176 SECTIONAL ADDRESSES 
these changes with those produced in photoelectric cells. Furthermore, 
it is known that there is a chemical substance in the retina which is 
bleached by light. It would be a remarkable circumstance if this photo- 
chemical change were entirely divorced from the function of vision. 
Hecht has published a series of papers on the photochemical action of 
light on living organisms, and it has been shown that the data for dark 
adaptation are best explained on the basis of a bimolecular chemical 
change. The analytical factors in sensation are generally considered to 
be specialised receptor organs which receive the stimuli and cause nerve 
impulses. ‘These receptors act by having a low threshold to some mani- 
festations of energy while maintaining a high threshold to others. It 
is for this reason that receptors are of such importance in the physiology 
of the nervous system. 
‘THEORIES OF COLOUR VISION. 
If it is legitimate to regard all colour perceptions as being synthesised 
from three sensory mechanisms, we can return to the consideration of 
visual perceptions (p. 170.) 
Perceptions of form may be regarded as related to the optical patterns 
produced in both uniocular and binocular vision ; therefore they are 
related to the anatomical connections between areas of the retina and of 
the cerebrum. 
Perceptions of movement depend upon the presentation of successive 
patterns, such, for example, as shown by the cinematograph. 
Recognition of intensity differences is ascribed to the frequency with 
which impulses reach the central nervous system. ‘Therefore we have 
to consider how the three subqualities underlying colour vision can be 
conveyed by the optic nerves. If we could prove that different types of 
nerve impulse could pass up the same nerve, we could say that a single 
nerve fibre could serve for all colour perceptions, but if we must limit 
each nerve fibre to one type of impulse, then we must look for separate 
nerve fibres for each of the three colour sensation processes. In other 
words, of the six properties of vision we can relate three of them—form, 
movement, and intensity—to anatomical, temporal, and frequency relations 
respectively. The other three, namely colours, must be related to different 
groups of nerve fibres. It is possible to imagine frequency relations 
giving rise to colour sensations, but we would then have to abandon the 
experimental relation between frequency and intensity. 
The maximum frequency at which nerve impulses can pass up a nerve 
fibre is of the order 400 per second, whilst the frequency of light waves 
is from 400 to 750 billions per second. It seems difficult to imagine a 
relationship between such disproportionate frequencies. 
The relation between receptor organs in the retina and nerve fibres in 
the optic nerve is complicated by the synapses in the layers of the retina 
(Granit has been investigating these problems!*). If vision depends 
16 E. D. Adrian, British Association Report, p. 163 (1933). 
16 R. Granit, Arch. of Ophthal., 6, p. 104 (1931). 
