604 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



have been introduced at different times for the same purpose, 

 as few if any of them are so exact as those which have been 

 described. 



When the stage is in the exact position of extinction in other 

 words, when the directions of vibration in the crystal are parallel 

 to those of the nicols and therefore to the cross wires the posi- 

 tion of the latter is indicated in the sketch by thick lines traversing 

 the whole field, and the index reading is inserted on the right 

 extremity of the right and left cross wire, while the other 

 terminations of the cross wires are distinguished by the corre- 

 sponding angular numbers differing by 90 (see fig. 1). 



Pleochroism, etc. The light vibrating parallel to each direction 

 of vibration is differently affected by the structure of the crystal. 

 The velocity of transmission of the vibrations parallel to one is 

 greater than that of those parallel to the other and the index of 

 refraction is consequently less in the case of the former. At 

 the same time the absorption of light may differ considerably 

 both in the colour selected and in amount. For an examination 

 of these differences the crystal is observed with only one nicol 

 in place, and the stage is rotated in turn into each of the positions 

 in which a direction of vibration is parallel to the right and 

 left cross wire. This, as we have seen (p. 598) , should be the 

 direction of vibration of the nicol that is retained. The surface 

 of the mineral, whether it is rough or smooth, and its luminosity 

 and colour are observed in each case and noted in the sketch 

 at the right end of the thick line representing the corresponding 

 direction of vibration.* 



Sometimes the surface of the crystal is distinctly rougher in 

 one position than in the other. This indicates that there is 

 considerably more difference between the refractive index of 

 the light vibrating parallel to the right and left direction and 

 that of the medium in which the section is mounted (Canada 

 balsam or whatever it may be) in the former case than in the 

 latter. This phenomenon is well seen, when Canada balsam is 

 the medium, in calcite and the carbonates isomorphic to it, 

 as well as in the colourless micas. It causes a characteristic 

 twinkling effect when the lower nicol is rapidly rotated. 



Character of Directions of Vibration. We now proceed to 

 determine the character (or sign) of the extinctions or directions 



* See fig. 1. 



