352. =Camera Obscura.=--This instrument differs from the magic lantern in giving us diminished images of objects. An instrument of this kind can be arranged extemporaneously any where. Thus, if into a darkened chamber light be admitted through a small opening, inverted images of any objects in front of the opening will be formed upon a white screen in the opposite part of the chamber. Such an arrangement is represented in Fig. 242 (p. 273), C D being the chamber, L the opening, and _a b_ the image of the object A B. The images in such a case, however, are faint, because the opening must necessarily be small, and therefore but few rays, comparatively, come from the objects. By making the opening larger, and gathering the rays that enter it with a double convex lens, we can have well-defined and bright images of objects. Though the camera obscura may have various forms, I have described what is essentially the arrangement of the instrument. One form of it, for sketching either single objects or groups of them in landscapes, is represented in Fig. 243. Here the rays of light coming from objects strike upon a mirror, A B, and are reflected through a convex lens, C D, upon white paper on the bottom, E F, of the box, where the outlines of the images are traced by the sketcher. The light can enter only at the opening above, for on the side of the box which is open there hangs down a curtain on the back of the artist as he sketches. [Illustration: Fig. 244.] 353. =The Eye.=--The eye is essentially a camera obscura. It is a dark chamber in which images are formed upon a screen in its back part, and the light which comes from objects is admitted through an opening in front, where there is a double convex lens. That you may understand the manner in which the images are formed, I give you, in Fig. 244, a map of the eye. At _a_ is the thick, strong white coat called the _sclerotic_ coat, from a Greek word meaning hard. This, which is commonly the white of the eye, gives to the eyeball its firmness. Into this is fastened in front, like a crystal in a watch-case, _e_, the _cornea_. The sclerotic and cornea, you see then, make together one coat of the eye, the outer one. The cornea is the clear, transparent window of the eye through which the light enters. Next to the sclerotic coat comes the _choroid_ coat, which is dark, to prevent too much reflection back and forth in the eye. Then you have a very thin membrane, _c_, the _retina_, the screen on which the images are formed. This is composed chiefly of the fine fibres of the nerve of sight, _d_. To return to the front of the eye where the light enters--behind the cornea is the iris, _g g_, which is immersed in a watery fluid, _f_, called the _aqueous humor_. The light passing through the cornea and the aqueous humor comes to the crystalline lens, _h_, which, you see, is a double convex lens. Passing through this and through a jelly-like substance, called the vitreous humor, which