a more desirable and useful shape and size fig5 6 show a reflecting shell 220 according to a second embodiment of the present invention similar to the first embodiment the reflecting shell 220 includes a tapered upper body 222 and a tapered lower body 224 sequentially connected together the inner surfaces 2222 2242 of the two bodies 222 224 are approximately conoid on the assumption that the inner surfaces 2222 2242 of the two bodies 222 224 are conoid the taper angle u03b1 2 u03b2 2 of the inner surface 2222 2242 of each body 222 224 is in range of 10 u02dc 70 degrees the taper angle u03b1 2 of the inner surface 2222 of the upper body 222 is smaller than the taper angle u03b2 2 of the inner surface 2242 of the lower body 224 the inner angle u03b3 2 formed by the inner surfaces 2222 2224 is larger than 180 degrees one difference between this embodiment and the first embodiment is that the inner surface 2222 2242 of each body 222 224 is concave and thus the generatrix of the inner surface 2222 2242 of each body 222 224 is a curved line it is to be understood that the inner surface 2222 2242 of each body 222 224 can alternatively be convex in addition the outer surfaces 2224 2244 of the upper and lower bodies 222 224 are in the form of truncated pyramids in this embodiment the outer surfaces 2224 2244 of the two bodies 222 224 are in the form of twelve sided truncated pyramids in other.