POLARISATION OF LIGHT. 



33 



from red to violet, and back again, the 

 pole, with the rings about it, will also 

 move backwards and forwards, vibrat- 

 ing, as it were, over a considerable space. 

 If homogeneous rays of two colours be 

 thrown at once on the lens, two sets of 

 rings will be seen, bavins: their centres 

 more or less distant, and their magni- 

 tudes more or less different, according 

 to the difference of refransibility of the 

 two species of light employed." 



This description will be understood by 

 referring to Jig. 27, where we may sup- 

 pose the rings round A to be those 

 viewed by the observer. In violet light, 

 they will approach towards CD, and 

 within 28 of it, while, in red light, they 

 will recede from CD as far as^38, the 

 rin^s formed by intermediate colours 

 having intermediate positions, the centres 

 of all these systems of rings lying in one 

 plane, viz. that of the principal section of 

 the crystal passing through AB. These 

 results will be still better seen by using 

 a crystal, in which both the systems of 

 rings round A and B are seen at once. 

 The centres AB of the two systems will 

 approach to, and recede from, each other, 

 according as violet or red light is used ; 

 so that, when white light is used, all the 

 systems, when seen at once, will form a 

 most irregular system. 



In the rings of topaz, and of other 

 crystals, Dr. Brewster observed the tints 

 to commence from black, at points which 

 he called virtual poles, because they 

 were different from the real poles. These 

 virtual poles lie between the resultant 

 axes in crystals of the first class, and 

 beyond them in those of the second 

 class, and are easily explained by the 

 compensation which takes place in con- 

 sequence of the displacement of the rings 

 for different colours. 



In crystals where the displacement of 

 the rings is very great, the two oval cen- 

 tral spots shown my^.32 are drawn out, 

 as Mr. Herschel observes, into long 

 spectra or tails of red, green, and violet 

 light, and the extremities of the rings 

 shown in the same figure, are distorted 

 and highly coloured, exhibiting the ap- 

 pearance shown in Jig. 35. If we examine 

 these spectra with coloured media, which 

 absorb different colours, they will be 

 found to consist of well defined spots of 

 the several simple colours, arranged on 

 each side of the principal section, as 

 shewn in fir. 36. The length of the 

 spectra within the crystal is, in Rochelle 

 salt, no less than ten degrees. 



Fig. 35, 



The discovery of Mr. Herschel, which 

 we have now explained, is a complete 

 proof, if any were wanted, of Dr. Brews- 

 ter's theory, that all the tints are related 



. 



W 



Fig. 36. 



- 



Orange. 



Yellow. 

 f Green. 



Blue. 

 Indigo. 



9 Violet. 



to two rectangular axes, and that the 

 two apparent axes passing through the 

 centres of the systems of rings, are merely 

 axes of compensation, or resultant axes. 

 If this were not the case, Rochelle salt 

 would have two axes for each different 

 ray of the spectrum, and the axes of 

 crystals would have no definite position, 

 and no lelation to the crystalline form of 

 the substance.* 



In order to explain the cause of the 

 displacement of the systems of rings in 

 homogeneous light, let the rings be pro- 

 duced by two negative axes C, A., Jig. 37, 

 of which C is of such strength, that it 



* See Mr. Herschel'- original paper, in the Mem. 

 Camb. Phil. &jc., vol. j. 



D 



