216 On Colour-Vision by Very Weak Light. 
between the threshold of light-sensation and the threshold of colour- 
sensation. 
With the exception of the earlier experiments this research has been 
conducted partly at University College, Reading, and partly at the Physio- 
logical Laboratory, Oxford. The expense of some of the experiments has been 
defrayed from the Government grant. 
My thanks are due to Mr. W. W. Fisher for the loan of apparatus from 
the Marlborough Collection, and to Professor Gotch for the facilities afforded 
in his laboratory. And I desire to acknowledge the assistance rendered 
during my stay in the dark room by the laboratory attendant. 
