M 



NATURE 



[November 19, 1914 



Special features of the volume are the number 

 and quality of the illustrations. There is a yfood 

 index. 



The work is an cxceedingfly valuable contribu- 

 tion to anthropolog-ical literature, indispensable 

 for the student of primitive beliefs and cere- 

 monial. Sidney H. Ray. 



THE EVOLUTION OF THE PETRO- 

 GRAPHICAL MICROSCOPE. 



WHEN Henry Clifton Sorby laid the founda- 

 tion of the science of microscopical 

 petrology, in the year 185 1, the instrumental 

 means at his command were of the simplest 

 kind ; his microscope had attached to it two 

 Nicol-prisms, one above the eye-piece and the 

 other below the stag"e, the latter being- capable 

 of rotation, thus rendering it possible to study 

 the sections of minerals in rocks by plane polarised 

 light. Then, as is so often the case, necessity 

 became "the mother of invention," and Sorby 

 himself, as well as several of his followers, de- 

 vised additions to their microscopes which con- 

 verted them into more useful instruments for 

 investigating the optical properties of minerals, 

 as seisn in thin sections of rocks. The designers 

 of these improvements were, of course, dependent 

 on the able makers of optical instruments for 

 putting their suggestions into practical form. 



In the year 1876 the late Prof. Rosenbusch, of 

 Heidelberg, who had been led to the microscopical 

 study of rocks by Heinrich Fischer, one of the 

 earliest pioneers in this branch of research in 

 Germany, described "a new microscope for 

 mineralogical and petrologlcal researches." The 

 chief features in this microscope were an accur- 

 ately graduated, revolving stage, with verniers, 

 and a complex nose-piece enabling the objectives 

 to be rapidly changed. About the same time MM. 

 Fouque and Michel Levy — with the co-operation 

 of M. Emile Bertrand — had also turned their 

 attention to the improvement of this class of in- 

 struments. The eminent optical instrument-maker 

 of Paris, M. A. Nachet, carrying out their de- 

 signs, constructed a microscope which embodied 

 many advantageous features for petrographical 

 work. In this instrument the necessity for the 

 troublesome centring arrangements, for keeping an 

 object on the cross-wires of the field of view, is 

 got rid of by dividing the tube into two portions 

 moving independently, the upper section carrying 

 the eye-piece, analyser, and some accessory ap- 

 paratus, and the lower attached to the finely 

 graduated revolving stage bearing the objectives ; 

 these latter are easily changed by moving in a 

 slide with spring-catch. Another important addi- 

 tion to the instrument which we owe to the French 

 petrologists is the series of converging lenses with 

 a magnifying lens above, by which interference 

 figures may be viewed in the thin sections of 

 minerals in rock-slides. It is true that these 

 interference figures are only partial ones, but by 

 the aid of diagrams supplied by the authors of 



NO. 2351, VOL. 94] 



the method their interpretation is rendered 

 possible. 



Outside France, the Nachet instrument would 

 not appear to have come into very general use, 

 a fact which is perhaps accounted for by the 

 rather cumbrous arrangements necessitated by 

 the division of the uibe. In this country an 

 arrangement having the same object has been 

 devised by Mr. A. H. Dick, and has found much 

 favour with many petrologists. It consists in 

 having the rotating, polarising and analysing 

 prisms so connected that they can revolve to- 

 gether, while the stage is fixed. The forms of 

 the ordinary and Dick types of petrographical 

 microscope, as employed by the officers of the 

 Geological Survey of (ireat Britain are illustrated 

 in Figs. I and 2. 



Still more recently the celebrated Russian 

 crystallographer, Federow, has devised a form 

 of the mineralogical and petrographical micro- 

 scope, in which the crystal or section to be 

 examined is carried on a stage which is capable 

 of movement and inclination at exactly measurable 

 angles, by which means very important optical 

 determinations may be made. Instruments con- 

 structed on Federow 's plan are made by the 

 Societc Genevoise pour la Construction d'lnstru- 

 ments de Physique et de Mecanique, 87 \'ictoria 

 Street, London, S.W. 



As a matter of course, all the improvements in 

 the mechanical and optical arrangements in micro- 

 scopes introduced during the last fifty years have 

 been made available for the instruments con- 

 structed for mineralogical and petrographical 

 work. With the addition of many pieces of 

 accessory apparatus, such as sections and wedges 

 cut in definite directions from the crystals of 

 various minerals, stage goniometers, and special 

 arrangements for stage-movement, the instru- 

 ments of this class have now become, as will be 

 seen from the figures, more and more complicated 

 as the refinement of methods has increased. 



Not less important than these elaborate instru- 

 ments employed in research are the simpler forms 

 used in elementary and advanced geological 

 teaching, which must necessarily be produced at 

 much smaller cost. There are also special petro- 

 graphical microscopes made, which are adapted 

 for projection purposes in lecture-theatres, for 

 photographic work, for examining crystals and 

 sections while being heated and cooled, and for 

 studying the development of crystals in solutions 

 and fused materials. Examples of many of the 

 types of petrographical microscopes are exhibited 

 in the Science Museum at South Kensington. 



As a direct offspring of the petrographical 

 microscope, we may refer to the instruments now 

 so extensively employed in metallographical re- 

 searches. In 1864 Sorby, while studying sections 

 of meteorites, for the purpose of comparing them 

 with terrestrial rocks, was led to examine the 

 grains of nickel-iron in the " sporado-siderites," 

 after they had been etched, by reflected light. It 

 occurred to Sorby that the same method of study 

 might with advantage be employed in the case 



