Visual stimuli were used. The objects to give the perceptual basis for the judgments were small rectangular openings in cardboard seen, on exposure, by transmitted light. These rectangular windows in the cardboard were 2 cm. by 1 cm. and stood in the vertical position 1 cm. apart. The judgments were all based upon differences existing between these rectangles as shown. One of these differences was in length. They might be of the same length, or either the right or left might be 2 mm. longer than the other. Another difference was in shade. This was secured by different thicknesses of paper, pasted over the openings. Two shades were used. The opening on one side might be shown as either the same brightness, brighter, or less bright. The third difference was in the number of lines which crossed the rectangles. Two or three wires were placed across them horizontally and about 5 mm. apart. Thus they had the same number of lines, or one had fewer or more than the other.
The same large pendulum was used in these experiments. The moveable magnet on the curved steel bar was kept in one position throughout. It held the pendulum, ready for release, at twenty degrees from the position of rest. The adjustable weight on the pendulum was also kept in one position. The only adjustment which was changed during this series of experiments was the width of the slit in the window of the screen. This was varied from one millimetre to five. The whole time during which any part of the two rectangles was in view (the total exposure) with a 5 mm. slit was .033 sec.; with a 3 mm. slit .031 sec.; with a 1 mm. slit .029 sec. These times were measured with a Hipp's chronoscope. The entire visual field, embracing the two rectangles, was about 2 cm. by 3 cm., and was about three fourths of a metre from the observer's eye. It could be accurately fixated beforehand and fully exploited during the moment of exposure.
The observer was always instructed to give his judgments in terms of one of the two rectangles. If, for example, length was in question, he should say of the left-hand rectangle that it was longer, shorter, or of the same length as the right-hand one. The process of expressing the judgments was also facilitated by using the terms plus, minus, and equal, for all three sorts of judgments. This was a special aid to expression where two or more judgments were in question at the same time. In these cases the observer was always given an order beforehand, in which the judgments were to be given. This order for the three combined, for example, was always, "length, lines, shade," as in the following tables. If, then, the judgments were given "plus, minus, minus," it meant that the left-hand rectangle was longer, had fewer lines, and was less bright than the right. The process of making these interpretations, as well as the order, was made automatic with the observer, by practice, before experimenting.
Three observers, A, B, and Y, were used in this experiment. The judgments were made in series of ten. Each hour's work was distributed over (1) several series of single judgments, (2) two at a time, and (3) three at a time, the aim being to get an equal number of judgments of each kind, length, lines, and shade, under each of the three conditions. The results are given as general percentages of correct results. To properly weight these averages, the number of series (of ten judgments each) which are included in making up any average, is given just above the average.
TABLE X
| Single Judgment | Two Judgments | Three Judgments | |||||||
| Obs. | Length | Lines | Shade | Length | Lines | Shade | Length | Lines | Shade |
| A Number of series averaged | 11 | 12 | 10 | 15 | 17 | 14 | 11 | 11 | 11 |
| Per cent Correct Judgments | 95 | 93 | 96 | 90 | 91 | 83 | 94 | 91 | 89 |
| \———\/———/ | \———\/———/ | \———\/———/ | |||||||
| Average | 95 | 88 | 91 | ||||||
| B Number of series averaged | 8 | 8 | 7 | 9 | 10 | 9 | 7 | 7 | 7 |
| Per cent Correct Judgments | 93 | 80 | 90 | 76 | 77 | 90 | 75 | 70 | 75 |
| \———\/———/ | \———\/———/ | \———\/———/ | |||||||
| Average | 88 | 81 | 73 | ||||||
| Y Number of series averaged | 12 | 13 | 13 | 19 | 19 | 16 | 13 | 13 | 13 |
| Per cent Correct Judgments | 76 | 80 | 58 | 72 | 76 | 61 | 72 | 74 | 58 |
| \———\/———/ | \———\/———/ | \———\/———/ | |||||||
| Average | 71 | 70 | 68 | ||||||
Since these general averages for the single judgments are so close to those in pairs, it seemed possible that the presence of objective differences, other than the single one asked for, might be a distracting agent, and really interfere with the judgment process in question. For example, when judgment on length was in question, it might be possible to give it correctly a larger number of times, if there were no differences in shade or lines, than if these were present. Some careful test experiments were made with a view to clearing up this situation. The observers in no case knew the nature of the investigation, nor were they aware that other differences were absent in some of the cases. The results presented in Table XI certainly show that the presence of other differences than the one in question is no cause of interference.
TABLE XI
| Length | Lines | Shade | ||||
| Obs. | With Diffs. | Alone | With Diffs. | Alone | With Diffs. | Alone |
| A Number of series averaged | 5 | 5 | 5 | 5 | 5 | 5 |
| Per cent Correct Judgments | 98 | 96 | 96 | 96 | 98 | 94 |
| B Number of series averaged | 5 | 5 | 5 | 5 | 5 | 5 |
| Per cent Correct Judgments | 90 | 92 | 96 | 94 | 78 | 84 |
| Y Number of series averaged | 7 | 8 | 8 | 8 | 7 | 8 |
| Per cent Correct Judgments | 73 | 76 | 75 | 64 | 88 | 67 |