440 COMPLEMENTARY COLOURS.—DIRECTION OP MOVEMENTS. 
570. Upon these properties of the eye are founded the laws 
of harmonious colouring; a full knowledge of which should 
he possessed by artists of every kind who are concerned with 
contrasts of colour, whether in pictures, architectural decora¬ 
tions, or even in dress. All complementary colours have an 
agreeable effect when judiciously disposed in combination; 
and all bright colours which are not complementary have 
a disagreeable effect, if they are predominant: this is espe¬ 
cially the case in regard to the simple colours (red, blue, and 
yellow), strong combinations of any two of which, without 
any colour that is complementary to either of them, are ex¬ 
tremely offensive. Painters who are ignorant of these laws, 
introduce a large quantity of dull grey into their pictures, in 
^order to diminish the glaring effects which they would other¬ 
wise produce; but this benefit is obtained by a sacrifice of the 
vividness and force which may be secured in combination 
with the richest harmony, by proper attention to physiological 
principles. 
571. The Eye is endowed with common sensibility (§ 487) 
by the fifth pair of nerves; and when this is paralysed, all 
parts of it are completely insensible to the touch, although 
the power of vision may remain unimpaired. It seldom pre¬ 
serves its healthy condition in this state, however; for the 
lachrymal and mucous secretions which protect its surface, are 
no longer formed as they should be ; and inflammation, often 
terminating in the destruction of the eye, is the result. 
572. The visual sensations obtained through the Eye have 
numerous and varied purposes among the lower animals. 
That they chiefly serve to direct their movements, is evident 
from observation of these movements; and from the fact, 
that, when the eyes are covered or destroyed, most animals 
make little attempt at determinate motions, though they fre¬ 
quently exhibit a great deal of restlessness. There are a few 
Yertebrata, however, which do not possess perfectly-formed 
eyes, and which are consequently guided in their movements 
by other senses. This is the case with the Mole, which 
spends its whole life in burrowing beneath the ground; and 
also with the Proteus, and some others of the lower Am¬ 
phibia, which inhabit the dark recesses of subterranean lakes 
and channels. 
573. In the Articulated series of animals, we meet with 
