A body has a configuration in which a first ellipse has a first and second focal
points and a second ellipse having a third and fourth points are arranged such
that the second and third focal points coincide. This body is equivalent to an
optical system having an optical path in which light gathered to the first focal
point passes through the first focal point, and is reflected by a first concave
reflecting surface, being a part of the first ellipse, to go through the second
focal point, and then reflected by a second concave reflecting surface, being a
part of the second ellipse, to be gathered to the fourth focal point.
