J. W. N. LE HEUX: «'Explanation of some Interference-Curves of Uni-axial and 

 Bi-axial Crystals by Superposition of Elliptic Pencils". 



Fie. 2. 



Fig. 4. 



Fié. 3. 



Fig. 5. 



Fig. 6. 

 Proceedings Royal Acad. Amsterdam. Vol. XXV, 1922. 



