A convex lens then has the power of magnifying objects when brought near the eye, and its action is clearly seen in Fig. [35], where the upper figure shows the arrow at as short a distance from the eye as it can be seen distinctly with an ordinary eye, and the lower figure shows the same arrow brought close to the eye, and rendered distinctly visible by the lens when a magnified image is thrown on the retina, as if there was a real larger arrow somewhere between the dotted lines at the ordinary distance of distinct vision. It is also obvious that the nearer the object can be brought to the eye-lens the more magnified it is, just as an object appears larger the nearer it is brought to the unaided eye.
We have been hitherto dealing with the effect of a convex lens on the rays passing to the eye. We will now deal with a concave one.
We found that the power of adjustment of the normal eye was sufficient to bring parallel rays, or those proceeding from a very distant object, and also slightly diverging rays, to a focus on the retina. Parallel or slightly divergent rays are most easily dealt with, and slightly convergent rays can also be focussed on the retina; but if the eye-lens is too convex, as is the case with short-sighted people, Fig. [36], a concave lens of slight curvature is used to correct the eye-lens and bring the image to a focus on the retina instead of in front of it.
Fig. 36.—Action of Short-sighted Eye.
If the rays are very convergent, as those proceeding from a convex lens and coming to a focus, the lens of a normal eye will bring them to a focus far in front of the retina, as if the person were very short-sighted. But by interposing a sufficiently powerful concave lens the rays are made less convergent or parallel, and the eye-lens brings them to a focus on the retina, as if they came from a near object, so the use of convex and concave lenses placed close to the eye is to render divergent or convergent rays nearly parallel, so that the eye-lens can easily focus them, and therefore one of the conditions of the telescope is that the rays which come into our eye shall be parallel or nearly so.
CHAPTER VI.
THE REFRACTOR.
In the telescope as first constructed by Galileo there are two lenses, so arranged that the first, a convex one, A B Fig. [37], converges the rays, while the second, C D, a concave one, diverges them, and renders them parallel, ready for the eye; the rays then, after passing through C D, go to the eye as if they were proceeding along the dotted lines from an object M M, closer to the eye instead of from a distant object, and so, by means of the telescope, the object appears large and close.
Fig. 37.—Galilean Telescope. A B, convex lens converging rays; C D, concave lens sending them parallel again and fit for reception by the eye.