124 



RADAR SCOPES 



the width of the visual field (cf. Fig. 10 

 below) and the speed of eye movements. 



Retinal Distance from Line of Sight 



Although in general it is kno\\'ii that in 

 good light, f oveal vision is better than periph- 

 eral, the effect of eccentric vision on radar 

 detection had to be detennined directly on 



subtended only about r48" at the eye and 

 the largest pip only six times this, the ex- 

 tension of the pip itself into the fixation area 

 could account for but a fraction of the equal 

 visibility band. It is also interesting to note 

 in Fig. 10 that the slopes of the curves are 

 different for different background bright- 

 nesses. As might be expected, the steepest 



DISTANCE FROM FIXATION POINT 

 Fig. 10. Visibility at different retinal positions 

 Binocular fixation was at zero. A 1 /zs x 1° pip (retinal subtense, about 3^' x 7^') was shown at various 

 distances in a horizontal plane.. . The rise at 4 inches is probably an artifact of method. No video 

 noise. The data from the upper and lower sections were taken in independent experiments. 



radar scopes. Data taken from Williams' 

 study (53) are shown in Figs. 10 and 11. 

 It is evident that for the smallest pip avail- 

 able, visibility steadily declines as the pip 

 is moved away from the point of visual fixa- 

 tion and that there is no real area of equal 

 visibility. For a larger pip there is a small 

 area of equal visibility, although it is some- 

 what less than shown because of the fact 

 that the distance from the fixation point to 

 the pip was measured from center to center. 

 Considering, however, that the smallest pip 



slope is with the brightest screen and the 

 least slope with the dimmest. These data 

 confirm, in general, those of Craik and Mac- 

 pherson's (10) similar study, but differ in 

 yielding a slightly larger effect. Whether 

 the difference is due to background bright- 

 ness is not known, for no mention of this 

 factor is made by them. There is an in- 

 teresting implication of these data for radar 

 practice. For example, it had been pre- 

 viously noted by the author that if an at- 

 tempt is made to trick an operator by getting 



