ClipArt ETC
"...represents two parabolic reflectors, mn and op. It is a peculiarity of such reflectors that rays starting from the focus, as F will be reflected as parallel rays, and that parallel rays falling upon sucha reflector will converge at the focus, as F'. Hence, two such reflectors may be placed in such a sposition that sound waves starting from one focus shall, after two reflections, be converged at the other focus...Two reflectors so placed are said to be conjugate to each other." -Avery 1895

Parabolic Reflectors

"...represents two parabolic reflectors, mn and op. It is a peculiarity of such reflectors that rays…