178 SECTIONAL ADDRESSES 
retina with a residual yellow effect due to both these ranges of radiation 
stimulating a yellow sensation-producing mechanism. Burridge’s state- 
ment that there is an increase or decrease in rhythmical activity does not 
indicate how colour sensations are produced. 
Edridge-Green describes a theory which is quite different from all 
others. He says that the rods do not cause visual sensations, their only 
activity being to produce visual purple. Visual purple is passed into 
solution and, when decomposed by light, acts upon and produces stimula- 
tion of the cones. He seems to regard each cone and each nerve fibre as 
capable of giving rise to a number of different colour sensations ; this 
suggestion requires a modification of the view that a single nerve fibre 
can conduct only one type of impulse. 
Another suggestion is that put forward by Schultz (1866),1” namely, 
that there is one photochemical substance but different coloured filters to 
distinguish the various regions of the visible spectrum. Such filters have 
been found in amphibia, reptiles, birds and marsupials, but have not been 
found in other mammals. The coloured filters in the birds’ retinz 
would explain the type of colour vision found in man. For instance, by 
reducing the intensity of red pigment in the red filters the various degrees 
of hypochromatic vision would be produced, but in a single human eye 
examined by me no such filters could be seen. 
My own work leads me to suppose that the types of receptors which 
are stimulated by visible radiation are as follow :— 
The first type of receptor is one which is stimulated by all parts of the 
visible spectrum and gives rise to a sensation of violet 18 when stimulated 
strongly by itself.19 
The evidence for the first part of the above statement is the same 
as that which caused Hering to speak of a white-black substance and 
von Kries to describe a bluish-white sensation as due to stimulation 
of the receptors for achromatic scotopic vision: these usually being 
regarded as the rods. 
The evidence for the second part of the above statement is first of all that 
a narrow beam of any wave-length when shining slightly eccentrically gives 
rise to a violet sensation. ‘This has been called secondary excitation, 
implying that the sensation is due to stimulation of receptors by nerve 
impulses passing along fibres of the optic nerve. It is unlikely that such 
stimulation would occur, and if so, why should the sensation produced be 
17 M. Schultz, Arch. f. Mikr. Anat., 2, p. 255 (1866). 
18 Tt is with some hesitation that one states that violet is due to stimulation of 
a single receptor, as psychologically it suggests a mixture of blue with a little 
red. If violet is the sensation corresponding to stimulation of one type of 
receptor, we must regard the unitary sensation of blue as due to stimulation of 
the receptors for green and violet. It may be that blue is the sensation due to 
stimulation of the single receptor, and that violet is the result of stimulation of 
the receptors which give rise to blue and to red sensations. This matter must be 
left in abeyance, but the use of the term ‘ violet receptor’ is to be understood to 
mean either the receptor for violet or blue, owing to the fact that fatigue to ‘ red ’ 
causes violet to appear more blue. Wright believes that the single receptor gives 
rise to a sensation of blue. 
19 W. O. Sivén, Shand. Arch. f. Physiol., 17, p. 306 (1905). 
