806 F. K. Wright — Transmission of Light through 



central lino for an angle of rotation of 15° of the crossed nicols 

 is indicated by the curve I, fig. 16 ; it sposition for an angle of 

 rotation of 105° is shown by curve II. These two curves do 

 not coincide, and although such measurements cannot be made 

 very accurately, they show that a rotation of the crossed nicols 



Fig. 16. 



causes a slight shift of the axial bars of the interference figure 

 of a mounted crystal plate. The amount of shifting rarely 

 exceeds several degrees and is usually less, but it is often suffi- 

 cient to be perceptible and shows the importance of referring 

 the data, when plotting, to the correct position of the _ extin- 

 guishing plane of the upper nicol. It is, therefore, not imma- 

 terial which one of the principal nicol sections be chosen. If 

 the observations themselves were of a higher order of accu- 

 racy, this fact would be a serious objection to Professor Becke's 

 method. 



Anhydrite. — A series of observations (figs. 17a, b) on a cleav- 

 age plate of anhydrite, unmounted (17a) and mounted (175), cor- 

 roborates the conclusions stated in the last paragraph. The 

 degree of accuracy of the two methods in question is about the 

 same here as in muscovite. A rotation of the crossed nicols 

 through 90° also produced a slight shift of the axial bars on 

 mounted plates, as in muscovite, and it is important, therefore, 

 that the plotting be done with reference to the correct prin- 

 cipal nicol section. 



