58 
PEOFESSOE J. CLEEK MAXTTELL OX 
We may express this by saying that two compound coloui’s may be chromatically 
identical, but ojitically different. The ojytical properties of light are those which hare 
reference to its origin and propagation through media, till it falls on the sensitive organ 
of vision; the cliromaticcd properties of light are those which have reference to its 
power of exciting certain sensations of colour, perceived through the organ of wsion. 
The investigation of. the chromatic relations of the rays of the spectrum must there- 
fore be founded upon observations of the apparent identity of compound colours, as 
seen by an eye either of the normal or of some abnormal type ; and the results to which 
the investigation leads mmst he regarded as partaking of a physiological, as well as of a 
physical character, and as indicating certain laws of sensation, depending on the consti- 
tution of the organ of vision, which may be different in different individuals. We have 
to determine the laws of the composition of colours in general, to reduce the number of 
standard colours to the smallest possible, to discover, if we can, what they are, and to 
ascertain the relation which the homogeneous light of different parts of the spectrum 
hears to the standard colours. 
§ II. History of the Theory of Compound Colours. 
The foundation of the theory of the composition of colours v/as laid by XEWTOX^b 
He first shows that, by the mixture of homogeneal light, colours may be produced 
which are “like to the colours of homogeneal light as to the appearance of colour, but 
not as to the immutability of colour and constitution of light.” Eed and yellow give an 
orange colour, which is chromatically similar to the orange of the spectrum, but optic- 
ally different, because it is resolved into its component colours by a prism, while the 
orange of the spectrum remains unchanged. When the colours to be mixed lie at a 
distance from one another in the spectrum, the resultant appears paler than that inter- 
mediate colour of the spectrum which it most resembles ; and when several are mixed, 
the resultant may appear white. XEWTOEf is always careful, however, not to call any 
mixture white, unless it agrees with common white light in its optical as well as its 
chromatical properties, and is a mixture of all the homogeneal colours. The theory' of 
compound colours is first presented in a mathematical form in prop. 6, “ In a mixture 
of immary colours, the quantity and quality of each heing given, to hiow the colour of 
the com])Ound.” He divides the circumference of a circle into seven parts, proportional 
to the seven musical intervals, in accordance with his opinion about the proportions of 
the colours in the spectrum. At the centre of gravity of each of these arcs he places a 
little circle, whose area is proportional to the number of rays of the corresponding 
colour which enter into the given mixture. The position of the centre of gravity of all 
these circles indicates the nature of the resultant colour. A radius drawn through it 
pomts out that colour of the spectrum which it most resembles, and the distance from 
the centre determines the fulness of its colour. 
With respect to this construction, Newton says, “ This rule I conceive accurate enough 
* Optics, book i. part 2. props. 4, 5, 6, t 7tb and Stb Letters to Oldenburg. 
