DUANE— SIGHT AND SIGNALLING IN THE NAVY. 407 



cially the night semaphore. When, however, the ship is rolHng or 

 pitching a good deal, it is difficult to keep the glass on the object. 

 It then becomes quite hard to read the signals, and the strain on 

 the attention is considerably increased. And even when there is 

 no motion the use of a glass causes more strain than looking with 

 the naked eye. 



4. TJie Motion or Position of the Sliip. — This not only makes 

 the use of a glass difficult, but even with the naked eye interferes 

 with the observation of signals. So too, the ship may be so placed 

 that flag or semaphore signals are not easily seen or have a poor 

 background. 



5. Occlusion of Object by spars or funnels or by the sudden dip- 

 ping of the ship in a heavy swell. 



Influence of These Conditions in Special Cases. 



The visibility and legibility of the chief methods of signalling 

 and the demands they make upon the eyesight may be summarized 

 as follows : 



/. Hand Wig-Wag. — The regular wig-wag signal kit comprises 

 two-foot, four-foot, and six-foot flags, white, red and black. The 

 black flag is supposed to be used with a sky background, but is rarely 

 employed. The red flag theoretically would show better than the 

 white against a green or white background, but, as a matter of fact, 

 the white shows best under almost all circumstances. 



The wig-wag is visible a very considerable distance. Its legi- 

 bility varies very greatly with the technique of the sender and with 

 the background. Given a good sender and a good sky background, 

 the wig-wag messages with the large flags can be read over six 

 miles^ even with the naked eye. This method requires close atten- 

 tion, but is not particularly tiring to the eyes. 



Very legible is the night wig-wag with oil or electric lantern. 



2. Wig-Wag by Searchlight. — A searchlight projected against 



a cloud and waved from side to side like the hand wig-wag, can be 



seen and read practically any distance. Vessels have signalled in 



this w^ay from ocean to ocean across the Isthmus of Panama — the 



5 The statements in this and the following paragraphs regarding range 

 of visibility are to be taken as the maximum for expert receivers. For ordi- 

 nary signalmen, particularly when the weather conditions are not good, these 

 distances would be halved. 



