"But it is not there that we see it. As soon as the points of light from the pencil strike upon the retina, the thrill passes on along the optic nerve on, through the back of the eye to the brain; and our mind, following back the rays exactly as they have come through the lens, sees a pencil, outside the eye, right way upwards.
"This is how we see with our eyes, which adjust themselves most beautifully to our needs. For example, not only is the iris always ready to expand or contract according as we need more or less light, but there is a special muscle, called the ciliary muscle (cm, Fig. 11), which alters the lens for us to see things far or near. In all, or nearly all, perfect eyes the lens is flatter in front than behind, and this enables us to see things far off by bringing the rays from them exactly to a focus on the retina. But when we look at nearer things the rays require to be more bent or refracted, so without any conscious effort on our part this ciliary muscle contracts and allows the lens to bulge out slightly in front. Instantly we have a stronger magnifier, and the rays are brought to the right focus on the retina, so that a clear and full-size image of the near object is formed. How little we think, as we turn our eyes from one thing to another, and observe, now the distant hills, now the sheep feeding close by; or, as night draws on, gaze into limitless space and see the stars millions upon millions of miles away, that at every moment the focus of our eye is altering, the iris is contracting or expanding, and myriads of images are being formed one after the other in that little dark chamber, through which pass all the scenes of the outer world!
"Yet even this wonderful eye cannot show us everything. Some see farther than others, some see more minutely than others, according as the lens of the eye is flatter in one person and more rounded in another. But the most long-sighted person could never have discovered the planet Neptune, more than 2700 millions of miles distant from us, nor could the keenest-sighted have known of the existence of those minute and beautiful little plants, called diatoms, which live around us wherever water is found, and form delicate flint skeletons so infinitesimally small that thousands of millions go to form one cubic inch of the stone called tripoli, found at Bilin in Bohemia."
Fig. 13.
Arrow magnified by a convex lens.
a, b, Real arrow. C, D, Magnifying-glass. A, B, Enlarged image of the arrow.
"It is here that our 'magic glasses' come to our assistance, and reveal to us what was before invisible. We learnt just now that we see near things by the lens of our eye becoming more rounded in front; but there comes a point beyond which the lens cannot bulge any more, so that when a thing is very tiny, and would have to be held very close to the eye for us to see it, the lens can no longer collect the rays to a focus, so we see nothing but a blur. More than 800 years ago an Arabian, named Alhazen, explained why rounded or convex glasses make things appear larger when placed before the eye. This glass which I hold in my hand is a simple magnifying-glass, such as we used for focusing the candle-flame. It bends the rays inwards from any small object (see the arrow a, b, Fig. 13) so that the lens of our eye can use them, and then, as we follow out the rays in straight lines to the place where we see clearly (at A, B), every point of the object is magnified, and we not only see it much larger, but every mark upon it is much more distinct. You all know how the little shilling magnifying-glasses you carry show the most lovely and delicate structures in flowers, on the wings of butterflies, on the head of a bee or fly, and, in fact, in all minute living things."
Fig. 14.
