PSYCHOLOGICAL MEASUREMENTS. 



Table 174. — Squad B — Reaction time for finding serial numbers. 

 [Values in seconds.] 



607 



of a practice effect with both groups. Squad A shows a lengthening 

 of time on October 27, also a depression on December 8 and 19, and 

 a marked drop on January 26. Squad B shows a depression in the 

 curve on their first reduction date of January 13. This decline may 

 be a mere coincidence as there is continued improvement on the two 

 following dates, January 19 and 27. In general the curve for Squad B 

 is more regular than that for A, but it can not be stated that there is 

 any definite indication of an effect due to the reduction in diet. 



(12) SENSORY THRESHOLD FOR VISUAL EFFICIENCY. 



Extended series of observations with this apparatus and with trained 

 subjects have shown that this measure of visual efficiency is remark- 

 ably constant from day to day and shows a standard deviation of 

 approximately 4 per cent of the average threshold. The normal series 

 for 1917, previously referred to, contains threshold values for 61 sub- 

 jects, which range from 38" to 106" on the arc of vision. The average 

 was 68.7", with a standard deviation for the 61 men of 18.5", which is 

 27 per cent of the average threshold value. It should be noted that 

 good vision was one of the factors operative in the selection of the 

 normal group of men, 25 of whom (somewhat more than 40 per cent) 

 show thresholds of 60" or somewhat less.-^ It is something of a sur- 



^ In the test-letters on charts used to determine acuteness of vision, the width of one stroke 

 or of the space between two strokes composing a letter is the fundamental width or unit for vision. 

 A determination of |§ means that the subject distinguishes the direction of lines the stroke width 

 of which occupies 60" on his arc of vision. Similarly, as recorded on page 175, the width of one 

 dark band or of one light band, or to state it in another way, since the boundary between dark and 

 light is not sharp in our test object (see figure 51), the distance from the center of one dark band 

 to the center of an adjoining light band was taken as the unit width in terms of which the visual 

 efficiency was given. A threshold of 60" signifies that the subject became aware of lines in the 

 test field and could correctly indicate their direction (vertical, horizontal, etc.) when the unit 

 width, from center to center of adjoining dark and light bands equaled 60" on his arc of vision. 

 In a personal communication Captain Percy W. Cobb, M. R. C, states that workers at the Nela 

 Research Laboratory have regularly employed this same unit with these test objects and believe 

 that the results thus expressed are in closest conformity with the regular Snellen units. Dr. Cobb 

 states further that f § is quite common as an ophthalmologic finding and that in persons who have 

 no defect or are well corrected, it is not uncommon to get \% or better, i. e., 45" or less as visual 

 angles. 



