XXXIV 
INTRODUCTION. 
ingress of many rays at once. This contraction occasions an indistinctness 
of vision ; but though the pupil readily diminishes in the dazzling of the 
mid-day sun, it expands as instantaneously in the glimmerings of twilight. 
In birds in general, the organ of sight is peculiarly adapted for acuteness 
and strength of vision, by the power of expansion in the optic nerve; and 
as it forms a membrane (after perforating the sclerotic and choroid coats of 
the eye) which retains the impression of the exterior objects most vividly 
and distinctly. Hence the sight of birds is so perfect. The Eagle, though 
soaring in the clouds, discerns its prey on the earth. The Hawk, hovering 
in the atmosphere beyond the reach of our sight, darts unerringly upon the 
animal that is doomed to fall a sacrifice to its rapine. Besides, this organ 
is so advantageously situated, as to take within its compass the greater part 
of the horizon, although many of the races cannot discern a near object in 
a straight line with the bill. Though, to outward appearance, the eyes of 
most birds seem small, they are, in fact, comparatively large, each of their 
orbits being nearly one- third of the circumference of the head, whereas in 
man it does not exceed a sixth part. A slight variety exists in the colouring 
of the irides : they are, in most of the Sparrow kinds, hazle, or dark 
brown. In Woodpeckers, white ; in the Landrail, Shag, and Corvorant, 
faint green ; in the Waterrail, the Moorhen, and the Hedgesparrow, rufous 
orange ; and in the Jackdaw, whitish grey ; and in some of the Hawk 
tribes, bright yellow. But these colourings are not permanent. In 
the matured state, the eyes of the Cuckoo and the Henharrier are of a 
bright yellow ; though when young of a dark brown ; and the rufous iris 
of the Hedgesparrow is then dusky. External ears are not given to birds ; 
instead of which, they are furnished with a winding orifice, that catches 
the sound, and conveys it through the auditory canal to the tympanum. 
Around this orifice a tuft of light feathers is placed, as a guard against 
injury, and yet so delicately constructed, as to permit the undulations of sound 
to pass between them. The auditory bones of their ears, though four in 
number, appear as but one, so closely are they connected together. The 
Owl kinds have a flap, or fleshy protuberance, covering this opening, which 
