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five then known forms of this common mineral he added no fewer 

 that 135 new ones, many of them with complicated indices deter- 

 mined through the skilful use of zones : as to the remarkabla 

 stereographic projection prepared by Des Cloizeanx to illustrate the 

 observed forms of quartz, Professor Ruskin, speaking as an artist 

 and critic, has more than once expressed to the present writer his 

 great admiration of the "patient labour and entire accuracy of 

 workmanship " therein displayed. 



From 1855 onwards his researches were mainly optical. It must 

 be remembered that at the time when M. Des Cloizeaux entered 

 upon the study of minerals, the instrumental appliances for the 

 optical examination of small crystals were in the rudimentary stage 

 of development, and were scarcely in use outside physical labora- 

 tories. He improved the polarising microscope of Norremberg, 

 giving it a new form, increasing the field of view, and making it a 

 convenient instrument for the examination of small sections. 

 Thereupon, not only did he entertain the colossal idea of determin- 

 ing the optical characters of all known crystals, whether natural or 

 artificial, but he began the work. He was soon led to emphasise 

 the importance of the " optical sign " for the discrimination of 

 minerals, and in his determination of the crystallographic system to 

 make constant use of the peculiarities of the distribution of colour 

 in the rings afforded by the sections of crystals in convergent 

 polarised light. In 1868 he published his observations of the 

 changes of optic axial angle resulting from changes of temperature. 

 In the case of orthoclase he found that, with a maximum tempera- 

 ture of 400 C. only temporary changes were induced, but that with 

 a temperature exceeding 700 800 permanent changes resulted. 

 Further, he pointed out that in the case of orthoclase from some 

 volcanic rocks this changed optical condition was already a character 

 of the specimens. 



He was the discoverer of the circular polarisation of cinnabar, 

 and showed that it was seventeen times that of quartz. He was the 

 first to find a substance (strychnine sulphate) which rotates the 

 plane of polarisation, both in the crystallised state and in solution. 

 He showed that benzil circularly polarises when in crystals, but is 

 inactive when fused or in solution, and that, on the other hand, 

 camphors are active only when in solution. He was the first to 

 show, by optical characters, that there are ortho-rhombic members 

 of the epidote, pyroxene, and amphibole groups, and that the three 

 types of humite have characteristic optical features. 



But more especially was he interested in the felspars, a group of 

 minerals of fundamental importance in the classification of rocks, 

 and to the investigation of which he gave more than twenty years of 

 his life. When he began this work it seemed unlikely that much 

 was left to be discovered in the case of so long known a group, and 



