Obituary. 433 



formed in a bead could be recognised. In this way he to some 

 extent foreshadowed the methods so beautifully developed by 

 Boricky, Behrens and others., known as " Microchemical Analysis." 



In the same way the examination of the polished and etched 

 surfaces of the metallic meteorites — and subsequently of artificial 

 irons and steels — led Sorby to devise that useful method of 

 illumination, the parabolic reflector. 



A method of determining the refractive index of substances 

 had been devised more than a century ago by the Due de Chaulnes. 

 But it remained nothing more than an interesting suggestion till 

 Sorby showed how, by adding a graduated circle to the tine-adjust- 

 ment and the employment of suitable gratings, the Microscope 

 could be converted into a refractometer of great value in identify- 

 ing minerals in the thinnest rock-sections. 



Subsequent devices, as shown in the pages of this Journal, 

 enabled him to solve the problem of determining double refraction 

 under similar conditions. 



In successive editions of Dr. Lionel Beale's useful manual, 

 " How to Work with the Microscope," Sorby supplied a series of 

 brief instructions concerning the new methods he had introduced 

 for making thin sections of rocks and minerals, for determining 

 refraction and double refraction, and for studying absorption and 

 other spectra with the Microscope. 



2. It is an almost impossible task even to enumerate the 

 highly curious, and often important, discoveries to which Sorby's 

 ingenious instrumental appliances and original methods conducted 

 their author. 



By the microscopical study of coals and limestones he was led 

 to highly important conclusions concerning the polymorphism of 

 carbon and calcium carbonate ; while his investigations of iron- 

 stones and dolomites showed how great a part is played by pseudo- 

 morphism in the determination of the characters of those rocks. 

 When he came to study slates and schists in thin slices under the 

 Microscope, the theories of cleavage and foliation, by which he will 

 always be remembered by geologists, suggested themselves to Ins 

 mind. And, in the end, his study of the minute cavities in the 

 crystals of rocks with their liquid contents — including super- 

 saturated alkaline solutions and carbon dioxide — resulted in his 

 far-reaching generalisations concerning the conditions under which 

 deep-seated and erupted igneous rocks must have consolidated. 



An examination of the curious phenomenon of impressed 

 pebbles was to Sorby the starting point in a series of ingenious 

 speculations, which culminated in the doctrine enunciated in his 

 Bakerian lecture, " On the Direct Correlation of the Mechanical 

 and Chemical Forces." 



In the same way the study of meteorites, by the aid of the 

 Microscope, led him to many ingenious deductions concerning the 



Aug. 19th, 1908 2 G 



