VISUAL SYSTEMS OF TRANSIENT COMMUNICATION 



259 



of very few of the various methods has been 

 explored fully in those situations where they 

 may be employed. 



The first set of direct systems includes 

 those in which the receiver sees some be- 

 havior of the sender that has become sym- 

 bolic, through the previous training of both 

 sender and receiver, and accordingly is re- 

 sponded to specifically and appropriately 

 by the receiver. 



Gestwe 



Gestures which have been employed as 

 stimuli in communication range from the 

 simple raised hand of the schoolboy who 

 wants to leave the room, to the elaborate 

 series of symbols employed in the referee- 

 ing of a game. A few experimental stud- 

 ies have been made on gesture, but these 

 have been restricted in scope. In military 

 situations, gestures have but rarely been 

 employed, owing to the critical lack of data 

 on the fineness of differentiation made by 

 the sender in producing gestures, and of the 

 receiver in discriminating them. Other 

 methods of visual communication have been 

 preferred for their greater precision, although 

 it is undoubtedly true that gesture is capable 

 of greater precision than is now shown. 



Sign Language 



Sign languages are essentially refinements 

 of gesture from gross bodily movements to 

 movement of specific parts of the body, 

 usually the arms and hands. The sign sys- 

 tems of the deaf indicate the great flexibility 

 of such behavior, as does the development 

 of a sign language like the lingua franca of 

 Indian tribes who did not share a spoken 

 language. Sign language is effective only 

 over very short visual ranges, even where 

 visual aids are available, and is a relatively 

 slow method. The difficulty of learning, 

 as well, necessarily eliminates sign lan- 

 guages, except very simple ones, from serious 

 consideration where auditory communica- 

 tion is impossible. 



Lip Reading 



Here, the signs of the sender are pro- 

 duced incidentally to voice communication 

 and are effective only over very short ranges, 

 where the receiver, either because of deaf- 

 ness or of high noise levels, is incapable of 

 auditory discrimination. As flexible as lan- 

 guage, it is both imprecise and difficult to 

 learn, and consequently is not at all suitable 

 for military exploitation. 



The second set of direct systems includes 

 those which we have termed supplemented. 

 Here, the sender directly controls the activ- 

 ities of some device. These activities are 

 directly seen by the receiver. In this class, 

 we find the basic methods of exterior visual 

 communication now employed by the Navy : 

 mirror signalling, searchlight blinker, sig- 

 nal flags and semaphore. 



The Heliographic Signalling Mirror 



The heliographic signalling mirror was de- 

 veloped to its present status during the re- 

 cent war as a means of signalling aircraft in 

 air-sea rescue. Its advantages lie in the 

 extreme simplicity of the apparatus, and 

 the ease with which it may be used. How- 

 ever, the information transmitted is of the 

 simplest sort: it is signalling to the receiver 

 of the presence of the sender at the point 

 of origin of the flash produced when the 

 mirror reflects the image of the sun into the 

 receiver's eyes. Although the use of such 

 mirrors has been restricted to air-sea rescue, 

 there is no reason why mirror signalling 

 cannot be employed as an alternative 

 method of blinker signalling when the sun is 

 bright and high. Given the proper solar 

 elevation and good atmospheric conditions, 

 mirror signalling can supplement search-light 

 blinker by day. As a method of com- 

 munication, then, it can be expected to show 

 many of the characteristics of searchlight 

 blinkers. 



