal, the tints will be entirely 

 obliterated, and if their axes coincide ri- 

 gorously, the black cross, though oblite- 

 rated in each of them separately, will ap- 

 pear in their combination. 



In examining the Amethyst, Dr. 

 Brewster discovered that it possessed 

 the power in the same specimen of turn- 

 ing the planes of polarisation both to the 

 left and to the right ; and upon a close 

 inspection, he found that this curious 

 mineral was actually composed of al- 

 ternate layers of right and left handed 

 quartz. This singular structure is seen 

 by cutting a plate out of a crystal of 

 amethyst, by planes perpendicular to its 

 axis ; and when such a plate is exposed 

 to polarised light in the apparatus Jig. 39, 

 it has the appearance shown in Jig. 47. 

 The three sets of veins here represented 

 correspond to the alternate faces of the 

 Fig. 47. 



