MM refers to the focal length of the lens. In the old, old days of Paul Rudolph's original Tessar lens, "focal length" meant the distance between the middle of the lens and the "focal point" - the point at which all the rays of light coming out of the lens converge on one spot. That lens was first introduced in 1902, and since then we've learned that different focal lengths of lenses have different angles of view...so now, if a lens has an angle of view equal to that of a 50mm lens, we call the lens a "50mm" regardless of the actual length of the lens.

It is lens. The lens consists of the lens capsule, the lens epithelium, and the lens fibres.

The ocular is the upper lens and objective is the lower lens

)( is a concave lens
() is a convex lens

The objective lens