PSYCHOLOGICAL PROGRAM AND TECHNIQUE. 175 



the lines are going to appear, because the Hnes are made too small to see, 

 and the instrument is set for some one of these four directions in a chance 

 order. Do not try to anticipate the direction in which the lines will come. 

 When you are given the signal to be ready, look intently at the black dot in 

 the middle of the field. The instant you know the direction of the lines call 

 *stop' and give the direction. After you have done this, rest the eye by 

 closing it until the next 'ready' signal." 



The right eye of the subject was always used for the determination; 

 it was not refracted and glasses were never worn. In these experi- 

 ments it was not the object to attain absolute values or to reveal indi- 

 vidual differences, but to test the man, for comparative purposes, 

 against himself in successive sessions. Since the eye was not refracted 

 it is obvious that astigmatism would play a role in influencing readings 

 in certain axes. The time at our disposal for taking the measurements 

 on a single subject was approximately 10 minutes. In this interval 

 it was usually possible to make from 3 to 5 separate threshold deter- 

 minations for each of the four positions, right, vertical, left, and hori- 

 zontal. These followed each other in a chance order. After two or 

 three trials the experimenter knew, approximately, where to expect 

 threshold value. The micrometer wheel was, after this, advanced, at 

 first one turn in about 5 seconds, and then the rest of the way to thresh- 

 old at the rate of one revolution in 15 seconds, as the object was to get 

 the reading within an interval of less than a minute after the sub- 

 ject had been given the ''ready" signal and fixated on the black dot in 

 the middle of the test field. A comparison of results from this appa- 

 ratus, with standards previously established, will be considered in the 

 discussion of our results (see page 607) for the reduced diet experiment. 



It is most useful to state the threshold-test band width for any sub- 

 ject in terms of degrees on the arc of vision. In the Nutrition Labo- 

 ratory instrument the distance from tip to tip of the yoke 7 is 119 mm. 

 The separation of the ruled hnes on the glass plates is ytt inch or 0.1 0577 

 mm. According to the formula given by Behn, Ives, and Johnson, in 

 the articles cited, the constant for this instrument is therefore 1.573. 

 This factor divided by the micrometer scale reading on the instrument 

 gives in millimeters the width of the separation of the light bands. To 

 illustrate in another way, if the micrometer wheel is set to read 1.57, 

 measurement by a millimeter scale applied directly to the window of 

 the test object will show that both the light and dark bands are each 

 1 mm. in width, i. e., from the center of one dark band to the center 

 of the adjoining light band is 1 mm. A scale-reading of 3.14 will 

 show bands 0.5 nam. in width, and so on. In the arrangement em- 

 ployed the window of the test object is distant 170 cm. from the sub- 

 ject's eye. On the circumference of a circle with this radius an arc of 

 1 second = 0.008242 mm. The millimeter width of test bands just 

 distinguishable to the subject's eye may be divided by 0.008242 to 



