684 



NATURE 



{Oct. 24, 1878 



It is now proved that the great philosopher occupied an attic in 

 the house, No. 28, Paveljoens-gracht in Doublet Straat, opposite 

 to the Ploly Ghost House (hoffje). The house belonged to a 

 Heer van der Spyk. Shortly a marble tablet with a fitting 

 inscription will be placed over the entrance. 



The German Geological Society met at Gottingen in the last 

 week of September. The first lecture was delivered by Prof. 

 Credner, of Leipzig, who spoke on the granite of Geyer. Then 

 Prof. Klein, of Gottingen, spoke on a collection of thin sections 

 of minerals forming rocks. — Herr Levin presented a petrified 

 starfish {Asieria cilicia) found by him in the upper Muschelkalk 

 of the Hainberg. This specimen is particularly interesting, 

 being the first starfish which has ever been found in the North 

 German limestone. — A communication was read from Prof. 

 Martin, of Leyden, on the tertiary fauna of Java. — Dr. Hornstein 

 (Cassel) reported on a new treatise on Eozoon by Prof. Mobius 

 (Kiel), which will be published in Parts 5 and 6 of the 

 " Palaeontographica," Prof. Mobius arrives at the conclusion, 

 from long-continued investigations of numerous j'specimens of 

 Eozoon, that the latter is of inorganic nature. — Then fol- 

 lowed some minor communications from Herr von Groddeck, 

 Prof. Streng, and Prof. Weiss (Berlin), the latter pointing to a 

 recent discovery of coal in the so-called "Eherne Kammer," 

 some few miles to the south of Eisenach. The other speakers 

 at the meeting were Prof, von Seebach (Gottingen), Herren 

 Grotrian (Brunswick), Romer'r(Breslau), Schmid (Jena), and 

 Prof, vom Rath (Bonn). The Society will meet at Baden Baden 

 in 1879. 



According to a communication made at the Berlin meeting 

 of ornithologists by Dr. Brehm, the Crown Prince Rudolf of 

 Austria is about to publish a work on Eagles, in conjunction 

 with Drs. Brehm and von Homeyer. 



We have on our table the following works: — "Commercial 

 Products of the Sea," P. L. Simmonds (Griffith and Farran) ; 

 " Talks about Plants," Mrs. Lankester (Griffith and Farran) ; 

 "Notes on a Tour in America," H. Hussey Vivian (E. Stan- 

 ford); "Philosophical Fragments," Dr. Morell (Longmans); 

 " Geological Survey of Canada : Report of Progress for 1876 and 

 1877," "The Germ Theories of Infectious Diseases," J. Drys- 

 dale, M.D. (Bailliere) ; "Sedimentary Formation of New 

 South Wales," Rev. W. B. Clarke (Triitner) ; " Annual Report 

 of the Department of Mines of New South Wales for 1877" 

 (Triibner); "Australian Orchids," Parts 3 and 4, R. D. Fitz- 

 gerald (Triibner) ; " Geology of Ireland," G. H. Kinahan 

 (Kegan Paul, and Co.); "Animal Chemistry," C. H. King- 

 zett (Longmans); "Manuel du Voyageur," D. Kaltbrunner 

 (Wurster and Co., Zurich) ; "Report on the Geological Survey 

 of the United States," Dr. Hayden. 



The additions to the Zoological Society's Gardens during the 

 past week include a Squirrel Monkey {Saimaris sa'urea), from 

 Guiana, presented by Mr. Edward Calthrop ; an Emu {Dromaius 

 nova-hollandia:), from Australia, presented by Mr. C. Hampden 

 Wigram; two Radiate 1 Tortoises {Testudo radiata), from Mada- 

 gascar, presented by the Rev. G. H. R. Fisk, C.M.Z.S.; a 

 Water Rail {Rallus aqtiatiais), British Isles, presented bj Capt. 

 F. H. Salvin; a Michie's Tufted Deer {Elaphodus cephalophus), 

 from China; a Naked -throated Bell-bird {Chasinorhynchus 

 nudicollis); three Blue-bearded l^.-^?, (^Cyanocorax cyanopogoti) ; 

 a Dark- winged Buzzard {Leucopternis scoiopterus), from Bahia; 

 a Saturnine Mocking Bird {Mimus saturninus) ; two" Lined 

 Finches {Spermophila lineola)\ two Gutteral Finches {Spermo- 

 pkila gutteralis), from Pernambuco; a Pileated Song Spartrow 

 {Zonotrichia pileata), from Rio de Janeiro; a Pain Tanager 

 {Tanagra palmarum), from Monte Video, deposited; a Patas 

 Monkey {Cercopithecus ruber), from West Africa; two Ruddy 

 Sheldrakes ( Tadorna rutila), European, purchased. 



