F. E. Wright — New Petrographic Microscope. 407 



Art. XXXYI. — A New Petrographic Microscope ; by Feed. 

 Eugene Weight. 



Expeeience has shown that the so-called universal instru- 

 ments are as a rule unsatisfactory, and often do not accomplish 

 in a thoroughly competent manner any one of the several 

 purposes for which they are intended. To fulfill a given set 

 of conditions adequately, it is usually necessary that a special 

 instrument be designed for the purpose. Thus, a small calibre 

 rifle may be admirably suited for small game, but for 

 larger game it is totally inadequate and might even be of more 

 harm than service in an emergency; and vice versa, a large 

 calibre rifle is of little value in hunting small game. 



The same principle of adaptability applies equally to micro- 

 scopes and scientific apparatus in general. The ordinary 

 microscope, which is designed to aid in the examination of thin 

 sections of rocks and minerals, is satisfactory and convenient 

 for such purposes, but is less so for use in the investigation of 

 artificial preparations which are usually very fine-grained, the 

 diameter of the individual grains averaging often not more 

 than -01 mm . To determine satisfactorily and with fair accuracy 

 the optical constants of substances in such minute grains, special 

 methods of attack are required, and these in turn postulate 

 certain new conditions to be fulfilled by the petrographic 

 microscope. It was with this end in view, to construct a 

 microscope better adapted than the microscopes now available 

 for the examination and determination of fine-grained artificial 

 silicate preparations, that the present microscope was designed 

 and constructed in the workshop of the Geophysical Laboratory. 

 Several of its features appear to be of general application, and 

 a brief description of its essential parts is therefore justified. 



In the construction of the present instrument (fig. 1), a 

 Zeiss microscope No. 1 C for photomicrography served as base, 

 and on it extensive changes were introduced so that the result- 

 ing microscope resembles the original only slightly. This 

 particular model was chosen chiefly because of its wide upper 

 barrel, which was well adapted for the introduction of 

 diaphragms and the movable Bertrand lens. The changes which 

 have been introduced are essentially as follows.* 



(1) The nicols are revolvable simultaneously about the optic 

 axis of the microscope. They are connected rigidly by the 

 bar T of fig. 1, and their angle of revolution can be read off 

 directly by the vernier on the stage. This method obviates 

 the errors introduced by the usual system of gear-wheels with 

 accompanying lost motion in the moving parts. The details of 



* A still further change, made recently aud not shown in fig. 1, is the intro- 

 duction of a block just below the upper tube support. The distance between 

 the stage and the upper tube is thereby increased and the use of the univer- 

 sal stage facilitated. 



