THE MICROSCOPE IN GEOLOGICAL INVESTIGATION. 



315 



FIG. 493. 



709. In the application of the Microscope to Petrological and Minera- 

 logical research, the employment of Polarized light is constantly re- 

 quired; and various means and appliances are needful for its most advan- 

 tageous application, which are not required by the ordinary Microscopist. l 

 An instrument having been recently brought out by M. Nachet, which 

 combines all that the large experience of MM. Fouque and Michel Levy 

 has led them to think desirable for Mineralogical and Petrological invest- 

 igation, an account of it is here subjoined. In all Microscopes previously 

 constructed for this purpose, the rotation of the object on the Stage be- 

 tween the Polarizing and the Analyzing prisms was liable to put it out 

 of position in regard to the cross-threads in the eye-piece; as the center- 

 ing of the Objective is scarcely 

 ever so perfect as not to pro- 

 duce some displacement, and, 

 if the centering be adjusted 

 so as to be perfect for one 

 Objective, it is likely to be 

 faulty for another. Now, the 

 peculiarity of M. Nachot's con- 

 struction is, that the Eye-piece, 

 with its cross-threads and ana- 

 lyzing prism, remains fixed 

 above (being carried upon a 

 separate arm), whilst the Body 

 and Stage (with the object it 

 carries) can be made to rotate 

 altogether around the optic 

 axis, above the Polarizing 

 prism which remains fixed be- 

 neath; the angular amount of 

 this rotation being measured 

 by a graduated ring, and a ver- 

 nier attached to the stage. By 

 this arrangement, the object is 

 made to rotate between the 

 two prisms of the Polarizing 

 apparatus, without changing 

 its position beneath the Ob- 

 jective, and therefore without 

 displacing its image from its 

 contact with the cross-threads 

 of the Eye-piece. The mode 

 in which this plan is worked- 



Oct., 1867; the Treatise of Vogelsang, " Philosophic der Geologie und Mikrosko- 

 pische Gesteinsstudien," Bonn, 1867; various subsequent Memoirs by the same; the 

 Treatises of Zirkel, " Mikroskopische Beschaffenheit der Mineralien u. Gesteine," 

 1873, and "Microscopic Petrography" (U. S. Geological Exploration of Fortieth 

 Parallel), 1876; the Treatises of Rosenbusch, " Mikroskopische Physiographie der 

 petrographisch-wichtigen Mineralien," 1873 and " Mikroskopische Physio 

 graphie der massigen Gesteine," 1877; that of Jenzsch, "Mikroskopische Flora 

 u. Fauna Krystallinischer Massengesteine," 1868; that of Von Lasaulx, "Elemente 

 der Petrographie," 1875; and the great work of MM. Fouque and Levy, " Mine- 

 ralogie Micrographique, Roches Eruptives Francaises," Paris, 1879. 



1 The description of a Microscope specially" devised for this purpose by Mr. 

 Rutley and made by Mr. Watson (of Pall Mall), will be found at p. 307 of his Text- 

 book. 



Nachet's Small Mineralogical Microscope. 



