These letters relate to:
Retinal Structure and Colour Vision by Willmer, E.N.
Introduction to the Study of Colour Vision by Parsons, J.H.
Magnetism and Electricity by McKenzie, A.E.E.
Radiation from Radio-Active Substances by Rutherford, E., Chadwick, J. and Ellis, C.D.
Textbook of Experimental Cytology by Gray, J.