10 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
THE PETROGRAPHIC MICROSCOPE. 
Although not strictly germane to the subject of optical methods, it has 
been found convenient to outline in this introductory section the essential 
features of the petrographic microscope and in particular to emphasize the 
principles underlying its construction. In the following descriptions con- 
stant reference will be made to the details of the optical system as sketched 
in this section, which, though necessarily incomplete and disconnected, 
serves to facilitate the presentation and to direct attention to certain prin- 
ciples which are essential to accurate work. 
In the preparation of this section the following books and papers have 
been found especially useful: 
Abbt, E. Die Lehre von der Bildentstehung im Mikroskop. Edited by O. Lummer and 
!'. Reiche, 1910. 
Beck, Conrad. The theory of the microscope. Cantor Lectures, Roy. Soc. of Arts, 1908. 
Beck, Conrad. The consideration of the equivalent planes of optical instruments. Proc. 
of Optical Convention, I, 9-18, 1905. 
Chwolson, O. D. Lehrbuch d. Physik, II, 1904. 
Czapski, S. Theorie d. optischen Instrumente, Aufl. 2, 1904. 
Dick, Allan. Notes on the polarizing microscope, London, 1894. Also Min. Mag., 8, 
160-163, 1888. 
Drude, P. Lehrbuch der Optik, 2d edition, 1906. 
Ferraris, G. Die Fundamental-eigenschaften der dioptrischen Instrumente. German 
translation by F. Lippich, 1879. 
Hastings, C. S. Light. New Haven, 1901. 
Heath, R. S. A treatise on geometrical optics, ad edition, 1895. 
Hockin, J. Journ. Roy. Micros. Soc. (2), 4, 337, 1884. 
Leiss, C. Die Opt. Instrumente, etc., 1899. 
Lummer, O. Muller-Pouillet, Lehrbuch d. Physik, roth edition, II, 2, 1907. 
Pelletan, A. Optique Appliquee, Paris, 1910. 
Rosenbusch-Wiilfing. Mikroskopische Physiographic, I, i, 1904. 
Schuster, A. The theory of optics, 2d edition, 1909. 
Southall, J. P. C. Principles and methods of geometrical optics, 1910. 
Spitta, E. J. Microscopy, 1907. 
Wright, A. Principles of microscopy, 1906. 
The petrographic microscope is designed to serve two purposes. 
(1) That of an ordinary microscope to magnify and to render visible 
details which would otherwise escape attention. 
(2) That of an instrument for measuring certain optical properties of 
minerals in the thin section. 
The first requirement is one common to all microscopes, and on it an 
extraordinary amount of care and thought have been spent in recent years, 
with the result that the lens systems of modern microscopes are highly cor- 
rected and satisfactory in practically every respect. The calculation of 
such lens systems is an exceedingly intricate task and one which does not 
primarily concern the working petrologist, who uses the microscope solely 
as a means to an end. Having been furnished with the optical system, it is 
essential, however, that he understand, in a general way at least, the func- 
tions of the different parts of the microscope if he desires to obtain the best 
possible results from his instrument. In the examination of thin sections 
of rocks and even of artificial silicate preparations, the details which must 
be resolved are not, as a rule, of such a nature as to require magnifications 
above 250 diameters. This is in part due to the fact that a practical limit 
is set to the thinness (o.oi to 0.02 mm.) to which rock sections can be 
