Acknowledgments 



The author wishes to record his thanks to Miss D. J. Taylor for 

 assistance in preparing the original illustrations. He also desires 

 to acknowledge his indebtedness to the Editorial Boards and 

 Proprietors of the following scientific journals, and to the authors 

 concerned, for permission to reproduce the following figures : 



Journal of Physiology: Figs. 1.9, 2.6, 2.7, 2.11, 2.13, 3.9, 5.8, 5.9, 6.1, 

 6.2, 6.3, 6.6, 6.7, 6.8, 6.9, 6.10, 7.1, 7.2, 7.4, 7.9 and Plate 1. 



Journal of General Physiology: Figs. 2.2, 2.3, 2.4, 2.5, 3.10, 3.11, 4.5, 

 4.6,5.2, 5.3,5.4, 5.6,5.7. 



Proceedings A of the Royal Society: Figs. 3.2 and 3.4. 



Biochemical Journal: Figs. 2.12, 3.7, 4.3. 



Optica Acta: Figs. 7.5, 7.10, 7.11. 



Science: Figs. 2.9 and 2.10. 



Acknowledgment is also made to Messrs. H. K. Lewis for per- 

 mission to reproduce Fig. 1.1 from Eugene Wolf's Anatomy of the 

 Eye and Orbit, and to the University of Chicago Press for permission 

 to reproduce Fig. 1.2 from S. L. Polyak's The Retina (Copyright 

 (1941) University of Chicago). 



