The speed 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 found to move very well through 19 deg. 38’ in 275[sigma], although, again, this is much more than a proportionate part of the total time (99.9[sigma]) given by Dodge and Cline for a movement of the eye through 40 deg..  The eye is in this case also moving slowly.  As before, it is permissible to let the pendulum run down till it swings too slowly for the eye to move with it; since any lessened speed of the pendulum only makes the reddish-orange phase more prominent.