: General Science by Clark Bertha May - Science

@FreeBooksTue 06 Jun, 2023

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The light from the sun, moon, and stars can reach us only by passing through the atmosphere, but in Section 76, we learned that the atmosphere varies in density from level to level; hence all the light which travels through the atmosphere is constantly deviated from its original path, and before the light reaches the eye it has undergone many changes in direction. Now we learned in Section 102, that the direction of the rays of light as they enter the eye determines the direction in which an object is seen; hence the sun, moon, and stars seem to be along the lines which enter the eye, although in reality they are not.

Lenses are very similar to prisms; indeed, two prisms placed as in Figure 69, and rounded off, would make a very good convex lens. A lens is any transparent material, but usually glass, with one or both sides curved. The various types of lenses are shown in Figure 71.

The position of the principal focus depends not only on the shape of the lens, but also on the refractive power of the material composing the lens. A lens made of ice would not deviate the rays of light so much as a lens of similar shape composed of glass. The greater the refractive power of the lens, the greater the bending, and the nearer the principal focus to the lens.

There are many different kinds of glass, and each kind of glass refracts the light differently. Flint glass contains lead; the lead makes the glass dense, and gives it great refractive power, enabling it to bend and separate light in all directions. Cut glass and toilet articles are made of flint glass because of the brilliant effects caused by its great refractive power, and imitation gems are commonly nothing more than polished flint glass.

We learned in Section 114 that a change in the position of the object necessitated a change in the position of the screen, and that every time the object was moved the position of the screen had to be altered before a clear image of the object could be obtained. The retina of the eye cannot be moved backward and forward, as the screen was, and the crystalline lens is permanently located directly back of the iris. How, then, does it happen that we can see clearly both near and distant objects; that the printed page which is held in the hand is visible at one second, and that the church spire on the distant horizon is visible the instant the eyes are raised from the book? How is it possible to obtain on an immovable screen by means of a simple lens two distinct images of objects at widely varying distances?

A nearsighted person is one who cannot see objects unless they are close to the eye. The eyeball of a nearsighted person is very wide, and the retina is too far away from the crystalline lens. Far objects are brought to a focus in front of the retina instead of on it, and hence are not visible. Even though the muscles of accommodation do their best to pull out and flatten the lens, the rays are not separated sufficiently to focus as far back as the retina. In consequence objects look blurred. Nearsightedness can be remedied by wearing concave glasses, since they separate the light and move the focus farther away. Concave glasses, by separating the rays and making the focus more distant, overbalance a wide eyeball with its tendency to focus objects in front of the retina.

An exact balance is required between glasses, crystalline lens, and muscular activity, and only those who have studied the subject carefully are competent to treat so sensitive and necessary a part of the body as the eye. The least mistake in the curvature of the glasses, the least flaw in the type of glass , means an improper focus, increased duty for the muscles, and gradual weakening of the entire eye, followed by headache and general physical discomfort.

Possibly the greatest danger of eye strain is among school children, who are not experienced enough to recognize defects in sight. For this reason, many schools employ a physician who examines the pupils' eyes at regular intervals.

The following general precautions are worth observing:--

PHOTOGRAPHY

Many housewives lower the window shades that the wall paper may not lose its brilliancy, that the beautiful hues of velvet, satin, and plush tapestry may not be marred by loss in brilliancy and sheen. Bright carpets and rugs are sometimes bought in preference to more delicately tinted ones, because the purchaser knows that the latter will fade quickly if used in a sunny room, and will soon acquire a dull mellow tone. The bright and gay colors and the dull and somber colors are all affected by the sun, but why one should be affected more than another we do not know. Thousands of brilliant and dainty hues catch our eye in the shop and on the street, but not one of them is absolutely permanent; some may last for years, but there is always more or less fading in time.

Sunlight causes many strange, unexplained effects. If the two substances, chlorine and hydrogen, are mixed in a dark room, nothing remarkable occurs any more than though water and milk were mixed, but if a mixture of these substances is exposed to sunlight, a violent explosion occurs and an entirely new substance is formed, a compound entirely different in character from either of its components.

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