258 



VISUAL COMMUNICATION 



upper limit in the number of stimuli for 

 communication purposes should be six, since 

 this is the approximate maximum that is 

 discriminated correctly when presented si- 

 multaneously ("subitized"). 



Spatial Patterning 



Spatial patterning of several simultaneous 

 stimuli includes all cases of the use of visually 

 presented words and sentences, and also of 

 dials, pointers, and similar visual displays. 

 The variables which determine legibility and 

 optimal instrument design are pertinent to 

 the use of such spatial patterning in visual 

 communication. A series of dials, indi- 

 cators, or words presented to the receiver 

 may conveniently serve as the means for 

 establishing man to man couples, just as 

 they do for machine to man couples. 



There has been occasion in the preceding 

 paragraphs to note several limitations on the 

 visual dimensions which may be employed in 

 visual communication. The limitations 

 stem from the following physiological func- 

 tions: (1) the photopic and scotopic visi- 

 bility curves, (2) the visual acuity function, 



(3) the level of adaptation of the receiver, 



(4) the intensity discrimination functions, 



(5) the hue-discrimination function, (6) the 

 flicker function, (7) the sensitivity functions 

 of the visual field. For exterior conmiunica- 

 tions, where the distance between stimuli 

 and receiver may vary, and where at- 

 mospheric attenuation adds yet another vari- 

 able, it is clear that the limits imposed by 

 (1), (2), (3), (4), and (5) will vary in an 

 irregular way, so that any discriminations 

 set up along these dimensions will yield un- 

 reliable communication. Temporal pattern- 

 ing is the only relatively stable method of 

 communication over a great and variable 

 range. 



For these reasons, studies of visual sig- 

 nalling have been largely concerned with 

 establishing the ranges at which stimuli of 

 varying brightness and color can be just 



seen, or just identified with respect to hue, 

 as a function of various meteorological vari- 

 ables. 



Where the range of observation is fixed, 

 however, and where atmospheric attenua- 

 tion is negligible, as in the interior of a vessel, 

 these physiological limitations on the pre- 

 cision of communication do not offer serious 

 obstacles, and the problems of establishing 

 a rapid and accurate system of visual com- 

 munication become those of designing the 

 system and of training the senders and re- 

 ceivers. 



The Present Status of Visual 

 Systems of Transient 

 Communication 



Direct Systems 



The variety of direct visual communica- 

 tion systems is relatively limited; the situa- 

 tions in which they have been most useful 

 are those in which attenuation by distance 

 has made voice communication impossible, 

 or unreliable without the intervention of a 

 mediating system. Consequently, they have 

 been used in military situations where the 

 sender is at some distance from the receiver, 

 so that the receiver is typically stimulated by 

 the sender's signal through a long column 

 of atmosphere. The optical transmission of 

 this column of air sets an unavoidable limit* 

 on the ranges over which direct visual com- 

 munication can be established and lends it 

 all of the predictable unpredictability of a 

 weather forecast, and all of the ultimate 

 uncontrollability of the weather itself. Con- 

 sequently, direct systems are becoming pro- 

 gressively less important as radio techniques 

 develop. On the other hand, the utility 



3 The attenuation of electromagnetic radiation 

 by passage through water clearly limits the range 

 over which under-water visual communication 

 can be effected. So sharp is this limit that it is 

 unfortunately unnecessary to devote space to the 

 possibility of submarine-to-submarine visual 

 communication when they are submerged. Ex- 

 perimental work in this direction has proven abor- 

 tive. 



