d of the pendulum must be determined, such that if _I_ lies in
the front groove (Fig. 5, _x_) and the eye is at rest, the image will
clearly show two phases of color when _T_ swings past on the pendulum.
With _T_ and _I_ as described above, a very slow pendulum shows the
image green, red (narrow), and green, in succession. A very fast
pendulum shows only a horizontal straw-yellow band on a green field
(Fig. 8:5). There is but one phase and no feeling of succession.
Between these two rates is one which shows two phases--the first a
green field with a horizontal, reddish-orange band (Fig. 8:3), the
second quickly following, in which the band is straw-yellow (5). It
might be expected that this first phase would be preceded by an
entirely green phase, since green is at first exposed. Such is however
not the case. The straw-yellow of the last phase is of course the
fusion-color of the red and green glasses. It would be gray but that
the two colors are not perfectly complementary. Since the arrangement
of colors in _T_ is bilaterally symmetrical, the successive phases are
the same in whichever direction the pendulum swings.

[Illustration: MONOGRAPH SUPPLEMENT 17. PLATE IV.
Fig. 8.
HOLT ON EYE-MOVEMENT.]

It is desirable to employ the maximum rate of pendulum which will give
the two phases. For this the illumination should be very moderate,
since the brighter it is, the slower must be the pendulum. With the
degree of illumination used in the experiments described, it was found
that the pendulum must fall from a height of only 9.5 deg. of its arc: a
total swing of 19 deg.. The opening of _T_, which is 9 cm. wide, then
swings past the middle point of _I_ in 275[sigma].

Now when the eye moves it must move at this rate. If the eye is 56 cm.
distant from the opening, as in the previous case, the 9 cm. of
exposure are 9 deg. 11' of eye-movement, and we saw above that 9 deg. 11' in
110[sigma] is a very slow rate of movement, according to the best
measurements. Now it is impossible for the eye to move so slowly as 9 deg.
11' in 275[sigma]. If, however, the eye is brought nearer to the
opening, it is clear that the 9 cm. of exposure become more than 9 deg.
11' of eye-movement. Therefore the eye and the fixation-points are so
placed that _EA_ (Fig. 5) = 26 cm. and _PP'_ = 18 cm. The total
eye-movement is thus 38 deg. 11', of which the nine-centimeter distance of
exposure is 19 deg. 38'. Now the eye is