110. The Window Pane. We have seen that light is bent when it pa.s.ses from one medium to another of different density, and that objects viewed by refracted light do not appear in their proper positions.

When a ray of light pa.s.ses through a piece of plane gla.s.s, such as a window pane (Fig. 67), it is refracted at the point _B_ toward the perpendicular, and continues its course through the gla.s.s in the new direction _BC_. On emerging from the gla.s.s, the light is refracted away from the perpendicular and takes the direction _CD_, which is clearly parallel to its original direction. Hence, when we view objects through the window, we see them slightly displaced in position, but otherwise unchanged. The deviation or displacement caused by gla.s.s as thin as window panes is too slight to be noticed, and we are not conscious that objects are out of position.

[Ill.u.s.tration: FIG. 67.--Objects looked at through a window pane seem to be in their natural place.]

111. Chandelier Crystals and Prisms. When a ray of light pa.s.ses through plane gla.s.s, like a window pane, it is shifted somewhat, but its direction does not change; that is, the emergent ray is parallel to the incident ray. But when a beam of light pa.s.ses through a triangular gla.s.s prism, such as a chandelier crystal, its direction is greatly changed, and an object viewed through a prism is seen quite out of its true position.

Whenever light pa.s.ses through a prism, it is bent toward the base of the prism, or toward the thick portion of the prism, and emerges from the prism in quite a different direction from that in which it entered (Fig. 68). Hence, when an object is looked at through a prism, it is seen quite out of place. In Figure 68, the candle seems to be at _S_, while in reality it is at _A_.

[Ill.u.s.tration: FIG. 68.--When looked at through the prism, _A_ seems to be at _S_.]

112. Lenses. If two prisms are arranged as in Figure 69, and two parallel rays of light fall upon the prisms, the beam _A_ will be bent downward toward the thickened portion of the prism, and the beam _B_ will be bent upward toward the thick portion of the prism, and after pa.s.sing through the prism the two rays will intersect at some point _F_, called a focus.

[Ill.u.s.tration: FIG. 69.--Rays of light are converged and focused at _F_.]

If two prisms are arranged as in Figure 70, the ray _A_ will be refracted upward toward the thick end, and the ray _B_ will be refracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect.

[Ill.u.s.tration: FIG. 70.--Rays of light are diverged and do not come to any real focus.]

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 gla.s.s, with one or both sides curved. The various types of lenses are shown in Figure 71.

[Ill.u.s.tration: FIG. 71.--The different types of lenses.]

The first three types focus parallel rays at some common point _F_, as in Figure 69. Such lenses are called convex or converging lenses. The last three types, called concave lenses, scatter parallel rays so that they do not come to a focus, but diverge widely after pa.s.sage through the lens.

113. The Shape and Material of a Lens. The main or princ.i.p.al focus of a lens, that is, the point at which rays parallel to the base line _AB_ meet (Fig. 71), depends upon the shape of the lens. For example, a thick lens, such as _A_ (Fig. 72), focuses the rays very near to the lens; _B_, which is not so thick, focuses the rays at a greater distance from the lens; and _C_, which is a very thin lens, focuses the rays at a considerable distance from the lens. The distance of the princ.i.p.al focus from the lens is called the focal length of the lens, and from the diagrams we see that the more convex the lens, the shorter the focal length.

[Ill.u.s.tration: FIG. 72.--The more curved the lens, the shorter the focal length, and the nearer the focus is to the lens.]

The position of the princ.i.p.al 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 gla.s.s. The greater the refractive power of the lens, the greater the bending, and the nearer the princ.i.p.al focus to the lens.

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

114. How Lenses Form Images. Suppose we place an arrow, _A_, in front of a convex lens (Fig. 73). The ray _AC_, parallel to the princ.i.p.al axis, will pa.s.s through the lens and emerge as _DE_. The ray is always bent toward the thick portion of the lens, both at its entrance into the lens and its emergence from the lens.

[Ill.u.s.tration: FIG. 73.--The image is larger than the object. By means of a lens, a watchmaker gets an enlarged image of the dust which clogs the wheels of his watch.]

In Section 105, we saw that two rays determine the position of any point of our image; hence in order to locate the image of the top of the arrow, we need to consider but one more ray from the top of the object. The most convenient ray to choose would be one pa.s.sing through _O_, the optical center of the lens, because such a ray pa.s.ses through the lens unchanged in direction, as is clear from Figure 74. The point where _AC_ and _AO_ meet after refraction will be the position of the top of the arrow. Similarly it can be shown that the center of the arrow will be at the point _T_, and we see that the image is larger than the object. This can be easily proved experimentally. Let a convex lens be placed near a candle (Fig. 75); move a paper screen back and forth behind the lens; for some position of the screen a clear, enlarged image of the candle will be made.

[Ill.u.s.tration: FIG. 74.--Rays above _O_ are bent downward, those below _O_ are bent upward, and rays through _O_ emerge from the lens unchanged in direction.]

If the candle or arrow is placed in a new position, say at _MA_ (Fig.

76), the image formed is smaller than the object, and is nearer to the lens than it was before. Move the lens so that its distance from the candle is increased, and then find the image on a piece of paper. The size and position of the image depend upon the distance of the object from the lens (Fig. _77_). By means of a lens one can easily get on a visiting card a picture of a distant church steeple.

