18 
BIRD-LIFE. 
moveable bony plates formed into a ring, which can be 
compressed together; this again presses forcibly on the 
globe of the eye. By means of this arrangement the latter 
can be rendered more or less convex; convexity giving 
long-sightedness, while flatness produces a sharp short- 
sight. From behind, the so-called “pecten” works with 
the same object, inasmuch as when injected with blood it 
swells, thus pressing the glass-like ball together, while, 
when it empties itself, the ball expands again, by which 
means the crystal lens moves backwards and forwards. 
Probably the various colouring of the iris, passing from 
silver-white on through yellow, grey, light blue and various 
shades of the darker colours, has little to do with the 
greater or less sharp-sightedness of birds, in most of which 
this gift is extremely powerful. A bird of prey can see 
a small object of the same colour as the ground on which 
it lies at a distance of a thousand feet: a Vulture will 
detect and distinguish a carcase from a height that is 
quite beyond our powers of sight, though the human eye 
is so large in comparison. Sight is the most valuable 
gift of the bird: it can exist without the remaining 
senses ; but without sight it must perish. 
