176 VITALITY AND EFFICIENCY WITH RESTRICTED DIET. 



state the threshold in seconds on the arc of vision. For example, a 

 micrometer-scale reading of 3.14 shows bands 0.5 mm. in width. This 

 width of 0.5 mm. represents to the subject's vision an angular separa- 

 tion of 60.7 seconds between the bright lines of the test field.^ 



(13) Sensory Threshold for Electric Shock. 



In a series of articles, Martin has described a method for using 

 induced current (faradic stimulation) to determine thresholds for 

 muscle and sensory processes. He and his co-workers have used this 

 in several researches.^ The same method has been used at the Nutri- 

 tion Laboratory with apparatus which is practically a duplicate of 

 Professor Martin's. Two researches have been published, one by 

 Dodge and Benedict, the other by Miles, in which the sensory threshold 

 for faradic stimulation was a factor under investigation. According 

 to the theory and formula given by Martin, threshold determinations 

 taken with different resistances in the secondary circuit with the fingers 

 of the subject should fall on a straight line when plotted. In the results 

 which have been obtained at this Laboratory it has seldom been found 

 that the thresholds of a subject fall on this theoretical straight line. 

 Our difficulty may have been subjective, due to lack of careful coopera- 

 tion and attention of the subject measured. On the other hand, it is 

 not impossible that some of the trouble was instrumental or had to do 

 with the technique. One point in the method seems particularly un- 

 satisfactory, i. e., the determination of the tissue resistance. This was 

 done by means of balancing on a simple Wheatstone bridge. The tis- 

 sue, usually the finger-tips, together with a known resistance, was 

 placed in one arm of the bridge, and against this a variable resistance 

 was balanced, the final adjustment being made on a slide-wire. A 

 telephone was used as an indicator of the point of balance on this 

 wire. As a matter of fact, a good balance-point which gives anything 

 near silence in the telephone can never be found; the operator has to 

 resort to balancing quality against quahty in the two ends of the shde- 

 wire. This is a very difficult proposition with the telephone as an 

 indicator, since the telephone membrane has characteristics which 

 make it respond to certain vibration frequencies with greater degrees 

 of intensity.^ When the string galvanometer was used as an indicator 

 in the bridge in place of the telephone, the reading was far from agree- 

 ing with that obtained by the telephone. The balance in the bridge 

 was somewhat improved by using an adjustable capacity in the varia- 



^ In practice the constant 1.573 for the instrument and the unit width 0.008242 mm. for an arc 

 of 1 second are, by dividing the latter into the former, combined into one factor of 191, 

 which, when divided by the micrometer-scale reading, gives directly the angular separa- 

 tion of the bright bands in degree-seconds. 



2 Martin, Am. Journ. Physiol., 1908, 22, p. 116, and 1910, 27, p. 226; also. The Measurement of 

 Induction Shocks, New York, 1912. See also, Martin, Porter and Nice, Psychol. 

 Rev., 1913, 20, p. 194. 



^See Meyer and Whitehead, Proc. Am. Institute of Elec. Engineers, 1912, 31, p. 1023; Kennelly 

 and Affel, Proc. Am. Acad. Arts and Sci., 1915, 51, p. 419. 



