32 



POLARISATION OF LIGHT. 



CHAPTER IX. 



Deviation of the polarised tints from 

 those of Newton's scale Mr. Her- 

 schel" s discovery of the different posi- 

 tions of the optic axes in the same 

 crystal for differently coloured rays 

 Deviation in crystals with one axis 

 Rings of Apophyllite, fyc. Pheno- 

 mena of Glauberite. 



HAVING considered in the preceding 

 chapter the form of the rings and the 

 law of their production, we come now 

 to consider the colours of which they 

 consist. M. Biot had taken it for granted, 

 in all his investigations, that they were 

 the same as the colours of thin plates ; 

 but in 1813, Dr. Brewster showed, in 

 his table of the colours of the rings in 

 topaz*, not only that they varied in dif- 

 ferent azimuths, but that there were even 

 colours developed at the extremity of the 

 two resultant axes. In his paper on the 

 laws of polarisation, written in 1817, 

 he remarks, " that in almost all crystals 

 with two axes, the tints in the neigh- 

 bourhood of the resultant axes, when 

 the plate has a considerable thickness, lose 

 their resemblance to those of Newton's 

 scale, as will be more minutely described 

 in another paper ;" (Phil. Trans., 1818, 

 p. 243,) and in April, 1817, he com- 

 municated to the Royal Society of Kdin- 

 burgh, his discovery of the extraordinary 

 system of rings in the apophyllite from 

 the Tyrol, in which the colours had not 

 the slightest resemblance to those which 

 appear in carbonate of lime, apatite and 

 beryl. In the prosecution of the subject, 

 he found that in biaxal crystals the 

 deviation was strong, as in tartrate of 

 potash and soda, and in acetate of lead ; 

 and that the crystals in which it was 

 found might be divided into two classes ; 

 viz. 1st. those that had the red ends of 

 the rings inward, or within the resultant 

 axes, and the blue ends outwards, or 

 without the resultant axes ; and 2nd. 

 those that had the red ends outwards, 

 and the blue ends inwards. Those crys- 

 tals in which the deviation is very 

 striking, are given in the following 

 table : 



CLASS I. Red ends inwards. 

 Nitre. 



Sulphate of Strontia. 

 B ary ta . 



* Phil. Trans. 1814, p. 204 20?. The phenomena 

 i represented in coloured drawings now in the 

 possession of the Royal Society. 



Phosphate of Soda. 



Tartrate of Potash. 



Hyposulphite of Strontia (Herschel). 



Sugar (Herschel). 



Arragonite. 



Carbonate of Lead. 



Sulphato-bi-carbonate of Lead. 



CLASS II. Red ends outwards. 

 Topaz. 

 Mica. 

 Anhydrite. 



Tartrate of Potash and Soda. 

 Native Borax. 

 Sulphate of Magnesia. 

 Arseniate of Soda. 



Crystals unclassed. 

 Chromate of Lead. 

 Muriate of Mercury. 



Copper. 



Oxynitrate of Silver. 



Sugar. 



Crystallised Cheltenham Salts. 



Nitrate of Mercury. 



Zinc. 



Superoxalate of Potash. 

 Oxalic Acid. 

 Sulphate of Iron. 

 Cymophane. 

 Feldspar. 

 Benzoic Acid. 

 Chromic Acid. 

 Nadelstein. 



This curious branch of the polarisa- 

 tion of light attracted the particular no- 

 tice of Mr. Herschel, who, by examining 

 the phenomena in homogeneous light, 

 discovered that the resultant axes differ 

 in situation within one and the same 

 crystal for the differently refrangible 

 homogeneous rays. " To make this evi- 

 dent," says he, " to popular inspection, 

 take a crystal of Rochelle salt (tartrate 

 of potash and soda), and having cut it 

 into a plate perpendicular to one of its 

 optic axes, or nearly so, and placed it in 

 a tourmaline apparatus, let the lens be 

 illuminated with the rays of a prismatic 

 spectrum, in succession, beginning with 

 the red, and passing gradually to the 

 violet. The eye being all the time fixed 

 on the rings, they will appear for each 

 colour of perfect regularity and form re- 

 markably well defined, and contracting 

 rapidly in size as the illumination is 

 made with more refrangible light; but, 

 in addition to this, it will be observed 

 that the whole system appears to shift 

 its place bodily, and advance regularly 

 in one direction as the illumination 

 changes ; and, if it be alternately altered 



