186 



The famous experiment ofVonReusch 1 ) 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 

 of 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 2 ), and 

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

 mental investigations, among others those of Wyroubof f 4 ), regar- 

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

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

 sulphate, strychnine- selenate, diacetyl-phenolphtaleme , benzile, ethylene- 

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

 of sqme 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^, described by Wyrou- 

 bof f 5 ), the crystals of which are endowed with a strong rotatory power. 



If an individual be studied composed of several intergrown lamel- 

 lar crystals (fig. 143), local triangular spots are met with, consisting 

 of lamellae interwoven at angles of 60, which become extinguished 

 between crossed nicols at any angles; but these are just the parts 

 endowed with rotatory power. There cannot be the least doubt 

 here, that the rotatory power of these parts is governed by the crossing 

 of the biaxial laminae, in full accordance with Mallard's theory. 



In general it has become clear from these researches, that a great 



J ) E. von Reusch, Pogg. Ann. der Phys., 138, 628, (1869). 



2 ) L. Sohncke, Pogg. Ann. der Phys., Erganz. End., 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 to treat theoreti- 

 cally once more the problem of the optical effect of a pile of regularly arranged, 

 infinitely thin, biaxial lamellae. The result of his very general reasonings is in 

 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. 



4 ) 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, (1896). 



5) C. Wyrouboff, Bull, de la Soc. Miner. 13, 217, (1890). 



