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24 nature to present a strong resistance to any violence from without.

Immediately beneath the cornea and in contact with it is the aqueous humour, a thin transparent liquid occupying a small portion of the front of the eye.

Next comes the iris, a circular disc perforated with a round hole in the middle, and coloured with various shades of blue, brown, and grey.

The opening in the centre, which appears like a black spot when the eye is examined, is not really an object, but simply an aperture, capable of changing its size according to the quantity of light striking the eye. This change of size in the opening, or pupil, as it is popularly called, is effected by the contraction or expansion of the iris, which thus possesses the peculiar property of exactly proportioning the amount of light that enters the eye, so that there is never too much or too little. It is through the pupil that the rays of light proceeding from the various objects around us pass into the interior of the eye, and form an image upon the retina, as will be afterwards explained.



Immediately behind the pupil is, a bi-convex lens to transmit the rays of light to the retina. It is generally called the crystalline lens.