fills all that large space _i_, it strikes upon the retina, _c_, where it forms the images of the objects from which it came. You see now how the eye is like a camera obscura. You have in it the dark chamber with its screen, the opening through the iris, the pupil, for the admission of the light, and just behind this opening the lens for gathering or concentrating the light before it falls upon the retina. The refraction of the light is not, however, done wholly by this lens. The projecting cornea, with its contained aqueous humor, refracts it considerably, for it forms a convex lens. [Illustration: Fig. 245.] [Illustration: Fig. 246.] 354. =Distinct Vision.=--In order that vision may be perfectly distinct, it is necessary that the rays coming from each point of the object which is seen should, on converging, meet together, or be brought to a focus on the screen of the eye, the retina. Thus, in Fig. 245, the rays which come from _a_, the end of the arrow, meet on the retina at _b_, and those from _c_, the other end, are brought to a focus at _d_. Now the muscles of the eye have considerable power in adjusting the eye to objects at different distances, so as to bring the rays in most cases together exactly at the retina. They fail to do it with objects that are very near. You can see that this is so if you bring any object, as your finger, nearer and nearer to the eye. You will at length find that you can not see it distinctly. The reason is, that the rays from it diverge so much that the cornea and lens can not make them converge enough to meet at the retina. This divergence of rays at different distances is illustrated in Fig. 246. Suppose that you are looking at some very minute object. The nearer you bring it to the eye the better you can see it, till you come to a certain point. There the rays are so divergent, as you can readily see by the figure, that the lenses of the eye can not make them converge sufficiently for distinct vision. Now just here the microscope comes in to help the eye by causing these divergent rays to come nearer together before they enter the window of the eye, the cornea. [Illustration: Fig. 247.] [Illustration: Fig. 248.]