PETROLOGICAL ABSTRACTS AND REVIEWS 483 



light has commonly a considerable divergence; (3) In any given case, the 



observed intensity of illumination is equal to the average intensity of that 



portion of the interference figure bounded by the limits of the pencil of 



light used. 



Albert Johannsen 



Wright, Fred Eugene. "A New Ocular for Use with the Petro- 

 graphic Microscope," Am. Jour. Sci., 4th ser., 1910, XXIX, 

 415-26. Figs. 10. 



A description of an ocular with which the birefringence, the optic 

 axial angle, when one or both optic axes appear within the field of vision, 

 and the extinction angle of a mineral in thin section may be measured. 



Albert Johannsen 



Wright, Fred Eugene. "A new Petrographic Microscope," Am. 

 Jour. Sci., 4th ser., 1910, XXIX, 407-14. Figs. 4. 



The author describes a new petrographic microscope, designed and 



constructed in the workshop of the Geophysical Laboratory at Washington, 



and specially adapted to the investigation of the optical properties of 



minute grains of artificial preparations. The principal differences between 



this and the usual petrographic microscopes are: (i) Both nicols revolve 



simultaneously, the connection between the two being a rigid bar; (2) 



The upper nicol always remains in the tube and the substage nicol is inserted 



or withdrawn on passing from ordinary light to crossed nicols; (3) The 



sensitive plate is inserted just below the condenser; (4) It has a simple 



mechanical stage; (5) The Bertrand lens is mounted in a sliding collar 



which permits of different magnifications of the interference figure; (6) 



An iris diaphragm immediately below the ocular is intended for use when 



observing interference figures directly by the Lasaulx method without 



the ocular and Bertrand lens; (7) An Abbe condenser is used, and with it a 



large nicol prism, or an Ahrens prism, 15 mm. edge, after the manner of the 



Fuess microscope No. la. With this arrangement the entire condenser 



lens system remains in position and its upper lens need not be removed 



when low-power objectives are used. 



Albert Johannsen 



