C. H. Warren — Calcium Carbide. 125 



(optically) on each other, the integrated effect being 

 almost zero. 



Interference figures. — If the dark areas last described 

 are examined in convergent polarized light, interfer- 

 ence figures are obtained which resemble the so-caUed 

 uniaxial /figures in charac^ter, viz., a dark cross s'ur- 

 rounded (in white light) by colored curves, in appear- 

 ance, approximately circles, which as the preparation is 

 rotated, either remains stationary, or rotates with the 

 preparation about the axis of the instrument. The figin-e 

 is positive in optical character. In a great number of 

 the figures, rotation of the section causes a slight opening 

 of the dark cross. A fading of the dark bars is character- 

 istic, and in certain cases may cause them to wholly dis- 

 appear. Furthermore, it was found necessary, in order 

 to get a definite figure, to have the pin-hole diaphragm, 

 through which the objective image of the figure is viewed, 

 in a certain position in each case. A slight movement of 

 the cap sideways or back and forth in general cause distor- 

 tion of the figure or breaks it up altogether. Professor 

 Moses obtained interference figures from certain spec- 

 ially prepared sections of very coarsely crystalline ma- 

 terial, which in his opinion were truly uniaxial, and his 

 opinion was shared by certain of the other observers. 



In figures where a distinct opening* of the cross is to be 

 seen, it has been noted that one arm of the cross is heav- 

 ier than the other and that the direction of the line join- 

 ing the center of the hyperbolae is parallel to the twinning 

 striae. This feature was especially emphasized by Pro- 

 fessor Kraus. 



From other fragments showing distinct double refrac- 

 tion and parallel extinction, interference figures have 

 been obtained which appear to be normally biaxial in 

 character, and the axial angle as judged by the distance 

 between the dark hyperbolae, is very variable for different 

 fragments; in some instances it has been observed as 

 high as 30°. The dispersion of the optic axes is 

 red < violet. Here also the line joining the centers of the 

 hyperbolae is parallel to twinning striae which are parallel 

 or at right angles to the cleavages, viz. the axial plane is 

 parallel to stride. 



From thicker grains of obviously very composite char- 

 acter, interference figures are obtained which in parallel 

 position appear biaxial in character with one bar strik- 



Am. Jour. Sci. — Fifth Series, Vol. II, No. 8. — August, 1921, 

 9 



