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Yw the greatest extinction and correspond ing- angle we find : 



Figure 3 affords a general view of tlie results. Suppose tlie part 

 of the globe-surface, falling outside the parallel-circle of 60° but 

 within the isogyre of 1°, to be equal to the part falling within 

 the same circle outside the isogyre, then it appears that at about 

 Vs of the sphere an extinction of less than 1" is observed, so practi- 

 cally a straight extinction. Now the sections, yielding greater extinc- 

 tions, lie so much in the neighbourhood of the planes, making i-ight 

 angles with the optic axes, that they are for the greater i)art 

 impracticable for the determination of the direction of extinction. 

 A comparison of figures 2 and 3 shows the result that with rhombic 



