ets with a body, what takes place? What is said in reference to rays
of light in passing through the eye?

-=-=-=-=-=-=-=-=-=-=-=-=

920. The sclerotic coat not only gives form to the body of the eye,
but protection to the interior and more delicate parts. The choroid
coat seems to be chiefly composed of a tissue of nerves and minute
blood-vessels; the latter give nourishment to the different parts of
the eye. One of the uses of this coat is, to absorb the rays of light
immediately after they have passed through the retina. This is
effected by the black pigment that lines its inner surface. Were it
not for this provision, light would be too intense, and vision
indistinct.

_Observation._ In albinos, where there is an absence of the black
pigment, the rays of light traverse the iris, and even the choroid
coat, and so overwhelm the eye with light, that their vision is quite
imperfect, except in the dimness of evening, or at night. In the
manufacture of optical instruments, care is taken to color their
interior black, for the same object, namely, the absorption of
scattered rays.

921. The iris, by means of its powers of expansion and contraction,
regulates the quantity of light admitted through the pupil. If the
iris is thin, and the rays of light pass through its substance, they
are immediately absorbed by the uvea, and, if that layer be
insufficient, they are taken up by the black pigment of the choroid
coat.

_Observation._ When we look toward the bottom of the eye, the pupil
appears like a black spot, instead of an opening. This is caused by
seeing the black pigment through the retina and humors of the eye.

-=-=-=-=-=-=-=-=-=-=-=-=

920. What is the use of the sclerotic coat? Of what is the choroid
coat chiefly composed? What is the use of this coat? How is it
effected? What is said of albinos? What care is taken in the
manufacture of optical instruments? 921. What is the use of the iris?
When we look toward the bottom of the eye, why does the pupil look
like a black spot, instead of an opening?

-=-=-=-=-=-=-=-=-=-=-=-=

922. The cornea, and the aqueous, crystalline, and vitreous humors,
are transparent; so that rays of light traverse these parts of the
eye, and fall upon the retina. The office of these humors and the
cornea is to refract the rays of light in such proportion as to direct
the image in the most favorable manner upon the retina.

923. The office of the retina is to receive the impre