22 Mr. L. Wright on Spired Fin uses observable in Crystals, 



embrace within itself, in some visible form, optical characte- 

 ristics of the two axes thus brought, temporarily or perma- 

 nently, into coincidence. This object seemed most likely to 

 be obtained by the aid of quartz, or some other substance pos- 

 sessing similar properties of rotary polarization. Such sub- 

 stances having, apart from their ordinary doubly-refractive 

 effects, two different axial velocities or waves capable of being 

 brought into interference, and the two axes of a biaxial being 

 according to hypothesis dissimilar, one being principal and 

 the other secondary to it, it seemed probable that by proper 

 means the two axes might be made to exert some kind of 

 differential or selective action upon the two sets of waves 

 passing through the rotary substance. I was confirmed in 

 this expectation by the curious double spiral, first noticed by 

 Sir George Airy, as displayed by quartz itself when subjected 

 to circularly polarized light, the cause of which appeared to me 

 to be connected with this very matter, as we shall presently 

 see to be the case. After observing with more care than usual, 

 therefore, the effects of quartz in combination with other crys- 

 tals in various ways, most of which have been described by 

 various observers, I finally adopted the following arrange- 

 ment : — We introduce first, next to the polarizer, a quarter- 

 wave plate, then (in the convergent rays) a plate of calcite, 

 and next to this a plate of quartz 5 to 1\ millim. thick. The 

 result of this arrangement is the system of double spirals, 

 mutually enwrapping each other, now on the screen (fig. 7). 

 This figure only changes in colour, or moves to or from 

 the centre, as the analyzer is rotated — though there are of 

 course only certain complementary positions of the polarizer, 

 related to that of the quarter-wave plate, which produce them. 

 The point to be here observed is the double character of the 

 spiral in this uniaxial crystal. 



This figure, however, so closely resembled in all but the 

 number of its convolutions the one exhibited by quartz alone, 

 as described by Sir George Airy in the Cambridge Transac- 

 tions for 1831, that it might possibly be due to the quartz 

 itself in the convergent light; it was necessary to see if there 

 were any differential results with other crystals, and, finally, 

 to see if they remained when any such possible cause for them 

 was removed. A single axis of sugar was therefore next 

 placed in the crystal-stage in place of the calcite; and the 

 result is again before you (fig. 8). Observe that, with this 

 single axis of a biaxial, we no longer have the double spiral, 

 but a single one, corresponding exactly to the supposed rela- 

 tion of which we are in search, and also showing that the 

 figures are not proper to the quartz as such, but to some selec- 



