45 
thus produced by combining the whole of the green and violet rays 
is not the blue of the spectrum : four parts of green and one of 
violet make a blue, diH'ering very Utile from green ; while the blue 
of the spectrum appears to contain as much violet as green, and it is 
for this reason that red and blue usually make a purple, deriving its 
hue from the predominance of the violet." 
The doctor says, " It would be possible to exhibit at once to the 
eye the combinations of any three colours in all imaginable vari- 
eties; two might be laid down on a revolving surface, in form of 
triangles placed in opposite directions, and the third on projections 
perpendicular to the surface, which, while the eye remains at rest in 
any one point, obliquely situated, would exhibit more or less of their 
jndntefl sides, as they passed tiieir diti'erent angular positions; and 
the only further alteration that could be produced in any of the 
tints, would be derived from the dilFerent degrees of light only. The 
same effect may also be exhibited by mixing colours in different 
proportions by means of the pencil, beginning from the equidistant 
jioints or centres." 
This, I think, seems a conclusive argument of the ntility of the 
knowledge of the three primitives, viz. yellow, red, and blue; for it is 
impossible for the compound sensations, as I would call the green and 
violet, with or without the red, to produce yellow in any part of the 
triangle ; and this is confessed in the following explanation of Dr. 
Young's^?"-. 427 : "A triangular figure exhibiting in theory all pos- 
sible shades of colours; the red, green, and wiiite are single in their 
respective angles, and are generally shaded off towards the opposite 
sides ; a little j/e//o2<; and blue only are added in their places in order 
to supply the want of brilliancy in the colours which ought to com- 
pose them. The centre is gray, and the light of any two colours 
which are found at equal distances on opposite sides of it, would 
always very nearly make up together white light, as yellow and 
violet, greenish blue and red, or blue and orange." 
This triangle of Dr. Young's will only make a variety of light or 
dark browns, which any three colours, simple or mixed, would, if 
