The subject's task in this measurement was to look from one to the other of two marks successively and as rapidly as possible throughout a period of 5 seconds. The two fixation marks were definite and prominent and separated by a distance which amounted to 40° on the subject's arc of vision. The left eye was covered, the fixation-points being visible only to the right eye. The eye movements were recorded by Dodge's familiar photographic technique.2 The apparatus was the same as that used for recording the eye reactions and the subject occupied the same position (see figs. 30,32, and 35). The ground plan of the apparatus is indicated by figure 50; the subject occupies a position at E; one fixation mark is at R, to his right, the other at L. The angle subtended between these two marks is 40°. The mark R was always in position; L was placed in position when the eye-movement measurements were to be made. It was mounted on a screen F, which could be raised out of the way of the stimulus device for the eye reactions, as can be seen in figure 35, page 165. The frame 2 in figure 50 is the same as 2 in figure 35. When this was released and fell by the action of the small solenoid shown in the latter figure, it exposed fixation-point R, and this was the signal for the subject to look from R to L and so begin his series of movements. At the moment that R was exposed the beam of light was turned on the subject's eye and the photographic record was commenced. The beam of light was reflected from mirror M1 to mirror M2 just below the lens Z, and from this second minor to the cornea of the subject's eye, from which it reflected back through the lens and through the hood of the camera H to the falling plate as indicated by the arrows.

The subject very easily understood his task. He had to be especially warned, however, to hold his head still by keeping it pressed firmly against the rest and biting on the soft wooden peg with his teeth. He was repeatedly told that he must make his eye do all the moving and that he should see one mark clearly before looking for the other. The subject was instructed also to refrain from winking at the time of moving the eye.1 The black screen which enclosed the subject's field of view (S, figure 50; B, figure 35) was of particular service in eliminating all distracting objects, so that it was unnecessary to warn the subject to refrain from looking at other objects than the correct fixation-points. Two series of movements were recorded on each photographic plate. The camera was moved slightly to one side at the beginning of the second record, so that the tracings would not overlap sufficiently to make them illegible. Two plates were taken for each subject, giving a total of four series of movements. A plate from each of two subjects may be seen in figure 52. The records are to be read from the bottom upward. The subject's eye movements were along a horizontal line, the plate moving downward at right angles to the line of eye movement. Right and left indicated in figure 52 corresponds with R and L of figure 50. They are of course reversed since the record was taken through a lens. In beginning a series of movements the subject looked for an instant at the right-hand mark R after it was exposed; then the eye was moved to L at the left. A corrective movement was frequently necessary to direct the vision definitely at the fixation mark. The dashes between the fixation-points were caused by the time-interrupter in the beam of light, each dash and one interspace equaling 0.01 second. The corrective movements in the record of the left-hand plate show that this subject was unusually accurate in his fixation of the marks. The other man sacrificed accuracy for speed. In spite of instructions the subject who made the records in the right-hand plate did not accurately fixate the marks in turn. The difference can be clearly brought out by laying a rule along the left-hand boundary of these records. The fixation marks should fall on a straight line, which they closely approximate in the left-hand records.

Fig. 50. - Ground plan of the apparatus and arrangement for photographing eye movements.

S, black screen surrounding the subject's field of view; R and L, right and left fixation marks separated by 40°; H, hood of camera; Z, lens in front of subject's eye, E; Ml and M2, mirrors for reflecting beam of light indicated by arrows; 2, frame carrying shield, A, drops down and exposes eye to light as signal for movements to begin; C, cords for operating eye-reaction, stimulus device. The camera, located at the other end of the hood //, is not indicated in the diagram.

1Miles, Carnegie Inst. Wash. Pub. No. 266, 1918, p. 81.