[Illustration:  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.

[Illustration:  FIG. 75.—­The lens is held in such a position that the image of the candle is larger than the object.]

[Illustration:  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 assist 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.

[Illustration:  FIG. 77.—­The lens is placed in such a position that the image is about the same size as the object.]

Many familiar illustrations 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 pass from the object through the pupil P, go through the crystalline lens L, where they are refracted, and then pass onward to the retina R, where they form a distinct image of the object.