The lens that has a thin middle and makes objects appear smaller is a concave lens, also known as a diverging lens. It is thinner at the center than at the edges, causing light to spread out as it passes through the lens, resulting in the objects appearing smaller and farther away from the lens. Concave lenses are commonly used for correcting nearsightedness, and they are also used in microscopes and telescopes to reduce the size of the image.