ON A NEW METHOD OF STUDYING THE 

 OPTICAL CHARACTERS OF MINERALS^ 



A S is well known, the optical characters of minerals furnish 

 ■^ us with a most valuable means for identifying the various 

 species. The practical application of these phenomena has, 

 however, been much restricted by the difficulty of obtaining 

 crystals sufficiently large and transparent to be cut into appro- 

 priate sections, so that the properties of some of the commonest 

 minerals were very imperfectly known. By the methods hitherto 

 employed it was almost impossible to study the black or imper- 

 fectly transparent minerals constituting the chief part of rock 

 masses, and in fact little could be learned unless the specimens 

 were so large and perfect that the individual species could be 

 identified by other means. What we want is a method which 

 will enable us to ascertain the approximate value of the principal 

 optical constants, when we have at our disposal only detached, 

 small, and imperfect crystals in their natural state, or those 

 scattered about in thin sections of rocks, cut into plates Avhich 

 are inclined at every varying angle to the optic axes. 



The method now to be described satisfies these requirements 

 sufficiently well. Time would not permit me to give a full 

 description of the apparatus, the manner of using it, and the 

 conclusions to be drawn from the observed data. I must there- 

 fore avoid all unnecessary detail, and confine myself to such an 

 outline as will serve to indicate the general character of the 

 method. I may here say that I have received most valuable 

 assistance from Prof. Stokes in the mathematical part of the 

 subject. 



When, many years ago, I first commenced to apply the micro- 

 scope to the study of minerals, I was told by a well-known pro- 

 fessor of mineralogy that it would never be possible to learn 

 much by the use of that instrument. If we merely magnify a 

 portion of a pure transparent crystal, we do, indeed learn little 

 or nothing ; biit if, instead of viewing the crystal itself, we look 

 through it with a suitable magnifying power at some appropriate 

 object, we can learn more facts of interest and importance than 

 by any other single method whatever. The property possessed 

 by the object-glass of collecting divergent rays to form an image 

 gives rise to an entirely new class of phenomena, and converts 

 the microscope into a most valuable apparatus for optical 

 research. The object examined through the crystals is the 

 image of a small circular hole, or of rectangular lines ruled on 

 a piece of glass, formed at the focal point of a well- corrected 

 achromatic condenser fixed below the stage, and so arranged 

 that the image is placed either just below or just above the 

 lower surface of the crystal. The divergent rays passing through 

 it to the object-glass are bent so that the focal length is, as it 

 were, increased by an amount depending on the thickness of 

 the crystal and its refractive power. In order to see the hnes in 

 focus, it is therefore necessary to move back the body of the 

 microscope. If we know the thickness (7*) and the amount of 

 the displacement of the focal length (^), we can calculate the 

 value of the index of refraction (yu) from the equation 



T 



These values are measured by means of a scale and vernier 

 attached to the body of the microscope ; and, with care, there 

 ought to be no error greater than -sinrTtb of an inch. The thick- 

 ness of the specimen {T) is determined by measuring the differ- 

 ence in the focal points for particles of dust on the surface of 

 the supporting glass and on the upper surface of the mineral. 

 In a similar manner the value of d is determined by the differ- 

 ence in the focal length for the lines of the grating seen through 

 the supporting glass with or without the specimen under examin- 

 ation. From the value of the index thus determined a small 

 amount must be deducted, depending upon the aperture and 

 correction of the object-glass, and, when great accuracy is 

 desired, several precautions must be taken to avoid a number of 

 possible small errors, which it would be tedious to explain in 

 detail. 



T 



In the equation, /* = — , it is assumed that the substance 



7' - d ^ 



possesses no double refraction, as in the case of glass or crystals 

 belonging to the regular system. When viewed through such a 

 substance, only one simple and undistorted image of the circular 

 hole can be seen, and both sets of lines are in focus at the same 



' Address by the Chairman, H. C Sorby, F.R.S., Pres. G.S., &c., at the 

 meeting of the Yorkshire Geological and Polytechnic Society, held in Selby 

 on March 13. 