[Ill.u.s.tration: FIG. 75.--The lens is held in such a position that the image of the candle is larger than the object.]

[Ill.u.s.tration: FIG. 76.--The image is smaller than the object.]

115. The Value of Lenses. If it were not for the fact that a lens can be held at such a distance from an object as to make the image larger than the object, it would be impossible for the lens to a.s.sist the watchmaker in locating the small particles of dust which clog the wheels of the watch. If it were not for the opposite fact--that a lens can be held at such a distance from the object as to make an image smaller than the object, it would be impossible to have a photograph of a tall tree or building unless the photograph were as large as the tree itself. When a photographer takes a photograph of a person or a tree, he moves his camera until the image formed by the lens is of the desired size. By bringing the camera (really the lens of the camera) near, we obtain a large-sized photograph; by increasing the distance between the camera and the object, a smaller photograph is obtained.

The mountain top may be so far distant that in the photograph it will not appear to be greater than a small stone.

[Ill.u.s.tration: FIG. 77.--The lens is placed in such a position that the image is about the same size as the object.]

Many familiar ill.u.s.trations of lenses, or curved refracting surfaces, and their work, are known to all of us. Fish globes magnify the fish that swim within. Bottles can be so shaped that they make the olives, pickles, and peaches that they contain appear larger than they really are. The fruit in bottles frequently seems too large to have gone through the neck of the bottle. The deception is due to refraction, and the material and shape of the bottle furnish a sufficient explanation.

By using combinations of two or more lenses of various kinds, it is possible to have an image of almost any desired size, and in practically any desired position.

116. The Human Eye. In Section 114, we obtained on a movable screen, by means of a simple lens, an image of a candle. The human eye possesses a most wonderful lens and screen (Fig. 78); the lens is called the crystalline lens, and the screen is called the retina. Rays of light pa.s.s from the object through the pupil _P_, go through the crystalline lens _L_, where they are refracted, and then pa.s.s onward to the retina _R_, where they form a distinct image of the object.

[Ill.u.s.tration: FIG. 78.--The eye.]

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?

The answer to these questions is that the crystalline lens changes shape according to need. The lens is attached to the eye by means of small muscles, _m_, and it is by the action of these muscles that the lens is able to become small and thick, or large and thin; that is, to become more or less curved. When we look at near objects, the muscles act in such a way that the lens bulges out, and becomes thick in the middle and of the right curvature to focus the near object upon the screen. When we look at an object several hundred feet away, the muscles change their pull on the lens and flatten it until it is of the proper curvature for the new distance. The adjustment of the muscles is so quick and unconscious that we normally do not experience any difficulty in changing our range of view. The ability of the eye to adjust itself to varying distances is called accommodation. The power of adjustment in general decreases with age.

117. Farsightedness and Nearsightedness. A farsighted person is one who cannot see near objects so distinctly as far objects, and who in many cases cannot see near objects at all. The eyeball of a farsighted person is very short, and the retina is too close to the crystalline lens. Near objects are brought to a focus behind the retina instead of on it, and hence are not visible. Even though the muscles of accommodation do their best to bulge and thicken the lens, the rays of light are not bent sufficiently to focus sharply on the retina. In consequence objects look blurred. Farsightedness can be remedied by convex gla.s.ses, since they bend the light and bring it to a closer focus. Convex gla.s.ses, by bending the rays and bringing them to a nearer focus, overbalance a short eyeball with its tendency to focus objects behind the retina.

[Ill.u.s.tration: FIG. 79.--The farsighted eye.]

[Ill.u.s.tration: FIG. 80.--The defect is remedied by convex gla.s.ses.]

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 gla.s.ses, since they separate the light and move the focus farther away. Concave gla.s.ses, 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.

[Ill.u.s.tration: FIG. 81.--The nearsighted eye. The defect is remedied by concave gla.s.ses.]

118. Headache and Eyes. Ordinarily the muscles of accommodation adjust themselves easily and quickly; if, however, they do not, frequent and severe headaches occur as a result of too great muscular effort toward accommodation. Among young people headaches are frequently caused by over-exertion of the crystalline muscles. Gla.s.ses relieve the muscles of the extra adjustment, and hence are effective in eliminating this cause of headache.

An exact balance is required between gla.s.ses, 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 gla.s.ses, the least flaw in the type of gla.s.s (for example, the kind of gla.s.s used), means an improper focus, increased duty for the muscles, and gradual weakening of the entire eye, followed by headache and general physical discomfort.

119. Eye Strain. The extra work which is thrown upon the nervous system through seeing, reading, writing, and sewing with defective eyes is recognized by all physicians as an important cause of disease.

The tax made upon the nervous system by the defective eye lessens the supply of energy available for other bodily use, and the general health suffers. The health is improved when proper gla.s.ses are prescribed.

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:--

1. Rest the eyes when they hurt, and as far as possible do close work, such as writing, reading, sewing, wood carving, etc., by daylight.

2. Never read in a very bright or a very dim light.

3. If the light is near, have it shaded.

4. Do not rub the eyes with the fingers.

5. If eyes are weak, bathe them in lukewarm water in which a pinch of borax has been dissolved.