Note 3 — Chapter XII · “ART OF TRAVEL”

Anyhow, the optical principle on which it worked was pretty. A part of the flash struck one end of a strip cut out of the middle of a glass lens, and was brought by it to a focus (a burning spot) on an otherwise shaded porcelain screen. The eye looking through the other end of the strip saw the burning spot as a mock-sun. Now, by a well-known optical law, the apparent position of the burning spot is the same whatever be the part of the lens that makes it, or through which it is viewed. So the mock-sun seen by the eye covers the same part of the landscape that is simultaneously covered by the flash. The eye sees, it is true, only one portion of the mock-sun, whence the position of the rest has to be inferred.