convenient ray to choose would be one passing through
_O_, the optical center of the lens, because such a ray passes 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.

[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 greate