The Newton tradition - Duplicity Theory of Vision - page 12

whether the essential cause of the different qualities was to be found
in the nerves themselves or in the brain (see Müller, 1840 , p. 261).
2.4 Newton's gravitation principle applied
to colour mixture data
A third major contribution by Newton is represented by his ideas of
colour mixture. These ingenious ideas paved the way for Maxwell's
fundamental discovery of the triplicity of colour vision, that is, that
colour vision depended on three types of receptor at the retinal level
(Maxwell, 1855 , 1860 ).
Thus, in order to illustrate the results of colour mixture, Newton
represented the colours of the spectrum on the circumference of a
circle, making the colours gradually pass into one another as they do
when generated by a prism (see Newton, 1730 , pp. 154-158). Although
Newton held that there existed a very large number of different colour
sensations, he assumed that the number of primary colours was
restricted to seven (red, orange, yellow, green, blue, indigo and violet).
These seven primary colours were represented on the circumference
of the colour circle by dividing it into seven parts proportional to the
seven musical string intervals that establish an octave as illustrated
in Fig. 2.1. Taking advantage of his principle of gravitation, he could
then present the colour mixture laws graphically in a two-dimen-
sional colour space. Thus, in the middle of each of the seven arcs
he placed a small circle, the centre of which represented the centre
of gravity of the rays of the corresponding primary colour, and the
number of rays of the colour, the gravitational force. The common
centre of gravity of all the primary colours in the mixture, then,
represented the location of the resultant colour. When the common
centre of gravity was located on the circumference of the circle, the
colour would appear saturated to the highest degree, while it would
gradually appear more whitish and eventually become white as its
location approached the centre of the circle.
These ideas of colour mixture make Newton the principal
forerunner of modern colorimetry. Thus, it can be seen that both