181 



ti"ii, mini t<> tin rii^'ht or to ilu- left, must necessarily occur. 



Thi^ niuvption is however more a description of the pin-noun -non 

 than an explanation, because it includes no rational cause, rithrr 

 \\liv tin one ray should be retarded in the crystalline medium with 

 1 1 -(nit to the other, or why the phenomenon, so far from being 

 a general one for such uniaxial crystals, is on the contrary a rela- 

 tively rare one. 



UK- famous experiment of Von Reusch l ) in 1869, who succeeded 

 in exactly imitating the phenomenon of the rotatory power in uniaxial 

 crystals by the regular piling up of a great number of biaxial laminae 

 ot mica crossing under angles of 45 and 60, gave a first indication 

 in which direction a solution of the problem might be looked for. 

 In point of fact the theory of the optical effect of such piles of 

 lamellae was developed in its base outlines by Sohncke*), and 

 more fully by Mallard 3 ) in 1876, while a great number of experi- 

 mental investigations, among others those of Wyrouboff 4 ) regar- 

 ding the properties of the crystals of quartz, cinnabar, potassium-, 

 rubidium-, calcium-, strontium-, and lead-dithionates, strychnine- 

 sulphate, strychnine-selenate, diacetyl-phenol-phtaleine, benzile, ethylenc- 

 diamine-sulphate , guanidine-carbonate, sodium-chlorate and -bromate, 

 of some uranyl-double-acetates, and of several other substances, have 

 strikingly confirmed the correctness of these views in a great 

 number of cases. 



One of the most beautiful examples of this kind is unquestionably 

 the ammonium-lithium-sulphate: (NH )LiSO t , described by Wyrou- 

 boff 5 ), the crystals of wich are endowed with a strong rotatory power 



1) E. von Reusch, Pogg. Ann. der Phys. 188. 628. (1869). 



2) L. Sohncke, Pogg. Ann. der Phys. Erganz. Bnd. 8. 16. (1876). 



3) E. Mallard, Ann. des Mines (7). 10. 119. (1876); ibid. (1881). 



At my request, professor Lorentz in 1905 was kind enough once more to treat 

 theoretically the problem of the optical effect of a pile of regularly arranged, 

 infinitely thin, biaxial lamellae. The result of his very general reasoning 

 qualitative agreement with the results obtained by Mallard, while quantitatively 

 there are some differences in the final values for the rotation-angle. In every case 

 the theory of the superposed lamellae may certainly be regarded as based upon a 

 perfectly sane supposition. 



) G. Wyrouboff, Ann. de Chim. et Phys. (6). 8. 340. (1886); Bull, de la Soc. 

 Miner. 7. 10, 49, 86. (1884); Cf. also: A. Bodlander, Inaug. Dissert. Breslau, 

 (1882); F. Klocke, Neues Jahrb. f. Min. 2. 97. (1880); C. Pape, Pogg. Ann. 

 139. 229. (1870); W. Barlow, Zeits. f. Kryst. 27. 468. (1986). 



6) G. Wyrouboff, Bull, de la Soc. Miner. 18. 217. (1890). 





