Take a concave mirror of the diameter of 40 inches, and three in
concavity, made by cementing pieces of looking-glass on a wooden mould.
The focus will be three feet six inches or so from the mirror.
The image of a person standing before it will be so vivid and seemingly
solid that it will terrify even a strong-minded experimentalist.
As a reflector of light or a burning-glass, the effects are wonderful,
even with an English sun.