[Illustration: 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 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. 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 principal axis, will pass 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. [Illustration: 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 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.