PETROLOGICAL ABSTRACTS AND REVIEWS 253 



pensated by means of a lens above the analyzer, but the former has 

 hitherto been left uncorrected. In this paper the cause of the astig- 

 matism is discussed and lenses for its correction are suggested. Two 

 photographs of the same slide show the remarkable improvement in 

 definition with the corrected nicol. 



Berek, M. "tjber die Berechnung der Polarisationsverhaltnisse im 

 Gesichtsfelde der Polarisationsprismen," Verhandl. d. Deutschen 

 Physikal. GeselL, XXI (1919), 338-46. 



Bekek, M. "Die Scharfentiefe des Mikroskops," Zeitschr.f. wis- 

 sensch. Mikroskopie, XXXVII (1920), 120-22. Fig. i. 



Berek, M. "Uber die einfachen und zusammengesetzten charak- 

 eteristischen Konstanten der Mikroskopobjektive," Zeitschr.f. 

 wissensch. Mikroskopie, XXXVII (1920), 36-41. Figs. 2. 

 A discussion of the use of 250 mm. and A in the determination of 



the enlargement of objectives. 



Berkey, Charles P. "Geological Reconnoissance of Porto 

 Rico," Ann. N.Y. Acad. Sci., XXVI (1915), 1-70. Figs. 20, 

 maps and profiles 2. 



This is a report of the New York Academy of Sciences Expedition to 

 Porto Rico undertaken, in part, to determine the nature and origin of the 

 rock formation and to group them into series suitable for use in subse- 

 quent geological work. Two series are described — a younger, consisting 

 of Tertiary shales, reef limestones, and recent deposits, and an older 

 including tuffs, ashes, shales, conglomerates, limestones, and a great 

 variety of probably pre-Tertiary intrusives. Igneous rocks described 

 briefly are extrusive basalts and andesites, and intrusive andesite- 

 prophyry, granite-porphyry, granite, and diorite. 



BoEKE, H. E. "Die Eisenerze," Die Umschau, XXIII (191 9), 

 289-92, 

 A popular article on the occurrence of iron ore. 



