152 CARNEGIE INSTITUTION OF WASHINGTON. 



the variation of the one with the other be ascertained; in other words, a differ- 

 ential equation wliich, if it can l)o integrated, should furnish the desired 

 crystal force-function. 



The persistence of (;ertain crystal configurations in spite of thermodynamic 

 tendencies towartl other more stable relations is well shown in rocks. The 

 most remarkable fact in petrology is the relatively few rock-making minerals, 

 especially in igneous rocks. These few minerals persist the world over and 

 constitute the major i)art of the rocks of the earth's crust. This persistency 

 of a few mineral species, notwithstanding great diversity in conditions of 

 formation and in chemical composition, is fundamental, and indicates that 

 the dominating factor in the crj'stallization of a magma is the stability of 

 certain crystal types or configurations, and that these assert themselves, not- 

 withstanding tendencies toward other groupings which thermodynamically 

 are more stable. The study of crystals from the vieA\'point of crystal forces 

 is an integral part of geophysical and geochemical research. 



(20) Xotc oil the lithophysa; in a speciinen of obsidian from California. F. E. Wright. 



J. W.a.'^h. Acad. Sci., 6, 367-369 (1916). 



A brief description is given of the obsidian and its lithophysse from Little 

 Lake, about 40 miles south of Owen's Lake, Liyo County, California. The 

 lithophysse are similar in composition and appearance to the lithophysse in 

 the olisidian from Hrafntinnuhr3^ggur, Iceland. A detailed study of both 

 these occurrences has led to the conclusion that in the formation of the litho- 

 physal cavities volatile gases set free during the crystallization of the spheru- 

 lites were the active factor (causing not only the cavity but also the recrystal- 

 lization), and not a secondary phenomenon accompanying the opening of the 

 cavities by hj^drostatic tension. 



(21) Recent, improvements in the petrograpic microscope. F. E. Wright. .J. Wash. Acad. 



Sci. 6, 465-471 (1916). 



During the past five years improvements have been introduced on the 

 writer's petrographic microscope, either to increase the degree of precision of 

 the measurements or to facilitate the use of the microscope by means of time- 

 saving devices. The most important of these devices are : 



(1) Sliding objective changer. — This has been substituted for the ordinary 

 objective clamps which experience has shown to be unsatisfactory for rapid, 

 accurate work. The new objective changer consists of a brass slider in which 

 are mounted two objectives in eccentric, conical steel rings adjusted so that 

 there is no change of focus or centering in passing from one objective to the 

 second. The slider has wide bearing-surfaces and runs in a holder which is 

 rigidly attached to the microscope. 



(2) The removal of astigmatism introduced by the analyzer. — Siegfried Becher 

 has shown that a telecentric arrangement of the lens system of the microscope, 

 such that the rays pass through the analyzer as parallel beams, reduces to a 

 large extent the astigmatic errors introduced by the analyzer. The practical 

 ai^plication of this idea has been accomplished on the writer's microscope, 

 following a suggestion by Dr. H. Kellner, by means of a weak negative lens 

 below the analyzer and a weak positive lens above, the focal lengths of the 

 lenses so taken that the rays from the objective pass through the analyzer as 

 parallel beams and are then brought to the original convergence by means of 

 the positive cap lens. For the removal of astigmatism in the interference 

 figures a positive lens below the analyzer is required as well as the positive cap 

 lens. These new lens systems are so arranged on the writer's microscope that 

 the passage from one to the other is accomplished directly without appreciable 

 loss of time. 



