166 MEMORIES OF MY LIFE
travelling in Damara Land. However, without additional complications, it could not be made into a really serviceable instrument for transmitting verbal messages. I t would then require nearly as much trouble to carry as the present sun-signalling apparatus, while it would be less rapid and sure.'
I t is interesting to flash with a small mirror against a light-coloured surface that lies in shadow, as through an open window against the opposite wall of the room behind. The size and shape of the mirror is then seen to have very little influence on the size or shape of the mock-sun, even at moderate distances. In longrange signalling their influence is wholly inappreciable.
I may describe here another contrivance, partly belonging to Art-of-Travel matters, partly military, that I sent to the United Service Institution . It was appropriate to the days of " Brown Bess," but useless as a protection against modern musket bullets with their flat trajectories. I showed it was easy to provide a screen under which A. could hit B. at any distance beyond, say, 200 yards, while on the other hand B. could not hit A., although he might see him clearly. The balls of B. would be intercepted by the target. The principle on which the target gave pro
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-stin. Now, by a wellknown 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.