and see distinctly that which is submitted to it. But we have every reason to suppose that if the eye could be brought within an inch of a small object, and see it clearly, it would appear much larger than it does to the unassisted eye at the ordinary distance.
The balloon and the castle as they came nearer to the spectator kept increasing in size, according to a well-known law, and would have continued to do so had they not been stopped by the inability of the eye to see them distinctly at a lesser distance than its proper focus. There are, however, strong-sighted persons, whose eyes, having a much shorter focus than their neighbours, are able to look more closely into some matters than their less-gifted brethren. Such may be seen reading a favourite author with their noses in contact with the page. Their vision is, in fact, microscopic. They see everything on a larger scale than ordinary folks. Try them with the minutest specimens of caligraphy or printing, and they excite your envy by reading with ease that which you can barely decipher by the aid of a lens. Whence this power? It will be found, if we mistake not, in the difference of the organization of such an eye. Our friend the physiologist here comes to our aid: by the light he has thrown upon the subject we perceive that the cornea of the short-sighted eye has a deeper curve than the cornea of the ordinary eye, and this may also apply to the crystalline lens. Such a departure from the normal form tends greatly to increase the refractive power of the eye, and thus to shorten its focus. But whilst an eye of this description has an advantage in being able to see objects distinctly at a short distance, and consequently much magnified, it labours under the great disadvantage in being unable to distinguish a friend from a lamp-post on the opposite side of the street. Without the aid of concave glasses to elongate his vision it would be impossible for a short-sighted person to realize the poet's expression—
'Tis distance lends enchantment to the view.
As we who possess ordinary eyes cannot alter their natural organization, it is pretty clear, if we wish to increase their power, that we must seek for the material wherewith to do so in the region of optics.
Ordinary object are seen by the light they reflect from all points of their surfaces. Thousands of such rays emanate from the objects that are round about us, each ray bearing the form and colour of that particular spot from whence it has proceeded. These enter the eye through that small hole—the pupil—and, in the aggregate, form on its retina a perfect miniature picture. Such, however, is the result only when the eye is at a proper distance from an object; when it approaches too closely it fails to perceive it. There is no distinct vision, owing to the light's rays diverging at too great an angle. Many of them do not enter the eye at all, but shoot by it; while those rays that do gain an entrance are still too divergent for the eye to bring them into a proper condition for forming a perfect image on the retina. Now as the mind can receive no true idea of the form and colour of external objects except through the instrumentality of the eye, it will necessarily follow that distinct vision depends on the accuracy of the picture formed on its retina. Where the image produced is hazy and indistinct, the mental impression will be equally obscure. Knowledge is said to be power. It certainly is so when applied to optics. By our searches we become acquainted with the fact that when a ray of light, in an oblique or slanting direction, enters a transparent medium denser than that through which it has previously travelled (such as water or glass), it suffers refraction; that is, it is bent out of its original course. Taking advantage of this fact, we procure a piece of clear glass, give to each of its surfaces a curve like that described by a bent bow, and then highly polish them. This is termed a convex lens, and when mounted on an arm, or in some kind of handle, it becomes a hand-magnifier, or simple microscope. The action of such a lens on rays diverging from a billiant point (if that point be in its focus) is to render them parallel, or if the point be nearer to the lens than its true focus, the divergent rays will rendered less so. If parallel rays be transmitted they will come to a point at some distance beyond the lens. This is its true focus for parallel rays, and any small object placed here and properly illuminated would be more distinctly seen than in any other position. By giving to our piece of glass a deeper curve, or by selecting a denser medium than glass, such as a diamond or ruby, we obtain a lens still more powerful in its refractive properties. We have thus the means of providing ourselves with lenses of longer or shorter foci, and of different degrees of magnifying power. All lenses of high power have short foci, and must be held very close to an object in order to see it; in some instances almost in contact with it. Now observe how cunningly that animal, man, takes advantage of his own contrivance. He has by his skill constructed an
