In order to separate the resident organic factors from those presented by the fixed relations of the external world, an adaptation of the mechanism was made for the purpose of carrying on the observations in a darkened room.  For the cardboard disc was substituted a light carriage, riding upon rigid parallel vertical wires and bearing a miniature ground-glass bulb enclosing an incandescent electric light of 0.5 c.p.  This was encased in a chamber with blackened surfaces, having at its center an aperture one centimeter in diameter, which was covered with white tissue paper.  The subdued illumination of this disc presented as nearly as possible the appearance of that used in the preceding series of experiments.  No other object than this spot of moving light was visible to the observer.  Adjustment and record were made as before.  The results for the same set of observers as in the preceding case are given in the following table: 

TABLE II.

Subject.        Constant Error.   Average Deviation.   Mean Variation.
A   (50)    — 52.76          55.16               30.08
C   (30)    —  7.40          42.00               35.31
D   (50)    — 14.24          38.60               30.98
E   (50)    — 43.12          86.44               30.19
F  (100)    —  2.01          72.33               20.27
G  (100)    — 21.89          47.47               32.83
H   (50)    —  1.62          59.10               29.95
I   (50)    — 32.76          41.60               24.40
K   (50)    — 61.70         100.02               52.44
L   (40)    -128.70         128.90               27.83

   Average:  — 36.62 67.16 31.43

Changes in two directions may be looked for in the results as the experimental conditions are thus varied.  The first is a decrease in the certainty of judgment due to the simple elimination of certain factors upon which the judgment depends.  The second is the appearance of definite types of error due to the withdrawal of certain correctives of organic tendencies which distort the judgment in specific directions.  The loss in accuracy is great; the mean variation increases from 7.69 to 31.43, or more than 400 per cent.  This large increase must not, however, be understood as indicating a simple reduction in the observer’s capacity to locate points in the horizontal plane of the eyes.  The two series are not directly comparable; for in the case of the lighted room, since the whole visual background remained unchanged, each determination must be conceived to influence the succeeding judgment, which becomes really a correction of the preceding.  To make the two series strictly parallel the scenery should have been completely changed after each act of judgment.  Nevertheless, a very large increase of uncertainty may fairly be granted in passing from a field of visual objects to a single illuminated point in an otherwise dark field.  It is probable that this change is largely due to the elimination of those elements of sensation depending upon the relation of the sagittal axis to the plane against which the object is viewed.