170 SECTIONAL ADDRESSES 
hypothesis can be relied upon. The second aspect is the nature of the 
departures from normal colour vision of those with defective colour 
vision. The third aspect is a brief consideration of some theoretical 
views on the nature of colour-perceiving mechanisms. 
‘“Wuy is CoLour VISION OF IMPORTANCE IN PHYSIOLOGY ? 
All measurements depend upon perceptions ; many are concerned with 
visual perceptions, and some are based upon the perception of colour. 
Therefore the study of special sense physiology should be of interest to 
all branches of science. Colour is an attribute of vision ; therefore any 
views as to the perception of light by the eye must involve a consideration 
of the phenomena of colour. As colour sensation is interpreted in the 
brain, the study of colour vision involves not only the action of light on 
the retina, but also the transmission of impulses through the various 
layers of the retina and through the optic nerve, and the interpretation of 
the impulses in the brain. Some of these problems are common to other 
parts of the nervous system : therefore a thorough knowledge of colour 
vision may illuminate other sensory processes. 
As the retina is merely an outgrowth from the brain, and the optic 
nerve a tract of the central nervous system, the function of the layers of 
the retina and of the optic nerve should correspond with other parts of 
the nervous system. Therefore, the distinctive problem in the physiology 
of colour vision is to discover how light affects the retina and produces 
nerve impulses. On the other hand, it is difficult to separate experi- 
mentally those activities due to stimulation of the retina from the activities 
of the various layers of the retina or the central nervous system. 
STATEMENT OF THE:PROBLEM INVOLVED IN THE CONSIDERATION OF 
CoLourR VISION. 
The real problem that one has to consider in special sense physiology is 
how the threshold of stimulation can be lowered for certain stimuli, but 
left high for others. The nature and action of the receptors determine 
what stimuli will most easily give rise to nerve impulses in certain nerve 
fibres. The number of varieties of receptors depends upon the data 
which must be presented to the cerebrum for the proper perception of 
sensory stimuli. 
The first difficulty is to decide whether a single nerve fibre can convey 
impulses corresponding to more than one sensory datum. ‘This problem 
was mentioned by Prof. Adrian in his address last year, and although the 
thesis cannot be definitely proved, the evidence seems to indicate that a 
single nerve fibre cannot convey more than one kind of impulse.2 The 
velocity of transmission and other characteristics may vary from fibre to 
fibre, but it has not been shown that one fibre can convey different types 
of impulse. ; 
If the above assumption is a legitimate one, we must endeavour to 
2 E. D. Adrian, British Association Report, p. 163 (1933). 
