and Laboratory Methods. 1431 



NOTES ON RECENT MINERALOGICAL 

 LITERATURE. 



Alfred J. Moses and Lea McI. Luquer. 



Books and reprints for review should be sent to Alfred J. Moses, Columbia University, 



New York. N. Y. 



tenSiethoff, E. G. A. Eine einfache Construe- The accompanying rough tracing 



tiondersogenannte Interferenzkreuzes der ^^.^^ ten Siethoff's diagram will iUus- 



zweiaxigen-Ivrystalle. Centralblatt fur Min- " 



eral. Geol. und Palaent. 267, 1900. trate the simple method taken to 



make clear the interference cross in 



biaxial crystals. As is well known, this dark cross or curve obtained between 



crossed nicols is not due to interference, but to the total extinction of all rays 



the vibration directions of which are par&llel to the vibration direction of either 



nicol. In the diagram the little crosses represent the vibration directions (with 



X K >< X X X '"< X ^ ^ -'^- V 4~h i-^"A^>cX \< ^ ^ ^ ^ ^ y 



y. y. >c % y X X x^ X V V -^-+-4-+ -^^y-x ^-X'x>cV>r;ir^ v 

 i^-hi-i^ %% v^x X X v^ -v-f^-H- i-i-i-x. X y ^ )^ )^ ^ >^ ^ , 



+-4--h-h-H-^H--f 4- 4^-f-f 4-H--f- + -f#i- H--h4-4--h-h+'^ "^ 



-V-V-V'V-v^-^\'^><X T^f -f- f-+ \ :^ y y^i-^^i- ^i-"^-^ '^ 



^ A^^>r-Y>^x .x^x: % ^ f H-4- -t- A- >- X x^x^ ><. ^ "^ ^ y. -a ^ 

 A ^.^A- >rx X x> >c >cy ~A-4--f-i--v ^ ^ X X y^y^-^^^^^ 



\ k><X K>< X^x- xX/t^-H— f' + ^'V^^ X xx^^ ^^"^^ 

 \ X X xy x^x^x x-^"^ -7^ f-f— f + -vv^vx xxx^^^ ^^ 

 ^ >c X K ^XK^y. y. X -A -f ^-f-4- -I'-VA'V^^xxx^V^^^^^ 

 xx-x-^^xVx /.x^y^i^ + + 4+-V A- V ^xx X^x'^xX X ^^^^ 



convergent light) of the different rays emerging from a section cut normal to a 

 bisectrix ; the • points are the optic axes. As the plate is turned, the vibration 

 directions of different rays, that is, different series of little crosses, move into 

 parallelism with the vibration direction of the nicols, and together make up the 

 hyperbola or the cross. By turning the diagram, while resting on a pad the edges 

 of which may represent the vibration directions of the nicols, the series of little 

 crosses parallel to the edges of the pad at any moment is easily observed. The 

 fact that with an optic axis in the field the rotation of the plate in one direction 



