1 66 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. It would then require nearly as much 

 trouble to carry as the present sun-signalling apparatus, 

 while it would be less rapid and sure. 1 



It 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 long- 

 range 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 [12]. 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- 



1 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. 



