136 



AirA S f-TIRALS — FEESKEL S PRISM. 





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It will be easily conceived tliat a right 

 handed and a left-lianded crystal of equal 

 thickness?, superposed upon each other, 

 will produce a resultant rotation equal to 

 zero. But two such plates so super- 

 posed, examined in polarized light, ex- 

 hibit a remarkable spiral cross, such as is 

 seen in the figure annexed. These spi- 

 rals were first observed by Mr. Airy, and 

 are commonly known as Airy's spirals. 



Two contrary plates of unequal thick- 

 ness, superposed as above, produce an 

 amount of rotation proportional to their 

 difference of thickness. 



The power of rotation of the same 

 crystal is different for the different colors, 

 being, on the undulatory theory of light, an inverse function of the length of 

 the undulations. By employing the successive colors of the spectrum sepa- 

 rately, Mr. Biot determined the absolute rotatory power of a crystalline plate of 

 quartz one twenty-fifth of an inch in thickness, for each, as follows : 



Extreme red 17.4964 



Limit, red and orange 20.4798 



Limit, orange and yellow — .22.3138 

 Limit, yellow and green 25.6752 



Limit, green and blue 30.0460 



Limit, bhie and indigo 34.5717 



Limit, indigo and violet 37.6829 



Extreme- violet 44.0827 



'J'his property of rotatory polarization does not exist in plates of quartz cut 

 parallel to the axis. In such plates ordinary double refraction exists ; but it is 

 the extraordinary instead of the ordinary ray whose velocity is least, or the 

 crystal is a positive one. 



The physical cause of rotatory polarization is unknown. Mr. Biot supposed 

 it to belong to the ultimate molecules of the substance ; but this hypothesis Sir 

 David Brewster believed to be disproved by the fact that the property ceases 

 to appear in quartz Vi^hose crystalline structure has been destroyed by fusion. 

 This argument seems, nevertheless, not to be conclusive. If the property bo- 

 longs to the ultimate molecules, the fact that it does not appear when the crys- 

 tals are examined across the axis, proves that a regular arrangement of them, 

 presenting their similar sides in a common direction, is necessary for its display. 

 Fusion breaks up the regular arrangement, and thus destroys this essential 

 condition. The fact, however, that different crystals turn the ray, in different 

 directions, is apparently decisive against the hypothesis of Mr. Biot; and the 

 connexion of this difference of property with difference of crystallogi'aphic mcdi- 

 ficai ion, seems to indicate that the phenomenon is an effect of the structural 

 arrr.ngoment of the molecules. Indeed, it is observed, in, the fracture of quartz 

 crystals that there is occasionally something actually resembling a spiral ar- 

 rangement of parts. 



The di-uble reiraction.of quartz along its axis was experimentally analyzed 

 by Fiesr.el by means of a very ingenious arrangement. The difference of 

 ■\-elocity of theitwo rays being so slight as to render their separation by ordinary 

 expedients difficuli., he devised and constructed a compound prism by which to 

 double their dovergeacy. In the annexed figure, A13F and CDF represent 

 similar triangular prisms of right-handed quartz, with 

 the faces AB, CD, cut perpendicularly to the axis. 

 The obtuse-angled prism BFD, having the angle 

 BFD equal to the supplement of 2AFB, has its 

 base, BD, parallel to the axis of a crystal of left- 

 handed quartz. The incident ray II' falling per- 

 pendicularly upon AB, is separated into two, whose 



Fi- 20. 



