XXXV.] COLOURS AND THEIR COMBINATION. 115 



brightness ; that its green, for instance, is much brighter than All the 

 its purple. But this is not strictly accurate ; it is our faculty ^^^^ ^'"'^ 

 of vision which is so constituted as to be more forcibly impressed eciunlly 

 by the green rays than by the purple ones. This is a purely '^'''S"t to 

 physiological fact, and it is quite possible that there may be 

 animals which have eyes that are more forcibly impressed by 

 the purple than by the green.^ If all parts of the spectrum 

 were of equal brightness to our eyes, then equal arcs of tlie 

 circle would contain equal quantities of colour, and equal and 

 opposite arcs would neutralize each other, forming white. The 

 circle, as we have seen, is divisible into arcs of the following 

 pairs of complementary colours : — 



Eed and bluish green = White. 



Orange and azure =: "White. 



Yellow and indigo = White. 



Yellowish green and violet = White. 



Green and purple = White. 



Were a pair of complementaries always of equal brightness, 

 it is obvious that, in order to make white without leaving any 

 residual colour, they ought to occupy equal arcs of the circle. 

 But this is not the case ; the colours on the green side of the 

 circle are much brighter than those on the purple side ; so that 

 the combination of equal arcs (or, in other words, of equal 

 quantities) of the two would not produce pure white, but would 

 have a residual green tinge. The neutralization, or conversion 

 into white, of all the colours, without residual tinge from any 

 of them, is attained by the brightest colour of any pair of com- 

 plementaries occupying the smallest arc ; and, as the colours are 

 brighter on the green side of the circle than on the purple side, 

 this involves as a geometrical consequence that the colours which 

 we might have expected to find separated by an arc of 180°, are 

 a little nearer to each other by the green side of the circle. 



As stated above (see p. 112), all possible colours are contained 

 in the spectrum, except white, black, and grey; though many 

 tints, especially the various tints of brown, are so inferior to 



1 It has been suggested that cats and owls, and other animals which see 

 well in twilight, may have eyes unusnally sensitive to the very refrangible 

 rays, — the violet and purple, and the photographic rays invisible to us, — 

 which are relatively more abundant in twilight than the less refrangible 

 ones, 



I 2 



