PBCDB’P is the horizontal section of a semicircular perimeter of 30 cm. radius. E is an eye-rest fixed at the centre of the semicircle; CD is a square hole which is closed by the screen S fitted into the front pair of the grooves GG.  In the center of S and on a level with the eye E is a hole A, 2 cm. in diameter, which contains a ‘jewel’ of red glass.  The other two pairs of grooves are made to hold pieces of milk-or ground-glass, as M, which may be needed to temper the illumination down to the proper intensity. L is an electric lamp. B and B’ are two white beads fixed to the perimeter at the same level as E and A, and used as fixation-points.  Although the room is darkened, these beads catch enough light to be just visible against the black perimeter, and the eye is able to move from one to the other, or from A to either one, with considerable accuracy.  They leave a slight after-image streak, which is, however, incomparably fainter than that left by A (the streak to be studied), and which is furthermore white while that of A is bright red. B and B’ are adjustable along a scale of degrees, which is not shown in the figure, so that the arc of eye-movement is variable at will. W is a thin, opaque, perpendicular wall extending from E to C, that is, standing on a radius of the perimeter.  At E this wall comes to within about 4 mm. of the cornea, and when the eye is directed toward B the wall conceals the red spot A from the pupil. W can at will be transferred to the position ED. A is then hidden if the eye looks toward B’.

The four conditions of eye-movement to be studied are indicated in Fig. 3 (Plate 1.).  The location of the retinal stimulation is also shown for each case, as well as the corresponding appearance of the streaks, their approximate length, and above all their localization.  For the sake of simplicity the refractive effect of the lens and humors of the eye is not shown, the path of the light-rays being in each case drawn straight.  In all four cases the eye moved without stopping, through an arc of 40 deg..

[Illustration:  PSYCHOLOGICAL REVIEW.  MONOGRAPH SUPPLEMENT, 17.  PLATE I.
               Fig. 3. 
               HOLT ON EYE-MOVEMENT.]

To take the first case, Fig. 3:1.  The eye fixates the light L, then sweeps 40 deg. toward the right to the point B’.  The retina is stimulated throughout the movement, l-l’.  These conditions yield the phenomenon of both streaks, appearing as shown on the black rectangle.