IV.

The influence of this factor is also exhibited when judgments made with both eyes are compared with those made under conditions of monocular vision. The latter experiments were carried on in alternate series with those already described. The figures are given in the following tables:

TABLE VI.

JUDGMENTS MADE IN DIFFUSED LIGHT.

Observer. Constant Error. Average Deviation. Mean Variation. _A_ (50) - 28.46 29.04 8.87 _C_ " + 7.54 14.86 8.01 _D_ " + 39.32 43.28 13.83 _E_ " + 50.46 65.26 9.86 _F_ " + 62.30 62.30 1.60 _G_ " 0.00 45.28 9.66 _H_ " + 22.92 79.12 5.07 _I_ " + 14.36 51.96 8.02 _K_ " + 9.26 38.10 9.55 _L_ " - 61.10 61.10 6.36 Average: + 11.66 49.03 8.18

TABLE VII.

JUDGMENTS IN ILLUMINATED POINT.

Observer. Constant Error. Average Deviation. Mean Variation. _A_ (50) - 38.42 51.96 32.64 _C_ (30) - 29.03 41.23 35.75 _D_ (20) - 30.87 34.07 17.24 _E_ (50) + 65.30 75.86 29.98 _F_ " + 50.74 50.74 5.89 _G_ " + 66.38 88.10 44.98 _H_ " + 65.40 80.76 42.93 _I_ " - 0.02 80.22 47.53 _K_ " - 44.60 52.56 32.93 _L_ " - 71.06 73.30 31.86 Average: - 3.38 62.88 32.17

The plane of vision in judgments made with the right eye alone is deflected upward from the true horizon to a greater degree than it is depressed below it in those made with binocular vision, the respective values of the constant errors being -7'.70 and +11'.66, a difference of 19'.36. When the field of vision is darkened except for the single illuminated disc, a similar reversion of sign takes place in the constant error. With binocular vision the plane of the subjective horizon is deflected downward through 36'.62 of arc; with monocular vision it is elevated 3'.38, a difference of 40'.00, or greater than in the case of judgments made in the lighted room by 20'.64. This increase is to be expected in consequence of the elimination of those corrective criteria which the figured visual field presents. The two eyes do not, of course, function separately in such a case, and the difference in the two sets of results is undoubtedly due to the influence of movements in the closed eye upon that which is open; or rather, to the difference in binocular functioning caused by shutting off the visual field from one eye. The former expression is justified in so far as we conceive that the tendency of the closed eye to turn slightly upward in its socket affects also the direction of regard in the open eye by attracting toward itself its plane of vision. But if, as has been pointed out, this elevation of the line of sight in the closed eye is accompanied by a characteristic change in the process of binocular convergence, the result cannot be interpreted as a simple sympathetic response in the open eye to changes taking place in that which is closed, but is the consequence of a release of convergence strain secondarily due to this act of closing the eye.

Several points of comparison between judgments made with binocular and with monocular vision remain to be stated. In general, the process of location is more uncertain when one eye only is used than when both are employed, but this loss in accuracy is very slight and in many cases disappears. The loss in accuracy is perhaps also indicated by the range of variation in the two cases, its limits being for binocular vision +46'.29 to -56'.70, and for monocular +62'.30 to -61'.10, an increase of 20'.41. In the darkened room similar relations are presented. The mean variations are as follows: binocular vision, 31'.42; monocular, 32'.17. Its limits in individual judgments are: binocular, -1'.62 to -128'.70, monocular, +66'.38 to -71'.06, an increase of 10'.36. In all ways, then, the difference in accuracy between the two forms of judgment is extremely small, and the conclusion may be drawn that those significant factors of judgment which are independent of the figuration of the visual field are not connected with the stereoscopic functioning of the two eyes, but such as are afforded by adjustment in the single eye and its results.