44 
able from the neighbouring white light." If Dr. Young had had a 
jjerfect line of light, viz. with an equal appearance of centre and sides 
altogether resembling white, the three primitive tints would pro- 
bably have presented themselves to him as in my Tab. 2, Jig. 3. 
Therefore he was near seeing what might, in such hands, have proved 
useful. 
It is further observed that "the effect of white light on the sense 
of sight might be imitated by a mixture of colours taken from dif- 
ferent parts of the spectrum, notwithstanding the omission of some 
of the rays naturally belonging to white light. Thus, if we intercept 
one iialf of each of the principal portions into which the spectrum 
is divided, the remaining halves, when mixed together, will still 
preserve the appearance of whiteness ; so that it is probable that the 
different parts of these portions of the spectrum which appear of one 
colour, have precisely the same effect in the eye. It is certain that 
the perfect sensations of yellow and blue are produced respectively, 
by mixture of red and green and of green and violet light, and there 
is reason to suspect that those sensations are always compounded of 
the separate sensations combined ; at least this supposition simpli- 
fies the theory of colours. It may therefore be adopted with advan- 
tage until it be found inconsistent with any of the phenomena; 
and we may consider the white light as composed of a mixture of 
red, green, and violet only, in the proportions of two parts of red, 
four of green, and one of violet, with respect to the quantity or in- 
tensity of the sensations produced." Again: "From these three 
simple sensations with their combinations we obtain seven primitive 
distinctions of colours * ; but the different proportions, in which they 
may be combined, afford a variety of tints beyond all calculation. 
The three simple sensations being red, green, and violet, the three 
binary combinations are yellow, consisting of red and green ; crim- 
son, of red and violet ; and blue, of green and violet ; and the seventh 
in order is white light composed of all three united; but the blue 
* Thus we have seven colours diffeving from Sir Isaac Newton's. 
