Page:Optics.djvu/122

 two convex lenses of crown-glass, with a concave one of white flint-glass between them. In this case we may consider the two outer lenses as producing one single refraction, and the inner one as correcting it.

134. organ by which we are most usually, and most easily informed of the presence of external objects, and without which we should often be ignorant of their form, and always of their colour, is the eye, a most curious combination of parts so admirably contrived to answer all the purposes required, that nothing short of divine intelligence could have been capable of constructing it, and the mere imitation of it is far beyond the reach of human skill.

135.The eye is, in form, nearly spherical, as will be seen by referring to Fig. 147, which represents a horizontal section of the right eye.

Its several parts are as follows:

The cornea $$A$$ is a transparent membrane which covers the convexity in front of the eye. It is formed like a meniscus, being thickest in the middle.

The sclerotica $$HIK$$ is a thick tough coat, which covers the remainder of the eye, and is intimately united with the cornea round the edge of the convexity.

The choroid-coat $$EFG$$ lines the sclerotica; (but not the cornea): these two integuments are united rather loosely in general, except round the edge of the cornea, where they are firmly fastened together by a circular band, called the ciliary ligament.