816 
I feel it ineumbent upon me to tender my sincere thanks to 
Prof. Dr. L. S. Ornstein, under whose stimulating guidance | was 
able to make the above research; and to Dr. W.J. H. Morr and Dr. 
H. ©. Burger, whose continual interest and good advise have always 
been of great support to me. 
CONCLUSION. 
1. A method is described to measure the extinction in the photo- 
graphic way. This method is applied to liquid-crystalline phases. 
2. The two liquid-erystalline phases ex-solid and ex-liquid possess 
different extinction also in the visible spectrum. 
Utrecht, May 1920. Physical Laboratory, Institute for 
Theoretical Physics. 
